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    Abstract
    Introduction
    Invasive Methods
    Minimally Invasi...
    Non-Invasive Methods
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TUTORIAL Table of Contents   
Year : 2008  |  Volume : 11  |  Issue : 1  |  Page : 56-68
Cardiac output monitoring


Pondicherry Institute of Medical Sciences, Kalapet, Puducherry, India

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   Abstract 

Minimally invasive and non-invasive methods of estimation of cardiac output (CO) were developed to overcome the limitations of invasive nature of pulmonary artery catheterization (PAC) and direct Fick method used for the measurement of stroke volume (SV). The important minimally invasive techniques available are: oesophageal Doppler monitoring (ODM), the derivative Fick method (using partial carbon dioxide (CO 2 ) breathing), transpulmonary thermodilution, lithium indicator dilution, pulse contour and pulse power analysis. Impedance cardiography is probably the only non-invasive technique in true sense. It provides information about haemodynamic status without the risk, cost and skill associated with the other invasive or minimally invasive techniques. It is important to understand what is really being measured and what assumptions and calculations have been incorporated with respect to a monitoring device. Understanding the basic principles of the above techniques as well as their advantages and limitations may be useful. In addition, the clinical validation of new techniques is necessary to convince that these new tools provide reliable measurements.
In this review the physics behind the working of ODM, partial CO 2 breathing, transpulmonary thermodilution and lithium dilution techniques are dealt with. The physical and the physiological aspects underlying the pulse contour and pulse power analyses, various pulse contour techniques, their development, advantages and limitations are also covered. The principle of thoracic bioimpedance along with computation of CO from changes in thoracic impedance is explained. The purpose of the review is to help us minimize the dogmatic nature of practice favouring one technique or the other.

Keywords: Cardiac output, impedance cardiography, lithium dilution, oesophageal doppler monitoring, partial CO 2 breathing, pulse contour analysis, transpulmonary thermodilution

How to cite this article:
Mathews L, Singh KR. Cardiac output monitoring. Ann Card Anaesth 2008;11:56-68

How to cite this URL:
Mathews L, Singh KR. Cardiac output monitoring. Ann Card Anaesth [serial online] 2008 [cited 2016 Dec 3];11:56-68. Available from: http://www.annals.in/text.asp?2008/11/1/56/38455



   Introduction Top


Cardiac output (CO) is the primary compensatory mechanism that responds to an oxygenation challenge. Although Harvey discovered the circulation of blood more than 300 years ago, routine measurement of CO is available only since 1970's, when Swan, Ganz and colleagues reported that pulmonary artery catheter (PAC) insertion could be performed at the bedside by the use of specially designed balloon tipped catheter. [1]

Ideally, the technology which provides CO estimation should be noninvasive, accurate, reliable, continuous, compatible in adults and paediatric patients. At present, no single technique meets all these criteria. Recent advances in technology have led to the development of minimally invasive and non-invasive methods. The development of impedance cardiography and advances in electronics and signal processing has led to the development of totally non-invasive monitors, which can provide continuous measurement of haemodynamic parameters. Several reviews published in the recent past report clinical validation and limitations of the various minimally invasive and totally non-invasive techniques as well as their limitations. [2],[3],[4],[5] A recent review by the authors [6] explains the use of PAC in haemodynamic monitoring.


   Invasive Methods Top


Thermodilution technique

Apart from its pressure monitoring capabilities, undoubtedly the most important feature of PAC is its ability to measure CO using the thermodilution method. The thermodilution technique has become the defacto clinical standard for CO determination. This technique relies on principle similar to indicator dilution but uses heat instead of colour as an indicator. Cardiac output is calculated using the modified Stewart-Hamilton indicator dilution equation. The technique has recently been modified to calculate the continuous CO as well as monitoring of right ventricular function. The details of thermodilution technique for intermittent and continuous CO monitoring as well as monitoring of right ventricular function along with the details of PAC technique, pulmonary artery occlusion pressure (PAOP) measurements and major complications of PAC are given elsewhere. [6]

Fick's cardiac output measurement

The first method to find CO in humans was described by Adolph Fick in 1870. He postulated that oxygen uptake in the lungs is completely transferred to the blood and the total uptake or release of oxygen by lungs is the product of blood flow through the lungs and the arterio-venous oxygen content difference. Therefore cardiac output can be computed by relating oxygen consumption to arterial and mixed venous oxygen content using the equation



Where VO 2 is the oxygen content difference between inspired and exhaled gas, C a O 2 is oxygen content of arterial blood and C v O 2 is oxygen content of mixed venous blood. [7] This estimation is accurate when the haemodynamic status is sufficiently stable to allow constant gas diffusion during the mean transit time of blood through the lungs. The technique is often not applicable in critically ill patients because they require extreme ventilatory conditions of high fractional inspired oxygen or because their haemodynamic status is not stable. The arterio-venous oxygen content difference can be measured in-situ using a PAC having fibreoptic bundles incorporated in the catheter. Alhough the technique is not a practical bedside method any more, it is considered the most accurate method available to evaluate patients with low CO. [7]


   Minimally Invasive Methods Top


Doppler ultrasound

Ultrasound easily penetrates skin and other body tissues. As it encounters tissues of different acoustic density, a fraction of emitted ultrasound signal is reflected. When an ultrasound beam is directed along the path of the flow of blood in the aorta, using a probe, a fraction of the ultrasound signal is reflected by the moving red blood cells. The shift in the frequency of the reflected waves (Doppler shift) is proportional to the velocity of blood flow and is expressed by the equation:

F d = 2f 0 /C V Cosθ

Where F d is the change in frequency or Doppler shift, f 0 is the transmitted frequency, V is the velocity of moving blood, C is the velocity of ultrasound in blood, cosθ is cosine of the angle between the direction of ultrasound beam and blood flow.

The oesophageal Doppler monitoring (ODM) is a widely applied method of minimally invasive cardiac output monitoring performed currently in critically ill patients. [8] It is based on measurement of blood velocity in the descending thoracic aorta by means of Doppler transducer and either an estimated aortic cross sectional area close to the mean value during systole (derived from a nomogram stored in the computer based on age, sex, height and weight) or a measured cross-sectional area using an M-mode echo-transducer incorporated into the probe. This provides an immediate, comprehensive, left ventricular flow based assessment of the net effects of changes in the fundamental haemodynamic component i.e. stroke volume, heart rhythm, preload, contractility and afterload. [8],[9],[10],[11]

The volumetric flow rate of an established flow as a function of time passing through a blood vessel is expressed as the product of blood vessel cross-sectional area (at time t) and the spatial average velocity of blood over the entire cross-section. Therefore, at each instance, the two simultaneous local measurements required are a geometric parameter determined by anatomy (vessel cross-section) and a physiological parameter (blood velocity) reflecting the heart's performance as a pulsatile pump as modified by vascular tone. [12],[13]

The Doppler probe is inserted in to the oesophagus to an approximate depth of 35 cms from the incisors in an intubated patient. The probe is then rotated so that the transducer faces the aorta and characteristic aortic velocity signal is obtained. Probe position is optimized by slow rotation and alteration of depth to generate a clear signal with the highest possible peak velocity and or an optimum M-mode signal. Because the oesophagus and the descending aorta lie in close proximity and run essentially parallel to one another, the ultrasound transducer is mounted at a fixed angle, such that the ultrasound beam is oriented as much as possible in a direction that is parallel to the blood flow, that is known by the CO computer and is used to correct the resulting Doppler shift frequency to provide an accurate blood velocity measurement.

The spectral analysis of the Doppler shift gives velocity-time waveforms. Since distance traveled is velocity multiplied by time, the area under the curve gives the stroke distance i.e. the distance a column of blood travels along the aorta during each ventricular systole. It is obtained by multiplying blood velocity with left ventricular ejection time. The height of the waveform gives the peak velocity and the slope of the upstroke of the wave provides the mean acceleration. The cardiovascular indices, such as stroke volume (SV), CO are calculated from these.

Stroke volume = stroke distance × cross sectional area of aorta

Cardiac output = stroke volume × heart rate

The shape of the waveform allows assessment of the left ventricular preload, contractility and afterload. The base of the wave gives the flow time (left ventricular ejection time), which is corrected for the heart rate to give the corrected flow time.

The major assumption made using Doppler monitoring is that a constant percentage of cardiac output enters the descending thoracic aorta as measurements made in the descending thoracic aorta exclude coronary and cerebral circulations. But the percentage of the CO that passes into the descending thoracic aorta varies with disease states. For example in hypovolaemic state, there will be more flow to coronary and cerebral circulations so a greater portion of the blood will leave the aorta proximal to the descending thoracic aorta. Conversely, with lower limb regional block, vasodilatation due to loss of sympathetic tone will cause a greater portion of the blood to be delivered beyond the descending thoracic aorta. The presence of an intra-aortic balloon pump or severe aortic coarctation will produce turbulent flow in the descending aorta and excludes the effective use of ODM. A thorough description of the technique, its advantages and limitations are published in the recent past. [14],[15],[16]

Various studies comparing CO values obtained with ODM and thermodilution report good correlation and suggest ODM as a clinically useful alternative to thermodilution. [17],[18],[19],[20],[21] Estimation of preload by ODM can be obtained from the corrected flow time. [8],[18],[22] It has been observed that when the preload increased from hypovolaemic states, the corrected flow time increased and when preload was decreased due to haemorrhage or the use of nitrates in normovolaemia the corrected flow time decreased. [22] Estimation of preload can also be achieved by measuring the degree of left ventricular filling. This can be relatively easily assessed by measurement of the end-diastolic dimensions (for example end-diastolic area at the mid-papillary level). [23] A comparison of left ventricular preload assessment using the PAC and ODM made by Singer and Bennet [22] was found to give a more accurate measure of preload (end-diastolic volume) than the PAC (PAOP) which was significantly influenced by changes in ventricular compliance. A similar finding was made by Kincaid and coworkers. [24] Correlation of contractility with peak velocity and acceleration, afterload with shape and size of waveforms are observed by Singer and coworkers. [25] ODM has also been used as a tool to manage fluid resuscitation, thus improving the outcome in high-risk patients undergoing cardiac surgery, [26] hip surgery [27] and gastrointestinal surgery [28],[29] and guide early goal-directed therapy in sepsis and trauma patients. [30] Alhough reported use is limited, an oesophageal Doppler nomogram has been developed specifically for paediatric patients and subsequently validated. [31] Laousse and coworkers [32] used ODM to study the effects of caudal anaesthesia, in paediatric patients, on haemodynamics. Sloth and coworkers [33] have studied the use of ODM for CO monitoring, during cardiac surgery in 532 paediatric patients and reported its feasibility. A systematic review of use of various Doppler techniques for CO monitoring in paediatric patients found acceptable reproducibility. [34] A recent meta-analysis by Dark et al. , [35] based on all the validation studies using ODM, concludes that ODM estimates absolute CO values with minimum bias but limited clinical agreement. The limited clinical agreement is explained as due to the lack of accuracy of both ODM and thermodilution.

Transoesophageal echocardiography

Transoesophageal echocardiography (TOE) has become a widely used intraoperative tool to assess cardiac anatomy, left ventricular function, preload, myocardial ischaemia and myocardial infarction. Clinical studies support TOE as a reliable alternative for intraoperative CO measurement. [36] Initial studies used pulmonary artery blood flow to measure CO. This technique has the drawback of poor image quality because of interference from air in the left mainstem bronchus and it requires substantial probe manipulation. [37] Perino et al , [38] developed a TOE technique that gave excellent agreement with thermodilution. In this technique the probe is positioned to obtain a transverse plane, transgastric short-axis view of the left ventricle at the midpapillary level. By rotating the imaging array to ~120 o , the left ventricular out flow tract and ascending aorta were imaged lying parallel to the ultrasound beam. Aortic blood flow velocities were measured by a continuous-wave Doppler beam focused at the level of the aortic valve. The aortic valve area was measured by planimetry. The SV was then calculated. Using this technique from a single probe location one can monitor left ventricular regional and global wall motion using the short-axis view and measure CO by rotating imaging plane to measure aortic flow velocities. Another method uses the transgastric, apical view to assess aortic blood flow. This position provides an ultrasound beam oriented near parallel to blood flow within the aortic valve. A limitation of the approach is that obtaining the view is technically difficult. [39]

Partial CO 2 rebreathing

For many years it has been recognized that cardiac output can be estimated by using the Fick principle (with carbon dioxide as the marker gas) to measure pulmonary capillary blood flow. The advantages of using CO 2 as the marker gas is that CO 2 elimination is easier to measure accurately than oxygen uptake and that the estimation of arterial CO 2 concentration may be obtained from the gas exhaled from the lungs. The CO 2 concentration in the alveolar end-capillary blood can noninvasively be estimated by monitoring CO 2 concentration in the expired gas and relating it to the blood concentration with the help of CO 2 dissociation curve.

The first such monitor to employ this principle was introduced in 1999. The monitor called NICO 2 (Novametrix Medical Systems Inc, Wallingford, CT, USA) estimates CO, using respiratory gas analysis and pulse oximetry.

The partial CO 2 rebreathing technique uses the differential form of the Fick principle, applied to CO 2 produced by the body and eliminated through gas exchange in the lungs, for CO measurement. With partial rebreathing, a change in CO 2 consumption and an associated change in end-tidal CO 2 in response to a change in ventilation, are used in the Fick's calculation. [40] Since Fick's principle can be applied to any gas diffusing through the lungs, the equation applied with respect to CO 2 can be expressed as



Where VCO 2 , [CO 2 ] v and [CO 2 ] a are CO 2 consumption, venous CO 2 concentration and arterial CO 2 concentration respectively. Assuming that cardiac output remains unchanged under normal (N) and rebreathing (R) conditions, we can derive [5]



If a/b = c/d then (a - c)/(b - d) = a/b = c/d, therefore we derive



Since the diffusability of CO 2 in blood is 22 times faster than that of O 2, it can be presumed that there is no difference between the venous CO 2 concentration between normal and rebreathing conditions. Therefore,



The change in arterial CO 2 concentration can be approximated to change in end-tidal CO 2 multiplied by the slope (S) of the CO 2 dissociation curve (CO 2 volume versus partial pressure curve). Hence it can be written that



The NICO 2 system accomplishes the required change in ventilation by using the rebreathing valve and NICO 2 rebreathing loop. By temporarily adding a rebreathing volume to the breathing circuit, the patient inhales only a portion of the exhaled gases. The sequence of rebreathing and stabilisation is shown in [Figure - 1]. [4] During the baseline period, which lasts for 60 sec, the valve is in the non-breathing mode. During the rebreathing mode, which lasts for 50 sec, a quantity of exhaled gas equal to the volume of the expandable loop and the valve is inhaled. The inhaled CO 2 gas increases the alveolar CO 2 concentration, reduces the net flux of CO 2 diffusing in to the alveoli from the blood, reduces CO 2 elimination from the lungs and increases the arterial CO 2 content. During the restabilisation period, which lasts for 70sec the pressure falls back to the baseline. CO is calculated from the resulting changes in VCO 2 and end-tidal CO 2 using the above formula. [41] The precise breath-by-breath volumetric analysis provides continuous tidal volume and compliance assessment, facilitating adjustment of ventilatory parameters to optimize compliance with positive end expiratory pressure (PEEP) and tidal volume settings, while minimizing unnecessary auto-PEEP and CO depression. [42]

The main drawback of the partial rebreathing method is that since the changes in VCO 2 and Et-CO 2 reflect only the blood flow that participates in gas exchange, an intrapulmonary shunt can affect the CO estimation. If a correction factor for the amount of blood shunted through the lungs is then applied, CO can be derived non-invasively. Because intrapulmonary shunt can affect the CO estimation less precise values are obtained in critically ill patients with increased intrapulmonary shunt and poor haemodynamic stability. In addition, the lungs must be mechanically ventilated while measurements are made.

Several studies have compared the interchangeability of CO values obtained from NICO 2 and thermodilution. Initial validation studies reported a good agreement (bias ±1.8L/min). [43],[44] Botero and coworkers [45] compared CO values obtained with NICO in patients before, during and after cardio-pulmonary bypass (CPB), with those obtained with ultrasound transit-time flowmetry (considered as the true gold standard for CO measurement). The limits of agreement between NICO 2 and ultrasonic flowmetry remained unchanged even after CPB, whereas those between thermodilution and ultrasonic flowmetry were wider. These findings indicate that thermodilution is not the gold standard and is not a reliable reference. Therefore there is considerable doubt about the conclusions of other studies that have used thermodilution as a reference. A study by Levy and coworkers [46] concludes that NICO 2 is clinically acceptable in children with a body surface area of more than 0.6 m 2 and tidal volume greater than 300 mL, while discrepancy with thermodilution is more in smaller patients.

Transpulmonary thermodilution

The most common method of measuring CO involves the use of the PAC, utilising the thermodilution technique. However, in recent years, the use of PAC has become controversial. [47] A relatively new technique is arterial thermodilution or transpulmonary thermodilution. In this technique a bolus of cold indicator is injected into the central vein and the temperature change across the cardiopulmonary system is measured from a central artery, from which CO is calculated with the use of the Steward-Hamilton equation. [48] The injectate that is injected into the right atrium is mixed with the flowing blood in the right ventricle and the relative change in temperature reflects the CO flowing through the cardiovascular system. Many patients in the critical care unit and operating room have central venous and arterial cannulation. Measurement of CO by transpulmonary technique in these patients can be achieved without insertion of any additional invasive catheters. The technique requires insertion of the thermister-tipped catheter into a large artery, most commonly femoral artery. In patients, where the use of femoral artery cannulation is not possible, axillary artery cannulation is performed. The validity of the technique has been demonstrated in patients undergoing cardiac surgery [49] and critically ill patients. [50] Another advantage of this technique is its ability to measure extravascular lung water (EVLW) in patients with pulmonary oedema.

Lithium indicator dilution

In this technique a bolus of isotonic lithium chloride (LiCl) solution (150mM) is injected via a central or peripheral vein and the resulting arterial lithium concentration - time curve is recorded by withdrawing blood past a lithium sensor attached to an already existing arterial line. The CO is calculated from the lithium dose and the area under the concentration - time curve prior to recirculation using the formula

CO = {Li dose (mmol) × 60}/{Area × (1-PCV) (m mol/sec)}

where PCV is packed cell volume which is calculated as haemoglobin concentration (g/dl)/34: this correction is needed because lithium is distributed in the plasma and not into the red or white cells on the first pass to the arterial circulation. [51] Blood flows into the sensor assembly at a rate that is controlled by a battery operated peristaltic pump. The voltage across the ion-selective membrane is related to the lithium ion concentration in the plasma by the Nernst equation. A correction needs to be applied for the plasma sodium concentration because in the absence of lithium the base line voltage is determined by the sodium concentration in the plasma.

The dose of lithium marker needed is 0.15 to 0.30 mmol for an average adult. This dosage has no known pharmacological effect. [52] The technique is contraindicated in patients on therapeutic lithium. Since high doses of neuromuscular blocking agents can interfere with the sensing electrode, reliable readings may not be obtained if used intraoperatively. In such cases lithium calibration has to be performed prior to the use of neuromuscular blocking agents.

Pulse contour analysis

Arterial pulse contour analysis is a technique for measuring and monitoring SV on a beat-to-beat basis from the arterial pulse pressure waveform. If it is possible to know the degree to which aorta complies with a 1mm Hg pressure increment (change in aortic volume for unit change in pressure (dv/dp)) then SV could be computed by measuring the associated change in aortic pressure. The aortic pressure waveform is not obtained from the aorta itself but from a peripheral artery (invasively or non-invasively) or finger tip (non-invasively).

The SV is estimated from aortic pressure waveform using several methods based on knowledge models representing the systemic circulation, which include the lumped windkessel model, [53] the modified three element windkessel model [54] or advanced models that allow one to account for finite pulse wave velocity and wave reflection phenomena. [55] In these methods SV is estimated from the systolic, diastolic or both systolic and diastolic portion of the pressure waveform. The parameters considered are aortic impedance, compliance and peripheral vascular resistance. The aortic impedance represents the opposition to the puls atile flow from the contracting left ventricle. As the blood flows into the aorta the pressure increases and it depends on the aortic cross sectional area and elastic properties of the aorta. As the pressure increases the volume increases. The third element in the model is peripheral vascular resistance.

The basic windkessel and the modified three element models represent the arterial tree by two elements (arterial compliance and peripheral vascular resistance) and three elements (aortic impedance, arterial compliance and peripheral vascular resistance) respectively. The electrical analogue of the models is shown in [Figure - 2],[Figure - 3]. The voltage (V) is analogues to aortic pressure (P(t)), the capacitor (C A ) to whole body arterial compliance, the impedance (Z A ) to aortic impedance, resistance (R) to peripheral vascular resistance and electric current (i) to blood flow (Q(t)).

Diastolic pulse contour analysis

Diastolic pulse contour analysis is based on the basic windkessel model, in which it is assumed that the arterial compliance is constant over the physiological pressure range and peripheral vascular resistance does not vary with in the diastolic interval. Based on these assumptions the arterial blood pressure should decay exponentially during diastolic time intervals with a time constant (τ). The time constant, τ, is given as

τ = C A × R

The standard equation for the diastolic pressure (voltage) for the circuit shown in [Figure - 2] is

P(t) = A 1 exp[-(t-t 0 )/τ] + A 2

where, t, is the time, (A 1 + A 2 ) represent end-diastolic pressure, A 2 is mean circulatory pressure. [56] The time constant, τ, is obtained by fitting the above analytical expression into the diastolic portion of the waveform. By measuring the pulse wave velocity over the aorta (carotid to femoral) C A could be estimated. Knowing τ and C A, peripheral resistance R is calculated. From the mean arterial pressure (MAP) and R, using ohm's law flow (SV) is calculated. In this model since the arterial compliance is assumed to be constant, which is not in reality, the derived SV values does not reflect the true SV. For accurate values calibration against a standard method is carried out.

Systolic pulse contour analysis

In the windkessel model, the pulsatile systolic area (PSA) under the pressure curve above a horizontal line drawn from the diastolic point and bounded by a vertical line through the lowest point of incisura (area under the pressure curve from the start of the upstroke to the incisura as shown in [Figure - 4] and SV are related by means of characteristic impedance of the aorta (Z A ). Wesseling and coworkers developed a pulse contour analysis technique based on a transmission line model of the arterial tree. In this model the SV is related to PSA and Z A by the equation,

SV = PSA/Z A

PSA = ∫ ejection {P AO (t) - P ED}dt

where P AO (t) is aortic pressure at time t and P ED is end-diastolic pressure. [57] Although PSA can be found from the area under the curve there are no simple direct methods to establish the appropriate value of Z A . Taking into account the combined effects of varying heart rate and MAP Wesseling and coworkers formulated an equation for SV as,

SV = K (163 + HR - 0.48 MAP) ∫ ejection {P AO (t) - P ED}dt

Where K is an individual calibration constant, HR heart rate.

Since Z A is influenced by aortic cross section and compliance, both of which are pressure and age dependant [58],[59] and also a computer model of the circulation indicated that Z A is dependant on HR [57] and aortic properties change with age the above equation is modified as

SV = [PSA/(Z A ) ini ] [1,320 + HR × 10 - age × (0.28MAP-16)]/2000

Where the initial value for Z A , (Z A ) ini, is (90+age)/1,000.

In the Wesseling model a linear combination of HR, MAP and age multiplied by an individual calibration factor is used to estimate Z A. Antonutto and coworkers [60] used a multiple linear regression including pulse pressure (PP), HR and MAP to estimate Z A . Both the methods have been validated. Since Z A is not accurately known only uncalibrated SV is derived. For accurate SV values calibration against a standard method is carried out.

PiCCO system (Pulsion Medical Systems, Munich, ­Germany)

The PiCCO system is a continuous CO monitor whose working principle is based on the Wesseling's model and the software used have the original Wesseling algorithm. However, it deviates from Wesseling's modified method in that no age related corrections for pressure dependant non-linear changes in aortic cross sectional area are incorporated. In the second generation PiCCO equipment a more sophisticated algorithm that analyzes the actual shape of the waveform in addition to the PSA is used. From the shape of the arterial pressure curve after the dicrotic notch the exponential decay time constant (τ = C A × R) is calculated and R = MAP/CO. CO is obtained from the reference method. Since τ and R are known C A can be computed. SV is computed using the formula, [61]

SV = K [∫ ejection {P AO (t) - P ED}(t) + C A dp/dt] dt

Where K is the calibration factor and it is done by transpulmonary thermodilution.

Several clinical studies [62],[63],[64],[65] and an experimental study showed good agreement between PiCCO and thermodilution. Two different groups [66],[67] demonstrated that SV variation measured with PiCCO in patients before, during and after coronary artery bypass grafting (CABG) could predict fluid responsiveness. The technique may also be used in children. Peters and coworkers [68] reported the haemodynamic response to terlipressin in an 11 year-old patient with septic shock.

Modelflow pulse contour analysis

The Modelflow method computes an aortic flow waveform from the arterial pressure waveform using the three element model given in [Figure - 3]. The model is developed by Wessling and coworkers. [57],[69] The aortic impedance is a function of aortic cross sectional area, the instantaneous flow and compliance. Both compliance and impedance are nonlinear and depend on the elastic properties of the aorta. Unlike the previous models in Modelflow, the impedance and compliance nonlinearity are taken into account for SV calculations. The elastic properties of the human thoracic and abdominal arteries and changes in aortic cross sectional area with changing pressure were studied by Langewouters and coworkers. [58],[70] The aortic cross sectional area is described by them as a function of pressure by an arctangent relation with three parameters. Aortic impedance and arterial compliance are then presented in terms of the aortic pressure area relation and its derivative. In the Modelflow method, the two elements the aortic impedance and arterial compliance are computed making use of a built in data base of arctangent area-pressure relationships. Subject's gender and age were the input. Instantaneous impedance and compliance values so obtained are used in a model simulation to compute an aortic flow waveform. The peripheral vascular resistance is calculated for each beat and updated. Thus in short, the method computes arterial flow waveform from arterial pressure waveform with continuous nonlinear corrections for variations in aortic diameter, impedance and compliance during the arterial pulsation. Integration of flow waveform per beat gives SV.

The model assumes a normal human aorta and proper functioning of aortic valve. The maximal aortic diameter during ejection is the parameter included in the model that does not regress with age and its variability is considerable, hence derived SV does not reflect the true value. For accurate values calibration against a standard method is needed.

The commercially available system based on the above algorithm is Modelflow R (Netherland's Organization for Applied Scientific Research, Biomedical Instrumentation, BMI-TNO) and is calibrated by the mean of 3 or 4 conventional thermodilution measurements equally spread over the ventilatory cycle. Several clinical studies carried out in different clinical settings demonstrated good correlation with thermodilution. [69],[71],[72] The results of Jellema and coworkers [72] showed that once calibrated by thermodilution, Modelflow CO correlated well with thermodilution CO even after 48 hours of monitoring in ICU patients.

Pulse power analysis

The pulse pressure measured from the arterial trace is a combination of an incident pressure wave (ejected from heart) and reflected wave (from the periphery). In order to calculate SV the two waves need to be separated. This is further complicated by the fact that the reflected wave changes in size depending upon the proximity of the sampling site to the heart and also the patient's age. The algorithm used in pulse power analysis technique takes care of this aspect. [73] The algorithm used is based on the assumption that the net power change in a heart beat is SV minus blood lost to the periphery during the beat and there exists a relation between net power and net flow. Since the whole beat is taken for analysis the method is independent of the position of the sampling site. In this method the pressure wave is transformed to a volume wave and using the mathematical technique of autocorrelation of the volume waveform the net power and beat period are derived. [73] Net power is proportional to net flow (SV). It is only possible to calculate changes in SV rather than absolute values using this method. For accurate values, calibration against a standard method (Lithium dilution technique) is carried out.

The commercial equipment based on the pulse power analysis is the LiDCO™ plus (LiDCO Ltd, Cambridge, UK). Several studies have compared thermodilution and LiDCO™ plus and found agreement. [74],[75] Despite a negligible bias, the limits of agreement were close to (±1.5L/min).

Long time interval analysis of peripheral arterial blood pressure waveform

The varieties of pulse contour techniques described above are conceptually similar; the waveform analysis is performed only over time scales with in a cardiac cycle. However at such short time scales pressure waveforms are dominated by complex waves propagating back and forth the distributed arterial tree. According to the transmission line theory the confounding effects of the wave phenomena will diminish over large time scales. Based on the theory, Lu et al , [76] developed a novel algorithm in which a long time interval mathematical analysis of the waveform is carried out. From the analysis of the waveform over time scales larger than cardiac cycles, the arterial blood pressure response to a single solitary cardiac contraction is estimated. Then the time constant, τ, is estimated by fitting a mono-exponential function

P(t) = A exp[-t/τ] + w(t)

to the tail end of this response curve once the faster wave reflections are vanished. The parameters A and τ are estimated through least square minimization of the measured residual error [w(t)]. The validity of the technique was ascertained with respect to intra-arterial pressure waveforms obtained from open chest swines instrumented with aortic flow probes over a wide physiological state. This technique was also evaluated on humans based on previously published invasive and non-invasive arterial pressure data sets. [77],[78] Calibration against a standard method is necessary for measuring absolute SV.

Pulse contour cardiac output estimation without external calibration

Flo Trac/Vigileo (Edwards Life Sciences, Munich, ­Germany).

The pulse contour and pulse power based technologies described above require another standard technique to provide calibration constant compensating for the algorithms' inability to independently assess the ever changing effects of vascular tone on SV. Flo Trac/Vigileo™ system calculates CO, by using arterial pressure waveform characteristics in conjunction with patient demographic data, without external calibration. Preparation for monitoring consists only of entering the age, height, weight and gender into the unit and zeroing the transducer. The algorithm works on the principle that the pulse pressure (difference between the systolic and diastolic pressures) is proportional to SV and inversely proportional to aortic compliance. Aortic pressure is sampled at 100 Hz and analyzed and updated every 20 seconds. The standard deviation of the 2000 data points (SdAP) is proportional to pulse pressure, which is proportional to SV

SV = K(SdAP)

Where K is a constant quantifying vascular tone (arterial compliance and resistance) and is derived from patient characteristics (sex, age, height, weight, body surface area) according to the method described by Langewouters and coworkers. [69] A recent study indicated that the algorithm works satisfactory when compared with intermittent and continuous thermodilution techniques in post operative cardiac surgical patients. [79] Chakravarty and coworkers [80] compared CO values obtained with PAC-CCO, PiCCO and Flowtrac™ techniques, in 15 patients undergoing off-pump coronary artery bypass grafting. They found that the CO values obtained from all the techniques were interchangeable. The bias and precision were (in L/min), PAC (0.03, 0.06), PiCCO (0.13, 0.1) and Flowtrac™ (0.15, 0.04).

Pressure recording analytical method (PRAM)

PRAM is a technique based on the mathematical analysis of pulse profile changes based on the theory of perturbations, by which any physical system under the effect of a perturbation tend to react to achieve its own state of minimum energy. The basic principle of PRAM is that, in any given vessel, the volume changes occur mainly because of radial expansion of the artery in response to variations in pressure. PRAM measure absolute SV without external calibration by determining the parameters able to characterize the elastic properties of the arteries from the objective analysis of the pressure wave profile. [81],[82] SV is calculated using the formula

SV = PSA/(P/t) × K

Where P/t is pressure wave profile expressed as the variation of pressure over t during the entire cardiac cycle (dp/dt). K is a factor inversely proportional to instantaneous acceleration of the vessels cross sectional area. The value of K is obtained from the expected (theoretical) MAP, which is constant and measured MAP.

Only three studies have been carried out so far to prove its efficacy to determine absolute CO values. [81],[82],[83] The recent study is CO determination in animals under various haemodynamic conditions and has found to give CO values comparable to that obtained by electromagnetic flowmetry and conventional thermodilution. [83]


   Non-Invasive Methods Top


Electrical impedance cardiography

In the mid 1960s, the National Aeronautical and Space Administration (NASA) researchers and William Kubiceck developed the first practical method of impedance cardiography using the thoracic electrical bio-impedance to study the effects of zero gravity on the cardiac haemodynamics of astronauts. [84] The technique measures electrical resistance changes through the thorax as aortic blood volume increases and decreases during systole and diastole. Continuous measurement of the change in impedance caused by the fluctuation of blood volume through the cardiac cycle makes it possible to measure, calculate and continuously monitor SV, CO, myocardial contractility and total thoracic fluid status. The technique is a paradigm shift in haemodynamic monitoring.

In impedance cardiography four pairs of electrodes and a set of ECG leads measure haemodynamic parameters. Each pair of electrodes comprises of a transmitting and sensing electrodes. Two pairs are applied to the base of the neck on directly opposite sides and two pairs are placed at the level of the sternal-xiphoid process junction, again directly opposite from each other [Figure - 5]. The electrodes define the upper and lower limits of the thorax and the distance between them is measured to obtain the thoracic length (L).

A high frequency, low amplitude alternating current is introduced through the transmitting thoracic electrodes and the sensing thoracic electrodes measure impedance associated with the pulsatile blood flow in the aorta which occurs during the cardiac cycle. By measuring the impedance change generated by the pulsatile flow and the time intervals between the changes, SV can be calculated. Increased blood volume, flow velocity and alignment of red blood cells during systole reduce impedance.

The change in impedance is measured from the baseline impedance (Z 0 ) (overall thoracic resistance to flow of electrical current). It predominantly reflects total thoracic fluid volume. The magnitude and rate of the impedance change is a direct reflection of left ventricular contractility. This change in impedance related to time (dZ/dt) generates a waveform that is similar to the aortic flow curve. Simultaneous recording of the ECG creates a timing window for the evaluation of each cardiac cycle. During systole, a volume of blood leaves the thorax via the large arteries. The SV can be determined from the impedance curve by extrapolating to the impedance change (DZ) that would result if no blood were to flow out of the thorax during systole. If no blood were to flow away from the thorax during systole, the impedance would continuously decrease during systole, at a rate equal to the maximum rate of decrease of Z. Thus DZ can be approximated by drawing a tangent to the impedance curve at the point of its maximum rate of change. Then the difference between the impedance values of the tangent line at the beginning and the end of the ejection time is DZ. The value of DZ is easy to determine with the help of the first derivative curve of the impedance signal. Hence DZ is the product of the ejection time and absolute of the maximum value of the first derivative of the impedance signal. The SV is calculated using the formula,



where ρ - resistivity of blood, L - mean distance between the inner electrodes (the thoracic length), VET - ventricular ejection time, (dZ/dt) max - the absolute of the maximum value of the first derivative during systole and Z 0 - basal thoracic impedance. VET is obtained from the dZ/dt versus time curve [Figure - 6]. Heart is an electro-mechanical pump and ECG represents the electrical and impedance the mechanical activities. The mechanical events viz. the ventricular, aortic and atrial pressure variations and aortic blood flow during the cardiac cycle are shown along with DZ, dZ/dt, ECG and phonocardiogram in [Figure - 6]. CO is then calculated from the SV and heart rate. In addition to providing continuous measures of SV and CO, impedance cardiography also measures thoracic fluid content, left ventricular ejection time, systemic vascular resistance and left cardiac work index. [85] Recent trials with impedance cardiography have demonstrated acceptable intramethod reproducibility and intermethod comparison of accuracy with invasive methods. [86],[87],[88],[89],[90] Impedance cardiography accuracy has been validated in numerous clinical states, including heart failure, [86] post- coronary artery bypass, [87],[88] pulmonary hypertension [89] and critical illness [90]


   Conclusion Top


The desirable characteristics for haemodynamic monitoring techniques are accuracy, reproducibility, fast response, operator independency, continuous and ease of use with no increased mortality and morbidity. None of the technique combines all the criteria. Most of the techniques are still relatively invasive, requiring either sedation and mechanical ventilation or arterial and central venous access. The pulse contour and pulse power methods require frequent calibration. Fick/CO 2 method does not provide an instantaneous measure of CO, but rather a mean value every 3 min. Recent technological advances have allowed the development of completely non-invasive CO monitoring using impedance cardiography. This technique is ideal for continuous online and intermittent CO monitoring. However, large amount of thoracic fluid may interfere with the impedance signal making the haemodynamic data less reliable. No single method stands out or renders the others obsolete. By making CO easily measurable, these techniques should all contribute to improvement in haemodynamic management.


   Acknowledgement Top


The corresponding author thanks Dr. Tom Mathews, MS, M.Tech, Ph.D, Indira Gandhi Centre for Atomic Research, Department of Atomic Energy, Kalpakkam for the enlightening discussion on the correlation between the physics and physiological aspects underlying the pulse contour analysis and impedance cardiography as well as for explaining the windkessel model.

 
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Correspondence Address:
Lailu Mathews
Pondicherry Institute of Medical Sciences, Kalapet, Puducherry - 605 014
India
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DOI: 10.4103/0971-9784.38455

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[Pubmed] | [DOI]
3 Blood pressure and heart rate from the arterial blood pressure waveform can reliably estimate cardiac output in a conscious sheep model of multiple hemorrhages and resuscitation using computer machine learning approaches
Nehemiah T. Liu,George C. Kramer,Muzna N. Khan,Michael P. Kinsky,José Salinas
Journal of Trauma and Acute Care Surgery. 2015; 79: S85
[Pubmed] | [DOI]
4 Reduced systemic vascular resistance is the underlying hemodynamic mechanism in nitrate-stimulated vasovagal syncope during head-up tilt-table test
Byung Gyu Kim,Sung Woo Cho,Hye Young Lee,Deok Hee Kim,Young Sup Byun,Choong Won Goh,Kun Joo Rhee,Byung Ok Kim
Journal of Arrhythmia. 2015; 31(4): 196
[Pubmed] | [DOI]
5 Cambios en el perfil hemodinámico al instaurar la ventilación mecánica en pacientes con cardiopatía isquémica y enfermedad coronaria. Medición con biorreactancia torácica
Ivon Johanna Rodríguez,Juan Carlos Echeverry,Mauricio Abello,Luis Eduardo Cruz
Revista Colombiana de Anestesiología. 2014;
[Pubmed] | [DOI]
6 Changes in the hemodynamic profile when establishing mechanical ventilation in patients with ischemic heart disease and coronary disease: Measurement with thoracic bioreactance
Ivon Johanna Rodríguez,Juan Carlos Echeverry,Mauricio Abello,Luis Eduardo Cruz
Colombian Journal of Anesthesiology. 2014;
[Pubmed] | [DOI]
7 Hemodynamic monitoring in the critically patient. Recommendations of the Cardiological Intensive Care and CPR Working Group of the Spanish Society of Intensive Care and Coronary Units
A. Ochagavía,F. Baigorri,J. Mesquida,J.M. Ayuela,A. Ferrándiz,X. García,M.I. Monge,L. Mateu,C. Sabatier,F. Clau-Terré,R. Vicho,L. Zapata,J. Maynar,A. Gil
Medicina Intensiva (English Edition). 2014; 38(3): 154
[Pubmed] | [DOI]
8 Relationship between Stroke Volume and Pulse Pressure during Blood Volume Perturbation: A Mathematical Analysis
Ramin Bighamian,Jin-Oh Hahn
BioMed Research International. 2014; 2014: 1
[Pubmed] | [DOI]
9 Recent Advances in and Limitations of Cardiac Output Monitoring by Means of Electrical Impedance Tomography
Robert Pikkemaat,Stefan Lundin,Ola Stenqvist,Ralf-Dieter Hilgers,Steffen Leonhardt
Anesthesia & Analgesia. 2014; 119(1): 76
[Pubmed] | [DOI]
10 Bioimpedance and bioreactance methods for monitoring cardiac output
Dr. Djordje G. Jakovljevic,Mike I. Trenell
Best Practice & Research Clinical Anaesthesiology. 2014;
[Pubmed] | [DOI]
11 Measurement of Cardiac Output in Children by Pressure-Recording Analytical Method
Javier Urbano,Jorge López,Rafael González,María José Solana,Sarah N. Fernández,José M. Bellón,Jesús López-Herce
Pediatric Cardiology. 2014;
[Pubmed] | [DOI]
12 Comparison of thermodilution, lithium dilution, and pulse contour analysis for the measurement of cardiac output in 3 different hemodynamic states in dogs
Juan Morgaz,María del Mar Granados,Pilar Muñoz-Rascón,Juan Manuel Dominguez,Jose Andrés Fernández-Sarmiento,Rafael J. Gómez-Villamandos,Rocío Navarrete
Journal of Veterinary Emergency and Critical Care. 2014; 24(5): 562
[Pubmed] | [DOI]
13 Predicting fluid responsiveness with transthoracic echocardiography is not yet evidence based
Wetterslev, M. and Haase, N. and Johansen, R.R. and Perner, A.
Acta Anaesthesiologica Scandinavica. 2013; 57(6): 692-697
[Pubmed]
14 Estimation of cardiac output and systemic vascular resistance using a multivariate regression model with features selected from the finger photoplethysmogram and routine cardiovascular measurements
Lee, Q.Y. and Redmond, S.J. and Chan, G.S.H. and Middleton, P.M. and Steel, E. and Malouf, P. and Critoph, C. and Flynn, G. and OæLone, E. and Lovell, N.H.
BioMedical Engineering Online. 2013; 12(1)
[Pubmed]
15 Cardiac index assessment: Validation of a new non-invasive very low current thoracic bioimpedance device by thermodilution
Andrea Faini,Stefano Omboni,Marius Tifrea,Serban Bubenek,Ovidiu Lazar,Gianfranco Parati
Blood Pressure. 2013; : 1
[Pubmed] | [DOI]
16 Cardiac Index Assessment by the Pressure Recording Analytic Method in Critically Ill Unstable Patients After Cardiac Surgery
Luigi Barile,Giovanni Landoni,Marina Pieri,Laura Ruggeri,Giulia Maj,Caetano Nigro Neto,Laura Pasin,Luca Cabrini,Alberto Zangrillo
Journal of Cardiothoracic and Vascular Anesthesia. 2013;
[Pubmed] | [DOI]
17 Estimation of cardiac output and systemic vascular resistance using a multivariate regression model with features selected from the finger photoplethysmogram and routine cardiovascular measurements
Qim Y Lee,Stephen J Redmond,Gregory SH Chan,Paul M Middleton,Elizabeth Steel,Philip Malouf,Cristopher Critoph,Gordon Flynn,Emma O’Lone,Nigel H Lovell
BioMedical Engineering OnLine. 2013; 12(1): 19
[Pubmed] | [DOI]
18 Monitorización hemodinámica en el paciente crítico. Recomendaciones del Grupo de Trabajo de Cuidados Intensivos Cardiológicos y RCP de la Sociedad Española de Medicina Intensiva, Crítica y Unidades Coronarias
A. Ochagavía,F. Baigorri,J. Mesquida,J.M. Ayuela,A. Ferrándiz,X. García,M.I. Monge,L. Mateu,C. Sabatier,F. Clau-Terré,R. Vicho,L. Zapata,J. Maynar,A. Gil
Medicina Intensiva. 2013;
[Pubmed] | [DOI]
19 Predicting fluid responsiveness with transthoracic echocardiography is not yet evidence based
M. WETTERSLEV,N. HAASE,R. R. JOHANSEN,A. PERNER
Acta Anaesthesiologica Scandinavica. 2013; 57(6): 692
[Pubmed] | [DOI]
20 Fluid Management in Thoracic Surgery
Cait P. Searl,Albert Perrino
Anesthesiology Clinics. 2012; 30(4): 641
[Pubmed] | [DOI]
21 Evaluation of a minimally invasive non-calibrated pulse contour cardiac output monitor (FloTrac/Vigileo) in anaesthetized dogs
Rima N Bektas,Annette PN Kutter,Sonja Hartnack,Rahel S Jud,Manuela Schnyder,José M Matos,Regula Bettschart-Wolfensberger
Veterinary Anaesthesia and Analgesia. 2012; 39(5): 464
[Pubmed] | [DOI]
22 Técnicas disponibles de monitorización hemodinámica. Ventajas y limitaciones
M.L. Mateu Campos,A. Ferrándiz Sellés,G. Gruartmoner de Vera,J. Mesquida Febrer,C. Sabatier Cloarec,Y. Poveda Hernández,X. García Nogales
Medicina Intensiva. 2012; 36(6): 434
[Pubmed] | [DOI]
23 Estimating cardiac output. Utility in the clinical practice. Available invasive and non-invasive monitoring
X. García, L. Mateu, J. Maynar, J. Mercadal, A. Ochagavía, A. Ferrandiz
Medicina Intensiva (English Edition). 2012;
[VIEW] | [DOI]
24 Intraoperative transfusion threshold and tissue oxygenation: a randomised trial
K. Nielsen,B. Dahl,P. I. Johansson,S. W. Henneberg,L. S. Rasmussen
Transfusion Medicine. 2012; 22(6): 418
[Pubmed] | [DOI]
25 Techniques available for hemodynamic monitoring. Advantages and limitations
M.L. Mateu Campos,A. Ferrándiz Sellés,G. Gruartmoner de Vera,J. Mesquida Febrer,C. Sabatier Cloarec,Y. Poveda Hernández,X. García Nogales
Medicina Intensiva (English Edition). 2012; 36(6): 434
[Pubmed] | [DOI]
26 Intraoperative transfusion threshold and tissue oxygenation: A randomised trial
Nielsen, K. and Dahl, B. and Johansson, P.I. and Henneberg, S.W. and Rasmussen, L.S.
Transfusion Medicine. 2012; 22(6): 418-425
[Pubmed]
27 Fluid Management in Thoracic Surgery
Searl, C.P. and Perrino, A.
Anesthesiology Clinics. 2012; 30(4): 641-655
[Pubmed]
28 Evaluation of a minimally invasive non-calibrated pulse contour cardiac output monitor (FloTrac/Vigileo) in anaesthetized dogs
Bektas, R.N. and Kutter, A.P. and Hartnack, S. and Jud, R.S. and Schnyder, M. and Matos, J.M. and Bettschart-Wolfensberger, R.
Veterinary Anaesthesia and Analgesia. 2012; 39(5): 464-471
[Pubmed]
29 Techniques available for hemodynamic monitoring. Advantages and limitations [Técnicas disponibles de monitorización hemodinámica. Ventajas y limitaciones]
Mateu Campos, M.L. and Ferrándiz Sellés, A. and Gruartmoner de Vera, G. and Mesquida Febrer, J. and Sabatier Cloarec, C. and Poveda Hernández, Y. and García Nogales, X.
Medicina Intensiva. 2012; 36(6): 434-444
[Pubmed]
30 Assessing acute decompensated heart failure - Strategies and tools
Vazir, A. and Cowie, M.R.
European Cardiology. 2012; 8(2): 128-133
[Pubmed]
31 Study of impedance cardiography (ICG) in hypertensive patients
Parmar, C.V. and Prajapati, D.L. and Gokhale, P.A. and Mehta, H.B. and Shah, C.J.
International Journal of Pharmaceutical Sciences. 2012; 4(1): 1916-1927
[Pubmed]
32 The influence of limited dynamic response of the indicator detector in a Swan-Ganz catheter on the overestimation of cardiac output measurement by means of thermodilution
Gawlikowski, M. and Pustelny, T.
Metrology and Measurement Systems. 2012; 19(4): 751-758
[Pubmed]
33 Estimación del gasto cardíaco. Utilidad en la práctica clínica. Monitorización disponible invasiva y no invasiva
X. García, L. Mateu, J. Maynar, J. Mercadal, A. Ochagavía, A. Ferrandiz
Medicina Intensiva. 2011;
[VIEW] | [DOI]
34 Measurement of cardiac output with non-invasive Aesculon® impedance versus thermodilution : Non-invasive versus invasive measurements of cardiac output
Hedelin Petter, Agger Erik, Ekmehag Björn, Rådegran Göran
Clinical Physiology and Functional Imaging. 2011; 31(1): 39
[VIEW] | [DOI]
35 Measurement of Cardiac Output in Children by Bioreactance
Yolanda Ballestero, Jesús López-Herce, Javier Urbano, Maria José Solana, Marta Botrán, Jose M. Bellón, Angel Carrillo
Pediatric Cardiology. 2011; 32(4): 469
[VIEW] | [DOI]
36 Multivariate classification of systemic vascular resistance using photoplethysmography
Qim Y Lee, Gregory S H Chan, Stephen J Redmond, Paul M Middleton, Elizabeth Steel, Philip Malouf, Christopher Critoph, Gordon Flynn, Emma O’Lone, Nigel H Lovell
Physiological Measurement. 2011; 32(8): 1117
[VIEW] | [DOI]
37 An Uncalibrated Pulse Contour Method to Measure Cardiac Output During Aortic Counterpulsation :
Sabino Scolletta, Federico Franchi, Fabio Silvio Taccone, Katia Donadello, Bonizella Biagioli, Jean-Louis Vincent
Anesthesia & Analgesia. 2011; 113(6): 1389
[VIEW] | [DOI]
38 Cardiac Output Assessed by Invasive and Minimally Invasive Techniques
Allison J. Lee, Jennifer Hochman Cohn, J. Sudharma Ranasinghe
Anesthesiology Research and Practice. 2011; 2011: 1
[VIEW] | [DOI]
39 Cardiac Index Assessment by the Pressure Recording Analytic Method in Unstable Patients With Atrial Fibrillation
Giulia Maj, Fabrizio Monaco, Giovanni Landoni, Luigi Barile, Davide Nicolotti, Marina Pieri, Giulio Melisurgo, Federico Pappalardo, Alberto Zangrillo
Journal of Cardiothoracic and Vascular Anesthesia. 2011; 25(3): 476
[VIEW] | [DOI]
40 Estimating cardiac output. Utility in the clinical practice. Available invasive and non-invasive monitoring [Puesta al día en medicina intensiva. monitorización hemodinámica en elpaciente crítico]
García, X. and Mateu, L. and Maynar, J. and Mercadal, J. and Ochagavía, A. and Ferrandiz, A.
Medicina Intensiva. 2011; 35(9): 552-561
[Pubmed]
41 An uncalibrated pulse contour method to measure cardiac output during aortic counterpulsation
Scolletta, S. and Franchi, F. and Taccone, F.S. and Donadello, K. and Biagioli, B. and Vincent, J.-L.
Anesthesia and Analgesia. 2011; 113(6): 1389-1395
[Pubmed]
42 Comparing the accuracy of ES-BC, EIS-GS, and ES Oxi on body composition, autonomic nervous system activity, and cardiac output to standardized assessments
Lewis, J.E. and Tannenbaum, S.L. and Gao, J. and Melillo, A.B. and Long, E.G. and Alonso, Y. and Konefal, J. and Woolger, J.M. and Leonard, S. and Singh, P.K. and Chen, L. and Tiozzo, E.
Medical Devices: Evidence and Research. 2011; 4(1)
[Pubmed]
43 Measurement of cardiac output during adult donor care
Powner, D.J. and Hergenroeder, G.W.
Progress in Transplantation. 2011; 21(2): 144-151
[Pubmed]
44 Impaired hemodynamic response to meal intake in insulin-resistant subjects: An impedance cardiography approach
De Kreutzenberg, S.V. and Fadini, G.P. and Boscari, F. and Rossi, E. and Guerra, S. and Sparacino, G. and Cobelli, C. and Ceolotto, G. and Bottero, M. and Avogaro, A.
American Journal of Clinical Nutrition. 2011; 93(5): 926-933
[Pubmed]
45 Comparison of transthoracic electrical bioimpedance cardiac output measurement with thermodilution method in post coronary artery bypass graft patients
Sharma, V. and Singh, A. and Kansara, B. and Karlekar, A.
Annals of Cardiac Anaesthesia. 2011; 14(2): 104-110
[Pubmed]
46 Diurnal and position-induced variability of impedance cardiography measurements in healthy subjects
Tomsin, K. and Mesens, T. and Molenberghs, G. and Gyselaers, W.
Clinical Physiology and Functional Imaging. 2011; 31(2): 145-150
[Pubmed]
47 Propranolol decreases cardiac work in a dose-dependent manner in severely burned children
Williams, F.N. and Herndon, D.N. and Kulp, G.A. and Jeschke, M.G.
Surgery. 2011; 149(2): 231-239
[Pubmed]
48 Measurement of cardiac output with non-invasive Aesculon® impedance versus thermodilution
Petter, H. and Erik, A. and Björn, E. and Göran, R.
Clinical Physiology and Functional Imaging. 2011; 31(1): 39-47
[Pubmed]
49 Physical Model of the Pulmonary Circulation Designed for Investigation on Cardiac Output Measurement by Means of the Thermodilution Method
M. Gawlikowski,T. Pustelny
Acta Physica Polonica A. 2011; 120(4): 798
[Pubmed] | [DOI]
50 Measurement of cardiac output during adult donor care
David Powner,Georgene Hergenroeder
Progress in Transplantation. 2011; 21(2): 144
[Pubmed] | [DOI]
51 Pulmonary arterial thermodilution, femoral arterial thermodilution and bioreactance cardiac output monitoring in a pediatric hemorrhagic hypovolemic shock model
Yolanda Ballestero, Javier Urbano, Jesús López-Herce, Maria J. Solana, Marta Botrán, Diego Vinciguerra, Jose M. Bellón
Resuscitation. 2011;
[VIEW] | [DOI]
52 Propranolol decreases cardiac work in a dose-dependent manner in severely burned children
Williams, F.N., Herndon, D.N., Kulp, G.A., Jeschke, M.G.
Surgery. 2011; 149(2): 231-239
[Pubmed]
53 Classification of low systemic vascular resistance using photoplethysmogram and routine cardiovascular measurements
Lee, Q.Y., Chan, G.S.H., Redmond, S.J., Middleton, P.M., Steel, E., Malouf, P., Critoph, C., (...), Lovell, N.H.
2010 Annual International Conference of the IEEE Engineering in Medicine and Biology Society, EMBCæ10 ,. 2010; art(5628062): 1930-1933
[Pubmed]
54 Sympathetic and cardiovascular responses to glossopharyngeal insufflation in trained apnea divers
Heusser, K., Dzamonja, G., Breskovic, T., Steinback, C.D., Diedrich, A., Tank, J., Jordan, J., Dujic, Z.
Journal of Applied Physiology. 2010; 109(6): 1728-1735
[Pubmed]
55 Application of ultrasound dilution technology for cardiac output measurement: Cerebral and systemic hemodynamic consequences in a juvenile animal model :
Willem P. de Boode, Arno F. J. van Heijst, Jeroen C. W. Hopman, Ronald B. Tanke, Hans G. van der Hoeven, K. Djien Liem
Pediatric Critical Care Medicine. 2010; 11(5): 616
[VIEW] | [DOI]
56 Resting Measures and Physiological Responses to Exercise for the Determination of Prognosis in Patients With Chronic Heart Failure : Useful Tools for Clinical Decision-Making
Alberto Jorge Alves, Fernando Ribeiro, Moran Sagiv, Nir Eynon, Chen Yamin, Michael Sagiv, José Oliveira
Cardiology in Review. 2010; 18(4): 171
[VIEW] | [DOI]
57 LiDCO Systems :
Sugantha Sundar, Peter Panzica
International Anesthesiology Clinics. 2010; 48(1): 87-100
[Pubmed] | [DOI]
58 Cardiac output measurement using an ultrasound dilution method: A validation study in ventilated piglets :
Willem P. de Boode, Arno F. J. van Heijst, Jeroen C. W. Hopman, Ronald B. Tanke, Hans G. van der Hoeven, Kian D. Liem
Pediatric Critical Care Medicine. 2010; 11(1): 103-108
[Pubmed] | [DOI]
59 Monitoring in resuscitation: Comparison of cardiac output measurement between pulmonary artery catheter and NICO
Carretero, M., Fontanals, J., Agustí, M., Arguis, M., Martínez-Ocón, J., Ruiz, A., Rios, J.
Resuscitation. 2010; 81(4): 404-409
[Pubmed]
60 The non-invasive and continuous estimation of cardiac output using a photoplethysmogram and electrocardiogram during incremental exercise
L Wang, C C Y Poon, Y T Zhang
Physiological Measurement. 2010; 31(5): 715
[VIEW] | [DOI]
61 Fluid management
Day, A., Rockall, T.
Surgery. 2010; 28(4): 151-154
[Pubmed]
62 Monitoring in resuscitation: Comparison of cardiac output measurement between pulmonary artery catheter and NICO
Mªjosé Carretero,Jaume Fontanals,Mercé Agustí,Mªjosé Arguis,Julia Martínez-Ocón,Ana Ruiz,José Rios
Resuscitation. 2010; 81(4): 404
[Pubmed] | [DOI]
63 Fluid management
Andrew Day,Tim Rockall
Surgery (Oxford). 2010; 28(4): 151
[Pubmed] | [DOI]
64 LiDCO systems
Sundar, S. and Panzica, P.
International Anesthesiology Clinics. 2010; 48(1): 87-100
[Pubmed]
65 Relationship between maximum oxygen uptake and local muscle function
Uchiyama, K. and Terada, S. and Miyata, S. and Matsui, N. and Miaki, H.
Rigakuryoho Kagaku. 2010; 25(3): 391-395
[Pubmed]
66 Resting measures and physiological responses to exercise for the determination of prognosis in patients with chronic heart failure: Useful tools for clinical decision-making
Jorge Alves, A. and Ribeiro, F. and Sagiv, M. and Eynon, N. and Yamin, C. and Sagiv, M. and Oliveira, J.
Cardiology in Review. 2010; 18(4): 171-177
[Pubmed]
67 Non-invasive assessment of pulmonary blood flow using an inert gas rebreathing device in fibrotic lung disease
Corte, T.J. and Wells, A.U. and Gatzoulis, M.A. and Cramer, D. and Ward, S. and Macdonald, P.S. and Dimopoulos, K. and Wort, S.J.
Thorax. 2010; 65(4): 341-345
[Pubmed]
68 Choosing patient-tailored hemodynamic monitoring
Slagt, C. and Breukers, R.-M.B.G.E. and Groeneveld, A.J.
Critical Care. 2010; 14(2)
[Pubmed]
69 Cardiac output measurement using an ultrasound dilution method: A validation study in ventilated piglets
De Boode, W.P. and Van Heijst, A.F.J. and Hopman, J.C.W. and Tanke, R.B. and Van Der Hoeven, H.G. and Liem, K.D.
Pediatric Critical Care Medicine. 2010; 11(1): 103-108
[Pubmed]
70 Diurnal and position-induced variability of impedance cardiography measurements in healthy subjects : Variability of ICG in healthy subjects
Kathleen Tomsin, Mesens Tinne, Molenberghs Geert, Gyselaers Wilfried
Clinical Physiology and Functional Imaging. 2010; : no
[VIEW] | [DOI]
71 Cardiac output monitoring in pediatric patients
Anneliese Nusmeier, Johannes G van der Hoeven, Joris Lemson
Expert Review of Medical Devices. 2010; 7(4): 503
[VIEW] | [DOI]
72 Cardiovascular Regulation During Apnea in Elite Divers
Heusser, K. and Dzamonja, G. and Tank, J. and Palada, I. and Valic, Z. and Bakovic, D. and Obad, A. and Ivancev, V. and Breskovic, T. and Diedrich, A. and others
Hypertension. 2009; 53(4): 719-724
[Pubmed]
73 Impedance cardiography: a role in vasovagal syncope diagnosis?
S. W. Parry, M. Norton, J. Pairman, M. Baptist, K. Wilton, P. Reeve, K. Sutcliffe, J. L. Newton
Age and Ageing. 2009; 38(6): 718-723
[Pubmed] | [DOI]
74 Complications associated with pulmonary artery catheters: A comprehensive clinical review
Evans, D.C., Doraiswamy, V.A., Prosciak, M.P., Silviera, M., Seamon, M.J., Rodriguez Funes, V., Cipolla, J., Stawicki, S.P.
Scandinavian Journal of Surgery. 2009; 98(4): 199-208
[Pubmed]
75 Noninvasive cardiac output determination: Broadening the applicability of hemodynamic monitoring
Compton, F., Schäsignfer, J.-H.
Seminars in Cardiothoracic and Vascular Anesthesia. 2009; 13(1): 44-55
[Pubmed]
76 Noninvasive comparison of the effects of right and left thoracotomies on cardiac output [Saǧ ve sol torakotominin kardiyak outputa etkisinin noninvaziv olarak karşi{dotless
Dere, K. and Orhan, M.E. and Coşar, A. and Özkan, S. and Daǧli, G.
Gulhane Medical Journal. 2009; 51(1): 21-26
[Pubmed]
77 A novel functional haplotype in the human GNAS gene alters Gαs expression, responsiveness to β-adrenoceptor stimulation, and peri-operative cardiac performance
Frey, U.H. and Adamzik, M. and Kottenberg-Assenmacher, E. and Jakob, H. and Manthey, I. and Broecker-Preuss, M. and Bergmann, L. and Heusch, G. and Siffert, W. and Peters, J. and Leineweber, K.
European Heart Journal. 2009; 30(11): 1402-1410
[Pubmed]
78 Principles of non-invasive hemodynamic assessment with impedance cardiography [Principios de la evaluación hemodinámica no invasiva con cardiografía de impedancia]
Ochoa M, J.E. and McEwen O, J.G. and Dagnóvar Aristizábal, O.
Revista Colombiana de Cardiologia. 2009; 16(3): 91-102
[Pubmed]
79 A novel functional haplotype in the human GNAS gene alters G s expression, responsiveness to  -adrenoceptor stimulation, and peri-operative cardiac performance
U. H. Frey,M. Adamzik,E. Kottenberg-Assenmacher,H. Jakob,I. Manthey,M. Broecker-Preuss,L. Bergmann,G. Heusch,W. Siffert,J. Peters,K. Leineweber
European Heart Journal. 2009; 30(11): 1402
[Pubmed] | [DOI]
80 Relationship Between Cardiac Output and Onset of Succinylcholine Chloride Action in Electroconvulsive Therapy Patients :
Naoki Matsumoto, Akihiro Tomioka, Tomonobu Sato, Masakazu Kawasaki, Yuji Kadoi, Shigeru Saito
Journal of Ect. 2009; 25(4): 246-249
[Pubmed] | [DOI]
81 Noninvasive haemodynamic monitoring using finger arterial pressure waveforms
De Jong, R.M., Westerhof, B.E., Voors, A.A., Van Veldhuisen, D.J.
Netherlands Journal of Medicine. 2009; 67(11): 372-375
[Pubmed]
82 Analysis of N-terminal pro-B-type natriuretic peptide and cardiac index in multiple injured patients: a prospective cohort study
Kirchhoff, C. and Leidel, B. and Kirchhoff, S. and Braunstein, V. and Bogner, V. and Kreimeier, U. and Mutschler, W. and Biberthaler, P.
Critical Care. 2008; 12(5): R118
[Pubmed]
83 Cardiac output--have we found theægold standardæ?
Chakravarthy, M.
Annals of cardiac anaesthesia. ; 11(1): 1
[Pubmed]



 

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