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Table of Contents
Year : 2011  |  Volume : 14  |  Issue : 2  |  Page : 104-110
Comparison of transthoracic electrical bioimpedance cardiac output measurement with thermodilution method in post coronary artery bypass graft patients

Departments of Anaesthesiology and Critical Care, Escorts Heart Institute and Research Centre Ltd., Okhla Road, New Delhi, India

Click here for correspondence address and email

Date of Submission01-Nov-2010
Date of Acceptance04-Apr-2011
Date of Web Publication25-May-2011


Transthoracic electrical bioimpedance (TEB) has been proposed as a non-invasive, continuous, and cost-effective method of cardiac output (CO) measurement. In this prospective, non-randomized, clinical study, we measured CO with NICOMON (Larsen and Toubro Ltd., Mysore, India) and compared it with thermodilution (TD) method in patients after off-pump coronary artery bypass (OPCAB) graft surgery. We also evaluated the effect of ventilation (mechanical and spontaneous) on the measurement of CO by the two methods. Forty-six post-OPCAB patients were studied at five predefined time points during controlled ventilation and at five time points when breathing spontaneously. A total of 230 data pairs of CO were obtained. During controlled ventilation, TD CO values ranged from 2.29 to 6.74 L/min (mean 4.45 ± 0.85 L/min), while TEB CO values ranged from 1.70 to 6.90 L/min (mean 4.43 ± 0.94 L/min). The average correlation (r) was 0.548 (P = 0.0002), accompanied by a bias of 0.015 L/min and precision of 0.859 L/min. In spontaneously breathing patients, TD CO values ranged from 2.66 to 6.92 L/min (mean 4.66 ± 0.76 L/min), while TEB CO values ranged from 3.08 to 6.90 L/min (mean 4.72 ± 0.82 L/min). Their average correlation was relatively poor (r = 0.469, P= 0.002), accompanied by a bias of −0.059 L/min and precision of 0.818 L/min. The overall percent errors between TD CO and TEB CO were 19.3% (during controlled ventilation) and 17.4% (during spontaneous breathing), respectively. To conclude, a fair correlation was found between TD CO and TEB CO measurements among post-OPCAB patients during controlled ventilation. However, the correlation was weak in spontaneously breathing patients.

Keywords: Cardiac output, off-pump coronary artery bypass graft surgery, thermodilution, transthoracic electrical bioimpedance

How to cite this article:
Sharma V, Singh A, Kansara B, Karlekar A. Comparison of transthoracic electrical bioimpedance cardiac output measurement with thermodilution method in post coronary artery bypass graft patients. Ann Card Anaesth 2011;14:104-10

How to cite this URL:
Sharma V, Singh A, Kansara B, Karlekar A. Comparison of transthoracic electrical bioimpedance cardiac output measurement with thermodilution method in post coronary artery bypass graft patients. Ann Card Anaesth [serial online] 2011 [cited 2022 Jan 20];14:104-10. Available from:

   Introduction Top

Presently, the most common method of cardiac output (CO) measurement involves the use of pulmonary artery catheter (PAC), based on the principle of thermodilution (TD).

This technique, though considered a "gold standard", has often been questioned for risk-benefit ratio, cost-effectiveness, and the effect on outcome consequent to its use. The ideal technology for measurement of CO should be non-invasive, accurate, reliable, continuous, reproducible, and compatible in adult and pediatric population. [1] At present, no single technique meets all these criteria.

Transthoracic electrical bioimpedance (TEB), a non-invasive technique to measure CO, has been increasingly explored for its use in post-cardiac surgical patients over the last four decades. With the TEB method, CO is calculated by detecting changes in the body's impedance to small electrical currents. Both blood and tissues impede electrical current, but the volume and impedance of the tissues remain constant during cardiac cycle. With each ejection, only the volume of blood within the chest changes, and the resulting change in thoracic blood volume causes changes in impedance. On the basis of these changes in impedance, the computer based on mathematical formulae and algorithms calculates CO.

Raaijmakers et al. [2] conducted an extensive meta-analysis concerning the validation of TEB technology in animals, healthy human subjects, and in patients with different clinical conditions. They observed an overall correlation coefficient of 0.67 between TEB CO and that measured by other methods. More recent studies using upgraded computer technology and refined algorithms have solved many of the problems associated with first-generation technology. [3],[4] Several papers published in the recent past report validation and accuracy of various minimally invasive and totally non-invasive techniques as well as their limitations. [5],[6],[7] When compared with other methods of CO monitoring, TEB has shown satisfactory [8],[9] to poor [10],[11] agreement in previous reports. The algorithm used by the device and the choice of reference method made have significantly influenced the agreement and precision of CO measurement.

We hypothesized that a new generation TEB hemodynamic monitor (NICOMON) would show a clinically acceptable agreement and precision with TD-derived CO in post-cardiac surgical patients. The study was undertaken to evaluate the accuracy and precision of measurement of CO by TEB using NICOMON, and compare it with TD method in patients after off-pump coronary artery bypass (OPCAB) graft surgery. Another objective of the study was to evaluate the effect of ventilation on the measurement of CO by the two methods.

   Materials and Methods Top

This prospective, non-randomized, clinical study was performed between July 2008 and March 2009 after obtaining approval from the institutional review board and written, informed consent from the patients. The study was undertaken in 46 patients admitted to cardiac surgical intensive care unit (ICU) after OPCAB surgery, who were chosen to have perioperative PAC placed electively as per the institutional guidelines. Exclusion criteria were: patients undergoing emergency surgery, and those with generalized edema, pulmonary edema, pleural effusion, chronic obstructive pulmonary disease (COPD), obesity (body mass index > 30 kg/m 2 ), congestive heart failure, arrhythmia, tricuspid regurgitation, intracardiac shunt, hemodynamic instability (Heart Rate > 120 beats/min, systolic BP < 90 mmHg and/or mean arterial pressure < 60 mmHg, and urine output < 0.5 mL/kg/hour), and on intraoaortic balloon pump support.

All the patients underwent a detailed preoperative clinical evaluation. Chest X-ray, electrocardiogram, and echocardiography were performed to rule out the presence of conditions such as edema, pleural effusion, and arrhythmias. Height, weight, chest circumference, and body surface area were noted for each subject. Standard anesthesia regimen, consisting of midazolam, fentanyl, vecuronium, thiopentone, and isoflurane in air, was used for all the patients. Routine cardiac monitoring, including the use of PAC, was employed. Postoperatively, all the patients were electively ventilated with Servo I ventilator (Siemens Elema AB, Goeteborg, Sweden) using a tidal volume of 7-10 mL/kg and positive end expiratory pressure (PEEP) of 4 cm H 2 O on volume control mode. Normocapnia and adequate oxygenation were ensured in all the patients, and they were subsequently weaned off the ventilator as per the institutional guidelines.

TEB CO was measured using the NICOMON instrument (Larsen and Toubro Ltd., Mysore, India). After rubbing and cleaning the skin with alcohol to minimize the skin-to-electrode impedance, eight color-coded electrodes were placed as follows: (i) two red colored electrodes were placed on either side of the neck just below the mastoid process, (ii) yellow electrodes were placed on the neck on either side just below the red electrodes, (iii) purple electrodes were placed on either side of the chest wall on anterior axillary line at the level of lower end of the sternum, and (iv) green electrodes were placed just below the purple electrodes in the same vertical plane. Strict aseptic precautions were taken while placing the electrodes near sterile dressings of central line and surgical wound. An adaptive filter was used to remove signals outside the cardiac frequency such as those due to respiration and movement.

NICOMON works on the principle of impedance plethysmography, wherein after calibrating the instrument, a 48-kHz, 2-4-mA alternative current is passed across the thorax by the outer pair of electrodes. Baseline impedance (Zo) is calculated from the voltage changes sensed by the inner pair of electrodes. The first derivative (dZ/dt) of impedance waveform is calculated from the time-impedance curve. As the blood flow through the aorta determines most of the changes in the impedance, it is possible to compute the main blood flow in the aorta from the changes in dZ/dt over time. Bioimpedance values are updated on the NICOMON monitor every 15 heart beats. Distortions or artifacts in the bioimpedance signal were detected and excluded from the study. With TEB, the left ventricular ejection time (LVET) was directly measured. Left ventricular stroke volume was calculated using the formula described by Sramek and Bernstein, [12] which is a modification of Kubicek formula: [13]

stroke volume (SV) = VEPT Χ (dZmax /Zo) Χ LVET

where VEPT was calculated from patient's data (weight, height, age, gender) and dZmax represented the amplitude of the systolic wave of the TEB. The mean value of three accepted SV measurements (falling within 10% of each value) was used as the SV for computation of CO (SV Χ heart rate). All determinations were made by the same person (VS) according to the manufacturer's instructions.

Similarly, another person (AS) determined CO by TD method. Ten milliliters of normal saline at room temperature was injected into the proximal injectate port of flow-directed PAC (Biosensors International Pte Ltd., Kallang Avenue, Singapore) within 3 seconds. Each TD CO value was an average of three measurements made at end-expiratory phase.

For the purpose of study, the measurements were taken in the postoperative ICU every 15 min after the patients were stabilized and warmed up to a core temperature of 36°C. CO measurements were taken in the supine, resting state, with the head of the bed elevated to 20°. Five readings were taken in the volume control mode of ventilation at 15-min intervals. Additional five readings were taken at 15-min intervals, 2 hours after the patients were extubated. All values were kept inaccessible to the participating investigators till the study was completed. Inotrope infusions were unchanged for at least 10 min before the measurement and were not changed during the measurement. All the patients were calm and restful throughout the study, and for those who were on ventilator, no changes were made in the ventilator settings.

On the basis of data from other reports, it was estimated that to recognize a clinically significant CO difference of 300 mL/min between the two methods with a power of 0.80, at least 140 paired samples were required to be compared. All values are presented as mean and standard deviation (SD). Pearson correlation coefficient (r) was determined for paired CO values. Mean CO, bias (mean difference between the CO from the paired values), and precision (±2 SD of the average of biases) were compared for each pair as recommended by Bland and Altman. [14] Percentage error (ratio of precision to mean CO of the two methods) to determine acceptable limits of agreement between both techniques of CO measurement was calculated using the formula given by Critchley and Critchley. [15] A P value of less than 0.05 was considered to be statistically significant. Analysis of the data was done by using SPSS version 13 (Chicago, IL, USA).

   Results Top

Forty-six patients admitted to cardiac ICU were enrolled for the study and data from all the 46 patients were used for analysis. Demographic characteristics of the patients are shown in [Table 1]. The mean age was 58 ± 9.4 years, and there were predominantly male patients (40/6). Mean left ventricular ejection fraction (LVEF) was 42.9 ± 11.3%. Body mass index was 20-24.9 in 31 patients and 25-29.9 in 15 patients. Thirty-one patients were on inotrope/inodilator therapy at the time of study.
Table 1: Demographic data

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A total of 230 pairs of CO measurements with TD and TEB methods were compared on volume control mode, and a similar 230 pairs were compared after the patients were weaned off the ventilator and extubated. On volume control mode, TD CO values ranged from 2.29 to 6.74 L/min with a mean of 4.45 ± 0.85 L/min, whereas TEB CO values ranged from 1.70 to 6.90 L/min with a mean of 4.43 ± 0.94 L/min. The average correlation (r) on volume control mode was 0.548 (P 0.0002), accompanied by a bias of 0.015 L/min (TEB CO was lower than TD CO by a mean of 15 mL/min), and precision of 0.859 L/min. 95% confidence interval of difference limits were narrow (−0.096 to 0.126 L/min) [Table 2]. A Bland-Altman plot comparing TD CO and TEB CO on volume control mode is shown in [Figure 1].
Table 2: Comparison of TD CO and TEB CO on volume control mode (230 paired measurements in 46 patients)

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Figure 1: Bland-Altman analysis of TD CO and TEB CO data on volume control mode. X axis - Mean CO [(TD CO + TEB CO)/2] (L/min) for each data pair; Y axis - CO difference [TD CO - TEB CO] (L/min). Correlation: 0.548 (P = 0.0002); bias: 0.015 L/min; precision: 0.859 L/min; percent error: 19.3% (TD, thermodilution; TEB, transthoracic electrical bioimpedance; CO, cardiac output)

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During spontaneous breathing, TD CO values ranged from 2.66 to 6.92 L/min with a mean of 4.66 ± 0.76 L/min, whereas TEB CO values ranged from 3.08 to 6.90 L/min with a mean of 4.72 ± 0.82 L/min. The average correlation was relatively weak (r = 0.469), though it was statistically significant (P 0.002). Average bias and precision in extubated patients were −0.059 (TD CO being lower than TEB CO by a mean of 60 mL/min) and 0.818 L/min, respectively, with 95% confidence interval of difference limits of −0.166 to 0.046 [Table 3].
Table 3: Comparison of TD CO and TEB CO during spontaneous breathing (230 paired measurements in 46 patients)

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Their Bland-Altman plot is shown in [Figure 2]. Both TD CO and TEB CO values were lower during mechanical ventilation than during spontaneous breathing (TD CO 4.45 versus 4.66 L/min, TEB CO 4.43 versus 4.72 L/min, respectively). The overall percent errors between TD CO and TEB CO measurements were 19.3% (volume control mode) and 17.4% (extubated patients), respectively, well within an acceptable limit of 30%. Twelve values (5.21%) in volume control mode and 16 values (6.95%) in extubated patients fell outside the 95% limits of agreement. No adverse effect of TEB technology for measurement of CO was observed in any of the patients.
Figure 2: Bland-Altman analysis of TD CO and TEB CO data during spontaneous breathing. X axis - Mean CO [(TD CO + TEB CO)/2] (L/min) for each data pair; Y axis - CO difference [TD CO - TEB CO] and TEB CO (L/min). Correlation: 0.469 (P = 0.002); bias: −0.059 L/min; precision: 0.818 L/min; percent error: 17.4% (TD, thermodilution; TEB, transthoracic electrical bioimpedance; CO, cardiac output)

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   Discussion Top

With the advancement in technology, there is a strong drive toward development of minimally invasive or non-invasive tests for measuring cardiac physiologic parameters. Since its introduction in the mid 1960s by the National Aeronautical and Space Administration researchers and William Kubicek, impedance cardiography using the TEB has been increasingly explored for its ability to measure CO and other hemodynamic parameters. [16] This study was performed to evaluate non-invasive method to measure CO by TEB technology and to compare it with TD CO method in ventilated and extubated patients after OPCAB surgery.

The main findings of the present study are that (i) correlation between non­invasive CO measurement with TEB technology and TD method is fair (r = 0.548) during volume control mode and (ii) the correlation is weak (r = 0.469) during spontaneously breathing. In statistical terms, a correlation coefficient of more than 0.7 is considered as a strong positive correlation between the two linear related variables, whereas an r of 0.5-0.7 is considered as fair, and an r of less than 0.5 is considered as weak correlation. [17] A meta-analysis of 154 studies, addressing accuracy of TEB by Raaijmakers et al.[2] observed a pooled correlation coefficient of 0.67 between TEB CO and that measured by other methods, and concluded that TEB might be useful for trend analysis of different groups of patients. They, however, cautioned the use of TEB for diagnostic purposes in cardiac patients, as the correlation coefficient in this subgroup of patients was much lower (r = 0.53), the value closer to our study.

Numerous published studies report a poor correlation and lack of agreement between CO measured by TEB and a reference technique (most often TD) in various subgroups of patients. [10],[11],[18],[19] Genoni et al. [11] found similar results during mechanical ventilation with PEEP in a small group of sedated and paralyzed patients with acute lung injury. To avoid the usual physiological and pathological variabilities (such as obesity and COPD) that limit the validity and reliability of bioimpedance monitoring, we chose to exclude such patients. Obesity is considered as an important factor in unreliable measurement of CO by TEB technology in cardiac surgical patients. [20] van der Meer et al.[20] found that TEB CO values weakly correlated with TD CO values in patients with body weight of more than 15% of their ideal body weight. Changes in thoracic geometry caused by obesity have also shown to significantly affect the bioimpedance computations. [21],[22] TEB monitoring has shown inaccurate results in COPD patients, as changes in pulmonary vascular flow and intrathoracic pressure along with hyperinflation, may change the passage of current emitted by TEB device through chest and thus modify the aortic component of dZ/dt. [23]

Our results are somewhat different from those reported by Sageman et al., [9] Gujjar et al., [24] Suttner et al., [25] and Chakravarthy et al., [26] who found TEB technology to be accurate and interchangeable with TD in post-cardiac surgical patients. However, assessment of CO during spontaneous breathing was not done in their studies. Preiser et al., [27] in a small study (n = 8) found TEB to be unreliable in mechanically ventilated patients and concluded that several assumptions are made while computing CO data from this technology, such as (i) measurement of aortic blood flow depends on blood-specific resistivity which can vary with hematocrit and other factors, (ii) TEB method regards thorax as being a perfect cylinder and assumes a fixed relationship between length and radius of the cylinder, and (iii) perfusion within thorax is homogeneous and the distribution of blood flow is largely influenced by the cardiovascular status of the patient. However, all these assumptions are yet to be proved clinically.

Recently, Fellahi et al.[21] compared cardiac index (CI) measurement with bioimpedance cardiography and Doppler echocardiography in resting, healthy volunteers, and found a poor correlation and lack of agreement between the absolute values and the changes in CI measured by the two techniques. They concluded that inaccuracies in bioimpedance LVET measurement were responsible for the lack of agreement between CI determination with bioimpedance cardiography and echocardiography. Another explanation for the poor correlation could be attributed to inconsistencies in the underlying algorithms of all commercially available bioimpedance devices. de Waal et al.[18] recently highlighted that common cylinder and cone-based models for bioimpedance SV calculation were oversimplification of the complex electrical events occurring inside the thorax during the cardiac cycle and were not reliable compared with TD measurements in cardiac surgical patients. The authors strongly suggested that a more robust mathematical approach was mandatory to consider all relevant anatomical variabilities before TEB CO monitoring can be recommended for clinical decision making. [18]

Presence of endotracheal tube, mediastinal tubes, pleural tubes, sternal wires, and alteration in physiology caused by mechanical ventilation and PEEP, have all been shown to affect the TEB measurements by affecting the rate of change of thoracic impedance in a study by Sageman et al. [22] We found lower values of CO during mechanical ventilation than during spontaneous breathing (TD CO 4.45 versus 4.66 L/min, and 4.43 versus 4.72 L/min, respectively). A decrease in CO during mechanical ventilation, with the use of PEEP, could possibly have resulted from reduction in venous return. A weak correlation in spontaneously breathing patients in this study could have resulted from occasional, small bodily movements or diaphoresis, leading to suboptimal contact of chest and neck electrodes.

We attempted to minimize all possible controllable sources of error while measuring CO. Room temperature injectate, rather than iced injectate, was used for TD CO determinations, because iced injectate can affect the heart rate and CO. All TEB measurements were performed by the same investigator, who had been trained by the manufacturer on proper placement of electrodes and use of the machine. Knowledge of the impedance waveform morphology (just as TD waveform morphology) is important because waves that appear unreal may generate spurious data.

There are a few limitations to this study. A relatively smaller number of female patients (6 versus 40 male patients) is a limitation, considering the fact that gender is one of the important patient characteristics that factors in the computation of CO. Therapeutic interventions based on CO values and calculated hemodynamic parameters in patients with poor LVEF are considered as one of the indications for the use of PAC . A comparison with TEB in this patient population is also needed, though not undertaken in the present study. Yet another weakness is that there is a time lag for measurement of cardiac output by TD. But for the TEB, there is hardly any delay. Therefore, the assessment was made in different frame of time.

In conclusion, we found a fair correlation between 230 paired readings of TEB CO and TD CO measurements among 46 ventilated post-OPCAB patients. Their correlation was, however, weak after they were extubated. Further studies are clearly needed before the NICOMON device can be recommended routinely for non-invasive CO monitoring at the bedside in cardiac surgical patients. The accuracy during changing hemodynamic parameters and response to therapeutic interventions also needs to be further assessed.

   References Top

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20.van der Meer BJ, de Vries JP, Schreuder WO, Bulder ER, Eysman L, et al. Impedance cardiography in cardiac surgery patients: Abnormal body weight gives unreliable cardiac output measurements. Acta Anaesthesiol Scand 1997;41:708-12.  Back to cited text no. 20
21.Fellahi JL, Caille V, Charron C, Deschamps-Berger PH, Vieillard-Baron A. Noninvasive assessment of cardiac index in healthy volunteers: A comparison between thoracic impedance cardiography and Doppler echocardiography. Anesth Analg 2009;108:1553-9.  Back to cited text no. 21
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24.Gujjar AR, Muralidhar K, Banakal S, Gupta R, Sathyaprabha TN, Jairaj PS. Non-invasive cardiac output by transthoracic electrical bioimpedance in post-cardiac surgery patients: Comparison with thermodilution method. J Clin Monit Comput 2008;22:175-80.  Back to cited text no. 24
25.Suttner S, Schollhorn T, Boldt J, Mayer J, Rohm KD, Lang K, et al. Noninvasive assessment of cardiac output using transthoracic electrical bioimpedance in hemodynamically stable and unstable patients after cardiac surgery: A comparison with pulmonary artery thermodilution. Int Care Med 2006;32:2053-8.  Back to cited text no. 25
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27.Preiser JC, Daper A, Parquier JN, Contempre B, Vincent JL. Transthoracic electrical bioimpedance versus thermodilution technique for cardiac output measurement during mechanical ventilation. Intensive Care Med 1989;15:221-3.  Back to cited text no. 27

Correspondence Address:
Ajmer Singh
Department of Anaesthesiology, Escorts Heart Institute and Research Centre Ltd., Okhla Road, New Delhi - 110 025
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0971-9784.81564

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