Year : 2012  |  Volume : 15  |  Issue : 3  |  Page : 180--184

A comparison of a continuous noninvasive arterial pressure (CNAP™) monitor with an invasive arterial blood pressure monitor in the cardiac surgical ICU

AM Jagadeesh, Naveen G Singh, Subramanyam Mahankali 
 Department of Cardiac Anaesthesiology, Sri Jayadeva Institute of Cardiovascular Sciences and Research, Jayanagar, Bangalore, India

Correspondence Address:
Naveen G Singh
Department of Cardiac Anaesthesia, Sri Jayadeva Institute of Cardiovascular Sciences and Research, 9th Block Jayanagar, Bangalore - 560 069


Accurate measurement and display of arterial blood pressure is essential for rational management of adult cardiac surgical patients. Because of the lower risk of complications, noninvasive monitoring methods gain importance. A newly developed continuous noninvasive arterial blood pressure (CNAP™) monitor is available and has been validated perioperatively. In a prospective study we compared the CNAP™ monitoring device with invasive arterial blood pressure (IAP) measurement in 30 patients in a cardiac surgical Intensive Care Unit (ICU). Patients were either mechanically ventilated or spontaneously breathing, with or without inotropes. CNAP™ was applied on two fingers of the hand contralateral to the IAP monitoring catheter. Systolic, diastolic and mean pressure data were recorded every minute for 2 h simultaneously for both IAP and CNAP™. Statistical analysis included construction of mountain plot and Bland Altman plots for assessing limits of agreement and bias (accuracy) calculation. Three thousand and six hundred pairs of data were analyzed. The CNAP™ systolic arterial pressure bias was 10.415 mmHg and the CNAP™ diastolic arterial pressure bias was −5.3386 mmHg; the mean arterial pressure (MAP) of CNAP™ was close to the MAP of IAP, with a bias of 0.03944 mmHg. The Bland Altman plot showed a uniform distribution and a good agreement of all arterial blood pressure values between CNAP™ and IAP. Percentage within limits of agreement was 94.5%, 95.1% and 99.4% for systolic, diastolic and MAP. Calculated limits of agreement were −4.60 to 25.43, −13.38 to 2.70 and −5.95 to 6.03 mmHg for systolic, diastolic and mean BP, respectively. The mountain plot showed similar results as the Bland Altman plots. We conclude CNAP™ is a reliable, noninvasive, continuous blood pressure monitor that provides real-time estimates of arterial pressure comparable to those generated by an invasive arterial catheter system. CNAP™ can be used as an alternative to IAP.

How to cite this article:
Jagadeesh A M, Singh NG, Mahankali S. A comparison of a continuous noninvasive arterial pressure (CNAP™) monitor with an invasive arterial blood pressure monitor in the cardiac surgical ICU.Ann Card Anaesth 2012;15:180-184

How to cite this URL:
Jagadeesh A M, Singh NG, Mahankali S. A comparison of a continuous noninvasive arterial pressure (CNAP™) monitor with an invasive arterial blood pressure monitor in the cardiac surgical ICU. Ann Card Anaesth [serial online] 2012 [cited 2021 Jul 28 ];15:180-184
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Full Text


Accurate measurement and display of arterial blood pressure (ABP) is essential for the rational management of adult cardiac surgical patients. This requirement is most commonly met by the placement of an intraarterial catheter and connecting it to a pressure monitoring system, which is considered as the "gold standard" of ABP measurement in patients who may experience rapid changes in hemodynamic status. However, the invasive arterial pressure (IAP) monitoring is associated with complications like thrombosis, occlusion of the vessel with limb ischemia, hemorrhage, infection and difficulty of insertion [1] and nonavailability of arteries. In view of these problems, investigators have been evaluating noninvasive monitoring systems based on oscillometry, arterial tonometry and finapress principle but, data have been inconsistent when compared with invasive measurements of ABP. [2]

Recent studies showed that continuous noninvasive ABP measurements generated by a new device, called Infinity® CNAP™ SmartPod® (Draeger Medical Systems Inc., Telford, PA, USA), have been superior to intermittent oscillometric measurements during sedation for interventional endoscopy, [3] spinal anesthesia for caesarean section [4] and during general anesthesia. [5]

The basic principle of CNAP™ is the vascular unloading technique described by Penaz. [6] CNAP™ monitors blood flow into the finger and transmits blood flow oscillations sensed by the encircling finger cuff into continuous pulse pressure waveform and beat to beat values of ABP. In this study we compared the agreement of the CNAP™ method of measuring ABP with measurements obtained invasively via an arterial catheter in patients of the cardiac surgical intensive care unit (ICU).

 Materials and Methods

This study was approved by the ethics committee of Sri Jayadeva Institute of Cardiovascular Sciences and Research. Written informed consent was obtained from patients during the preoperative visit. Patients above the age of 16 years undergoing cardiac surgery were recruited. Patients were excluded if they had absence of bilateral radial pulses, more than 5 mmHg difference in ABP between the upper extremities, congenital or acquired anatomic differences between wrists, systemic vasculopathies or an improperly working radial artery catheter. Baseline demographics, surgical procedure performed and vasoactive medications were recorded.

A radial artery catheter was placed in each patient before induction of anesthesia and was used for continuous monitoring during surgery or after arriving to the cardiac ICU. The arterial catheter was connected to a transducer that was calibrated at the level of the patient's right atrium. The tubing and transducer were inspected to ensure that there were no technical issues or air bubbles that could cause erroneous recordings.

The CNAP™ monitoring system consists of reusable finger cuffs, the cuff controller and the CNAP™ pod, which interfaces with the patient monitor. The finger cuffs are available in three sizes (i.e., small, medium and large), and consist of two semi-rigid cylinders covering two adjacent fingers, i.e. index and middle fingers. Inflatable cuffs, sensors and electronics are present inside the semi-rigid cylinders. The adequately sized finger cuff is connected to the cuff controller that contained the pneumatic control unit for the inflatable parts of the finger cuff. The finger cuff was applied contralateral to the radial artery catheter. A 2.5-m-long cable leads from the cuff controller to the CNAP™ pod which contains hardware and software to drive sensor cuff function, interprets sensor signal and reports data to monitor. The continuously changing finger cuff pressure is measured with a pressure transducer. The analogue signal of the CNAP™ pod is displayed on another Drager Infinity Delta monitor that also control a standard oscillometric upper-arm cuff for scaling purposes. CNAP™ is obtained by applying pressure via the finger cuffs such that the blood volume flowing through the finger arteries is held constant (i.e., volume-clamping). The CNAP™ is scaled to central non-invasive ABP every 15 min by a scaling function with noninvasive ABP values as arguments. After applying this scaling operation, CNAP™ values correspond to the values measured at the brachial artery. The noninvasive ABP measurements were performed on the same arm with the CNAP™ finger cuff. The analogue signals of the ECG, IAP and CNAP™ were simultaneously derived from the patient monitors.

Simultaneous systolic, diastolic and MAPs by CNAP™ and IAP were recorded for 2 h. Patients were either tracheally intubated with controlled ventilation or breathing spontaneously during this time depending on their perioperative condition and stability. Patients were supine during this period. Data was recorded every 1 min for a max of 2 h. No clinical decisions in the postoperative care of the patients were based on the CNAP™ readings.

Measurements of ABP from CNAP™ and invasive arterial line were analyzed for correlation. Between-technique variability of two different methods of measuring the same parameter was evaluated by bias as recommended by Bland and Altman. [7] The bias represents the systemic error between the two techniques, and standard deviation of bias represents the random error or variability between the two techniques. Bland Altman plots were constructed of systolic, diastolic and MAPs to enable visual observation of data for agreement between the two methods and to determine the 95% confidence limits (limits of agreement). In addition, folded empirical cumulative distribution plot (mountain plot) were also plotted. The advantage of the mountain plot is that it is easier to find the central 95% of the data and it is easier to estimate percentiles for large differences. [8] If two monitors are unbiased with respect to each other, the mountain will be centered over zero. Long tails in the plot reflect large differences between the methods. All statistical analysis was conducted with the SPSS statistical programme (version 18.0) and MedCalc software version 12.2.1.


Thirty patients were enrolled into the study. There were no dropouts from the study. Data from twenty one males and nine females were available for analysis. Sixteen patients underwent Coronary artery bypass grafting (CABG), six patients had aortic valve replacement, three patients had double-valve replacement, four patients had mitral valve replacement and one patient had atrial septal defect closure. There were no hemodynamic complications intraoperatively. Seven patients were without inotropes, while the remainder were on either Dopamine 5 mcg/kg/min or Dobutamine 5 mcg/kg/min or NTG 0.1 mcg/kg/min or Adrenaline 0.01 mcg/kg/min infusions postoperatively. A total of 3600 paired data were obtained. 1.2% of the patients received a small finger cuff, while 77% and 21.8% received medium and large cuffs, respectively. The system set-up time of CNAP™ is defined as the time between the start of the CNAP™ device and the occurrence of the first valid CNAP™ beat, took on an average of 5 min. The systolic ABP by CNAP™ was consistently lower than systolic IAP, and the CNAP™ diastolic ABP was consistently higher than diastolic IAP, yielding a systolic bias of 10.41 mmHg and a diastolic bias of −5.33 mmHg. CNAP™ MAP values were almost close to the IAP mean ABP value, yielding a bias of 0.039 mmHg [Table 1]. Frequency of ABP pairs within the limits of agreement was 94.5%, 95.1% and 99.4% for systolic, diastolic and MAP, respectively. Calculated limits of agreement were −4.6 to 25.43, −13.38 to 2.7 and −5.95 to 6.03 for systolic, diastolic and MAP, respectively [Table 1]. Bland Altman plot [Figure 1]a, [Figure 2]a and [Figure 3]a showed uniform distribution of the variances over all measured ABP values and a good agreement of ABP between CNAP™ and IAP values. Mountain plot which is generally used as complimentary to Bland Altman plot, also showed similar results [Figure 1]b, [Figure 2]b, and [Figure 3]b. The median (bias, center of the plot) between IAP and CNAP™ for systolic ABP was 11.5, for diastolic ABP was -5 and for MAP was 0. The MAP showed no large tails i.e., smaller difference between IAP and CNAP™ [Figure 3]b. No significant differences in performance were present between measurements with different-sized finger cuffs.{Figure 1}{Figure 2}{Figure 3}{Table 1}


In this study, we demonstrated acceptable agreement between CNAP™ measurements and measurements generated by an intraarterial catheter system in patients in the cardiac surgical ICU. In clinical practice, a new monitoring technique is acceptable for clinical use if both the applicability and the performance are comparable with those of an established method. The intraarterial measurement of ABP from radial or femoral artery has been the method of choice when continuous ABP monitoring during predictable hemodynamic instability is required. Nevertheless, given the fact that CNAP™ is a reliable device to assess the ABP continuously, there is a wide range of possible clinical applications. Its noninvasiveness facilitates its use for any operation with a need to assess, document and maintain hemodynamic stability.

The currently available recommendations for evaluation of the accuracy of noninvasive ABP monitoring devices consider the mean difference and the SD of differences between the test and the reference method, [9] or express agreement in terms of percentage of differences that decrease within certain thresholds. [10] Limits of agreement obtained were −4.6 to 25.43, −13.38 to 2.7 and −5.95 to 6.03 for systolic, diastolic and MAP, respectively. The observed frequency of ABP measurements within these limits was found to be 94.5%, 95.1% and 99.4% for systolic, diastolic and MAP, respectively.

Because CNAP™ provides reconstructed brachial ABP, and the invasive ABP measurements were performed in the radial artery, the observed bias may be explained by the inherent physiological difference between brachial and radial artery ABP. The systolic ABP in the radial artery increases as a result of wave reflection, whereas the diastolic ABP decreases due to resistance to flow, which is reflected by the systolic and diastolic bias of our study [Table 1]. This systematic bias is also considered by the FDA-standard for intermittent oscillometric sphygmomanometers in a metaanalysis that reported absolute differences of 0.68-13.4 and 0.8-18 mmHg for systolic and diastolic ABP, respectively. [9] The observed bias of 10.41 and −5.33 mmHg for systolic and diastolic ABP with this investigation lie within these ranges. Median values of 11.5 mmHg of systolic ABP and −5 mmHg of diastolic ABP obtained from mountain plot also lie within the above-mentioned ranges. Furthermore, the observed bias for mean ABP lies within the range of +/-5 mmHg for clinically acceptable agreement as recommended by the Association for the Advancement of Medical Instrumentation ANSI/AAMI SP10 (Manual, Electronic or Automated Sphygmomanoneters). From this close agreement of mean ABP values seen between CNAP™ and invasive ABP, we can advocate the use of CNAP™ during routine postoperative management and in patients where two different sites of monitoring are required.

Currently, commercially available devices for continuous noninvasive ABP measurement are based on arterial tonometry (T-line Tensymeter) and on the volume clamp method (Finapres® ). To obtain accurate pressure values by tonometry, it is important to find the optimal position over the artery to apply the sensor. [11],[12] Finapress was shown in many studies as unreliable reflection of invasive ABP in anaesthetized adults that may have been caused by an increase in venous blood volume distal to the finger cuff and by rapid contractions and dilatations of finger arteries in relation to psychological, physical, and chemical stress. In order to reduce these limitations, the CNAP™ device switches repeatedly between two adjacent finger cuffs that are controlled by concentrically interlocking loops for rapid detection of blood flow oscillation. [13]

Dinamap, which works on the oscillometric principle, cycles in a "stat" mode that enables measurements to be obtained approximately every 30 s under ideal conditions; however, inflation of the cuff is associated with pain and discomfort. The cuff is also not suitable to be used in the "stat" mode over an extended period of time because of reports of compression injuries. [14]

Comparison between CNAP™ and direct ABP monitoring was only performed once patients were in a stable condition after cardiac surgery, and there were minimal adjustments to subsequent hemodynamic or ventilatory support during the course of the study. CNAP™ measurements represent ABP at the brachial artery because of the calibration with upper-arm oscillometric measurements. Although the comparison between CNAP™ and invasive ABP in the brachial artery would have been more close to an ideal study design, we decided not to perform the invasive ABP measurements at this arterial site because of the complications associated with arterial puncture and the inflating and deflating of the upper-arm cuff on the same arm and near the intraarterial catheter.


CNAP™ is a reliable, noninvasive, continuous blood pressure monitor that provides real-time estimates of ABP comparable with those generated by an invasive intraarterial catheter system in patients of the cardiac surgical ICU.


The authors would like to acknowledge Jagannath PS, Statistician, Bangalore, for statistical assistance.


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