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An Uncalibrated Pulse Contour Method to Measure Cardiac Output During Aortic Counterpulsation

Scolletta, Sabino MD*; Franchi, Federico MD*; Taccone, Fabio Silvio MD; Donadello, Katia MD; Biagioli, Bonizella MD*; Vincent, Jean-Louis MD, PhD

doi: 10.1213/ANE.0b013e318230b2de
Technology, Computing, and Simulation: Research Reports
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BACKGROUND: Less-invasive monitoring systems, such as pulse contour methods, are increasingly being used to estimate cardiac output (CO). However, alterations in the arterial waveform caused by intraaortic balloon pump counterpulsation may affect the ability of pulse contour algorithms to determine CO. We investigated the reliability of an uncalibrated pulse contour method, the MostCare system, in patients with cardiac failure receiving intraaortic balloon pump counterpulsation by comparing its measurements of CO with those determined by an intermittent thermodilution method.

METHODS: The study included 15 patients requiring hemodynamic support with an intraaortic balloon pump after coronary artery bypass graft surgery. A pulmonary artery catheter was inserted and CO was determined by bolus thermodilution (ThD-CO). The MostCare device was directly connected to the standard monitoring system for analysis of the radial artery pressure wave and computation of CO (MostCare-CO). Data were collected at 3 different intraaortic balloon pump rates (1:1, 1:2, 1:4) and after intraaortic balloon pump removal.

RESULTS: One hundred six pairs of ThD-CO and MostCare-CO measurements were analyzed. There was a good correlation between ThD-CO and MostCare-CO (r = 0.90, 95% confidence interval [CI] = 0.86–0.93; P < 0.001). The mean bias of all CO measurements corrected for repeated measures was −0.2 L/min with limits of agreements of −1.31 to 0.91 L/min (lower 95% CI, −1.72 to −0.90; upper 95% CI, 0.50–1.32) and a relative percentage error of 24. There were close agreements between ThD-CO and MostCare-CO at the different intraaortic balloon pump rate settings. Changes in CO were calculated separately for the 2 methods and data comparison showed a correlation of 0.82 (95% CI = 0.76–0.87; P < 0.001) and a mean bias of 0.14 L/min with limits of agreement of −1.31 to 1.59 L/min (lower 95% CI, −1.62 to −1.00; upper 95% CI, 1.28–1.90).

CONCLUSION: The MostCare system provided measurements of CO that were comparable to ThD-CO in patients assisted with an intraaortic balloon pump. The reliability of the MostCare system is not significantly affected by changes in arterial waveform morphology caused by inflation and deflation of the intraaortic balloon pump.

Published ahead of print October 14, 2011 Supplemental Digital Content is available in the text.

From the *Department of Anaesthesia and Intensive Care, University of Siena, Siena, Italy; and Department of Intensive Care, Erasme Hospital, Université Libre de Bruxelles, Bruxelles, Belgium.

Conflicts of Interest: See Disclosures at the end of the article.

Reprints will not be available from the authors.

Address correspondence to Jean-Louis Vincent, MD, PhD, Department of Intensive Care, Erasme University Hospital, Route de Lennik, 808-1070 Brussels, Belgium. Address e-mail to jlvincen@ulb.ac.be.

Accepted July 20, 2011

Published ahead of print October 14, 2011

The intraaortic balloon pump is an important tool to support the heart in critically ill patients with severe heart failure. Monitoring of cardiac output (CO) is essential to guide treatment during intraaortic balloon pump therapy and weaning.1 The thermodilution method is regarded as the “gold standard” technique for CO (ThD-CO) measurement, but insertion of a pulmonary artery catheter is invasive and is associated with various risks and complications.2 As an alternative, several pulse contour methods have been developed that use specific algorithms to compute CO from analysis of the arterial pressure waveform.3

During intraaortic balloon pump assistance, the shape of the arterial pressure curve is distorted. First, the intraaortic balloon pump inflates with the appearance of the dicrotic notch and peak-augmented diastolic pressure is inscribed. The balloon then deflates just before opening of the aortic valve and creates a potential vacuum, which decreases the aortic end-diastolic pressure below the patient's baseline value. Deflation is maintained until the onset of a new diastole.4 These changes in pressure waveform morphology can affect the reliability of pulse contour methods when used in conjunction with an intraaortic balloon pump.

The MostCare system (Vytech Health, Padua, Italy) is a new, uncalibrated pulse contour method that has been validated in various clinical conditions.510 The aim of this study was to investigate the reliability of the MostCare system in patients with cardiac failure who were supported with an intraaortic balloon pump, by comparing its CO measurements with ThD-CO.

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METHODS

Patients

The study was approved by the IRB, and written informed consent was obtained from each patient before coronary artery bypass grafting. We studied consecutive patients who required hemodynamic support with an intraaortic balloon pump in the intensive care unit after elective bypass grafting. Exclusion criteria were the presence of arrhythmias with severe hemodynamic instability, aortic regurgitation or stenosis, tricuspid valve insufficiency, or ascending aorta pathologies, because all these situations can negatively affect the reliability of the pulse contour method studied.

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Thermodilution Data Acquisition

A thermodilution pulmonary artery catheter (7F; Baxter-Edwards, Irvine, CA) was introduced into the right internal jugular vein, as standard procedure. Five consecutive injections of 10 mL of 5% glucose cold (4°C) solution were made randomly during the respiratory cycle over a 5-minute period. The lowest and the highest CO values were discarded and ThD-CO was calculated as the mean of the 3 remaining measurements. In the absence of hemodynamic stability (discrepancy in the CO measurements >10%), the series of CO measurements was discarded and the process repeated until satisfactory measurements were obtained. To avoid variations among operators, the injections were performed by the same operator.

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MostCare System Data Acquisition

The MostCare system is a pulse contour method that provides a beat-to-beat estimation of stroke volume and CO, derived from the arterial pressure waveform. This device does not need external calibration and is powered by the pressure recording analytical method algorithm.5 Because the MostCare device does not require a dedicated catheter-transducer system, it was connected via a simple cable to the main monitoring system for continuous recording of the radial arterial pressure waveform. Arterial pressure was measured in each patient using an 18-gauge radial artery catheter connected to pressure tubing and a pressure transducer (Edwards Lifesciences, Irvine, CA). For each measurement of ThD-CO, a corresponding value from the MostCare monitor was obtained by averaging the individual stroke volumes over the time needed for each ThD-CO measurement (approximately 30–45 seconds). These values were automatically downloaded and recorded in a computer database for offline analysis. Finally, the 3 MostCare-CO measurements and the corresponding 3 ThD-CO measurements were averaged for data analysis.

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Study Intervals and Experimental Procedure

An intraaortic balloon pump was placed after coronary artery bypass surgery if the patient exhibited signs and symptoms of hypoperfusion associated with a low CO despite inotropic drug support.11 CO data collection began in the intensive care unit. For each patient, 2 sets of ThD-CO measurements were collected at each intraaortic balloon pump rate setting (1:1, 1:2, 1:4) under hemodynamic steady-state conditions. If hemodynamic conditions deteriorated during the reduction in intraaortic balloon pump rate setting, the ThD-CO measurements were not performed and the intraaortic balloon pump setting was immediately shifted to the previous rate. Two sets of ThD-CO measurements were also obtained after intraaortic balloon pump removal. After zeroing the pressure transducer and before each MostCare-CO measurement, the frequency response of the arterial blood pressure transducer was checked using a fast flush test to assess the adequacy of the damping of the arterial pressure waveform.12

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Statistical Analysis

Statistical analysis was performed using StatsDirect version 2.5.8 (Cheshire, UK), SigmaPlot for Windows version 11.0 (Systat Software, Inc., San Jose, CA), and R version 2.11.1 (2010-05-31; R Foundation for Statistical Computing, Vienna, Austria).13

The agreement between ThD-CO and MostCare-CO was assessed using the Bland-Altman method.14 The correlation coefficient, bias (mean difference between measurements), and 95% confidence interval (CI) were calculated. Limits of agreement (LoA) (calculated as 2.2 times the standard deviation of the bias) were computed as proposed by Ludbrook15 for small samples. The 95% CIs of the upper and lower LoA were also calculated. The percentage of error was calculated as the LoA (2.2 times the standard deviation of the bias) divided by the mean CO from the 2 methods, as proposed by Critchley and Critchley16: 100 × (2.2 × SD of bias)/[(meanThD-CO + meanMostCare-CO)/2].

To investigate changes in CO regardless of absolute accuracy, differences in CO (ΔCO) were calculated by subtracting the first from the second measurement, the second from the third, and the third from the fourth. We used the method suggested by Myles and Cui17 to adjust for the effects of repeated measurements in the Bland-Altman analysis. For this purpose, linear mixed models were used with variance component structure.18,19 Subject and method were introduced as fixed effects, and the crossed effects (method × subject and subject × replication) were introduced as random effects. Because not all patients had the same number of measurements, a plot of within-subject standard deviation against the mean of each subject for each method was used.17

Changes of ThD-CO and MostCare-CO at the different intraaortic balloon pump settings (1:1, 1:2, 1:4, and removal) were also tested via linear mixed models. The choice of the covariance structure and hence the retained model was based on the Bayesian Information Criterion.

The ability of the MostCare to reliably follow changes or trends in CO was assessed using concordance analysis.20 The differences between serial ThD-CO measurements (ΔThD-CO) and serial MostCare-CO measurements were analyzed to assess the percentage of concordance between the 2 methods, including and excluding ΔThD-CO <0.5 L/min.21,22 ΔThD-CO is plotted on the x-axis and ΔMostCare system is plotted on the y-axis.23 For all statistical tests, a P value <0.05 was taken to indicate significance.

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RESULTS

One hundred six pairs of thermodilution and pulse contour CO measurements from 15 patients were compared. Patient characteristics and operative data are shown in Table 1. Fourteen ThD-CO measurements were not performed because of unstable hemodynamic conditions in 3 patients when the intraaortic balloon pump was set at 1:2, and in 10 patients when the intraaortic balloon pump was at 1:4. The MostCare system calculates CO values at all times in all patients. CO values at each intraaortic balloon pump setting are reported in Table 2.

Table 1

Table 1

Table 2

Table 2

ThD-CO values ranged from 2.4 to 6.3 L/min and MostCare-CO values ranged from 2.6 to 6.7 L/min. Mean ThD-CO and MostCare-CO did not differ significantly at each intraaortic balloon pump rate setting (Table 2). There was a significant difference between mean ThD-CO and MostCare-CO after intraaortic balloon pump removal and those measured at different intraaortic balloon pump rate settings for both techniques (P < 0.001).

Overall, there was a good correlation between ThD-CO and MostCare-CO (r = 0.90, 95% CI = 0.86–0.93; P < 0.001). The mean differences between ThD-CO and MostCare-CO are shown in Table 3. Bland-Altman plots at each intraaortic balloon pump setting are shown in Figure 1.

Table 3

Table 3

Figure 1

Figure 1

The difference between serial measurements of ThD-CO (ΔThD-CO) and serial measurements of MostCare-CO (ΔMostCare-CO) are shown in Figure 2. The correlation coefficient between ΔThD-CO and ΔMostCare-CO was 0.82 (95% CI = 0.76–0.87; P < 0.001) The bias was 0.14 L/min with LoA of −1.31 to 1.59 L/min (lower 95% CI, −1.62 to −1.00; upper 95% CI, 1.28–1.90).

Figure 2

Figure 2

The concordance between ΔThD-CO and ΔMostCare-CO was 89.5% (60 of 67 pairs of ΔCO agreed) and it improved to 95.1% (39 of 41 pairs of ΔCO agreed) when 26 ΔThD-CO <0.5 L/min were excluded from the analysis. A polar plot was used to show the direction of CO changes (i.e., the trending ability) (Fig. 3).

Figure 3

Figure 3

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DISCUSSION

In 1976, Herzlinger24 described a simple method for determining CO on a continuous beat-by-beat basis by the analysis of blood pressure traces in 40 patients with an intraaortic balloon pump. He used a simple mathematical formula that included the pumping volume of the intraaortic balloon pump and changes in the pressure wave generated by ventricular ejection and balloon inflation/deflation). He reported good correlation with indocyanine green dye and thermal dilution CO.24 Høie25 performed a similar study in 13 patients with an intraaortic balloon pump. He found a correlation of r = 0.86 between CO determinations from pulse contour and thermodilution. However, his analyzer failed to give results in 7 patients (60%) because it was not possible to recognize the 3 pressure excursions within the arterial waveform. The author therefore concluded that his algorithm was useless as a method for CO monitoring.25 After this study, the idea of using the arterial trace to calculate CO in patients with an intraaortic balloon pump was rejected for more than 2 decades.

Janda et al.26 studied the impact of an intraaortic balloon pump on measurements of CO made using transpulmonary thermodilution (PiCCO; Pulsion Medical Systems, Irving, TX). These authors demonstrated that the close agreement (r = 0.94) between CO measurements obtained by transpulmonary and pulmonary arterial thermodilution was not affected by an intraaortic balloon pump interposed between the injection and detection site of the indicator over a wide range of pacemaker-induced changes in CO.26 However, continuous CO measurement, a major application of the PiCCO system based on the principle of pulse contour analysis, was not a viable option during the use of an intraaortic balloon pump because of the complete alteration of the arterial pressure curve caused by the inflation and deflation of the intraaortic balloon pump, and this problem could not be circumvented by repeated recalibration of the signal by transpulmonary thermodilution.26 Lorsomradee et al.27 compared CO values obtained by an uncalibrated pulse contour method (Vigileo monitor version 1.07; Edwards Lifesciences) with those from continuous thermodilution (CCO, Vigilance Monitor; Edwards Lifescience) in 52 patients undergoing cardiac surgery. There was poor agreement between the 2 techniques in the subgroup of 12 intraaortic balloon pump patients. In 80% of intraaortic balloon pump patients, the Vigileo monitor displayed the message “unstable signal” or “check arterial waveform” during different periods and failed to show CO values for several minutes.27 Zangrillo et al.10 recently evaluated the accuracy and precision of the MostCare system compared with thermodilution in 32 patients undergoing different cardiac surgical procedures, with two-thirds of patients assisted with an intraaortic balloon pump. They reported good agreement between MostCare-CO and ThD-CO for the whole patient population (r = 0.72, mean bias of 0.072 ± 0.41 L/min/m2). Unfortunately, the agreement between the techniques was not specifically evaluated for the intraaortic balloon pump subgroup.10

Our findings showed close agreement between ThD-CO and MostCare-CO. Several other studies have confirmed the accuracy of MostCare in CO measurement in different clinical scenarios.510,28,29 Some interesting findings from the present study should be highlighted. First, the MostCare system identified the dicrotic notch at each cardiac cycle and thus could calculate CO values in all patients. Second, there were no significant differences between the MostCare system and thermodilution (Fig. 1), and the agreement persisted despite changes in the intraaortic balloon pump rate setting. Third, the LoA and the relative errors were within the clinically acceptable limits proposed by Critchley and Critchley16 (Table 3). Fourth, the agreement and the relative errors did not change significantly for all 106 CO measurements considering the repeated-measures design of the data (Table 3, Fig. 4). Finally, the polar plot used to assess trending ability according to Critchley et al.23 showed good concordance between the techniques in the detection of change in CO (ΔCO) (Fig. 3).

Figure 4

Figure 4

Pulse contour methods may have advantages over pulmonary artery catheter–derived thermodilution measurements.30 In particular, the MostCare system is easy to use, has a fast response time (beat-to-beat readout), and may detect abrupt changes in CO more quickly than the thermodilution technique. Moreover, the MostCare system does not require external calibration by thermodilution, and requires no other additional invasive procedure. Because it does not require injection of a thermal solution, a central line is not required, saving time and avoiding potential complications due to the insertion of a central catheter.31

However, there are also some drawbacks and limitations. In particular, various factors may affect the analysis of arterial waveforms. Patient-related conditions can affect the pulsatility and contour of waveforms because of abnormal transmission of the arterial signal; for example, in aortic valve regurgitation and stenosis or vascular pathologies resulting in obstruction to the transmission of the pressure wave (i.e., stenosis along the arterial tree from the aortic valve to the sampling site). Furthermore, over- or underdamped arterial pressure waveforms may affect the precision of the pressure wave analysis by pulse contour methods.3234 Analysis of the blood pressure wave at 1000 Hz is also dependent on the operator, who needs to maximize the quality of the arterial signal to obtain a reliable pressure wave morphology. In a recent study, Paarmann et al.35 reported a very weak agreement between measurements of CO obtained by MostCare and those obtained using thermodilution in postoperative cardiac surgery patients, in contrast to our findings. However, the authors provided little information regarding the quality of the arterial pressure signal, which, if inadequate, could explain in part the poor agreement observed in their study and the differences between their results and ours.36

Another important limitation of these less-invasive methods in measuring CO is their potential unreliability in patients with cardiac dysrhythmias, especially atrial fibrillation. In such cases, pulse contour methods must compute stroke volume for each heartbeat over a fixed time interval (i.e., 1 minute) and then calculate CO by multiplying stroke volume by the number of heartbeats. This potential drawback was recently demonstrated by Maj et al.37 who studied 41 patients who developed atrial fibrillation and hemodynamic instability after cardiac surgery. The authors compared MostCare with thermodilution and found poor agreement between the techniques.

Finally, the pulmonary artery catheter can provide additional data on pulmonary artery and filling pressures and mixed venous oxygen saturation, which are not available with pulse contour methods and allow accurate interpretation (and therapy) of CO in complex patients. The absence of these measures may be a disadvantage of pulse contour methods, but is perhaps a penalty we have to accept for being less invasive.

Obviously, the range of CO values analyzed in our study was relatively narrow because patients supported with an intraaortic balloon pump do not have a high CO. The studied patients were a homogeneous coronary artery bypass graft group free from ascending aorta disease and aortic valve pathology, which could affect the arterial waveform morphology and its analysis. This may explain why we found a better agreement between ThD-CO and MostCare-CO than did Zangrillo et al.10 in their study that also included intraaortic balloon pump patients with aortic valve and ascending aorta pathologies.

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CONCLUSIONS

In our patient cohort, we demonstrated that the MostCare device gave results comparable to those obtained by conventional thermodilution during the use of an intraaortic balloon pump, despite alterations in the arterial pressure waveforms.

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DISCLOSURES

Name: Sabino Scolletta, MD.

Contribution: Study design, patient inclusion and conduct of the study, data analysis, manuscript preparation.

Conflicts of Interest: Sabino Scolletta has received research grants from Vygon and Vytech.

Name: Federico Franchi, MD.

Contribution: Patient inclusion and conduct of the study, data analysis.

Conflicts of Interest: Federico Franchi reported no conflicts of interest.

Name: Fabio Silvio Taccone, MD.

Contribution: Data analysis, manuscript preparation.

Conflicts of Interest: Fabio Silvio Taccone reported no conflicts of interest.

Name: Katia Donadello, MD.

Contribution: Data analysis, manuscript preparation.

Conflicts of Interest: Katia Donadello reported no conflicts of interest.

Name: Bonizella Biagioli, MD.

Contribution: Data analysis, manuscript preparation.

Conflicts of Interest: Bonizella Biagioli reported no conflicts of interest.

Name: Jean-Louis Vincent, MD, PhD.

Contribution: Data analysis, manuscript preparation.

Conflicts of Interest: Jean-Louis Vincent reported no conflicts of interest.

This manuscript was handled by: Dwayne R. Westenskow, PhD.

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ACKNOWLEDGMENTS

We thank Vytech Health (Padua, Italy) for providing us with the MostCare device.

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