Evaluation of a new advanced thoracic bioimpedance device for estimation of cardiac output

Techniques of cardiac output measurement during liver transplantation: arterial pulse wave versus thermodilution

In this study, we compared continuous cardiac output (CO) obtained from the femoral arterial pressure by simulation of an aortic input impedance model [model-simulated cardiac output (MCO)] to thermodilution cardiac output (TDCO) determined by bolus injection during liver transplantation. Both variables were measured in 39 adult patients (13 females) every 10th minute during liver transplant surgery. Paired measurements were compared during the 4 phases of surgery-dissection, anhepatic phase, early reperfusion (the first 15 minutes after reperfusion), and late reperfusion (15-60 minutes after reperfusion)-without the detection of any significant difference between the 2 estimates of CO. TDCO ranged from 2.3 to 17.2 L/minute, and the bias (the mean difference between MCO and TDCO) prior to calibration was -0.4 +/- 1.6 L/minute (mean +/- standard deviation; 1309 paired measurements; 95% limits of agreement: -3.4 to 2.6 L/minute). After calibration of the first determined MCO by the simultaneously determined TDCO, the bias was 0.1 +/- 1.5 L/minute, with 57% (n = 744) of the comparisons being less than 1 L/minute and 35% (n = 453) being less than 0.5 L/minute; this was independent of the level of CO, and the mutual correlation coefficient was 0.812 (P < 0.001). This study indicates that during liver transplantation surgery, MCO reflects TDCO throughout the operation. Thus, for CO, this less invasive method appears to provide a reliable uninterrupted measurement during orthotopic liver transplantation.

Comparison of electrical velocimetry and transpulmonary thermodilution for measuring cardiac output in piglets

BACKGROUND

Monitoring of cardiovascular function is essential during major pediatric and pediatric cardiac surgery. Invasive monitoring of cardiac output (CO) and oxygen delivery is expensive and sometimes associated with adverse events. Therefore, we investigated the accuracy of a new noninvasive CO monitoring device using electrical velocimetry (EV) in comparison with the more invasive transpulmonary thermodilution (TPTD) method.

METHODS

In five fasted, anesthetized and mechanically ventilated piglets, CO was measured simultaneously using EV and TPTD under normal conditions, volume loading, inotropic support and exsanguination.

RESULTS

In five piglets, 169 measurements could be performed. The correlations between EV-CO and TPTD-CO were significant for absolute values (P < 0.0001, r = 0.82) and relative changes from baseline (P < 0.0001, r = 0.93). The receiver operating characteristic (ROC) curve analysis of the relative changes of the EV-CO values in relation to the first EV-CO measurement showed a sensitivity of 91% and specificity of 94% (AUC 0.974, 95% CI 0.96-0.99). Changes in TPTD-CO greater than 15% lead to a change of EV-CO in the same direction in 93%. Bland-Altman analysis showed a mean difference between the two methods of -0.63 l x min(-1) with an sd of 0.64 l x min(-1). The lower and upper limits of agreement were -1.88 and 0.62 l x min(-1), percentage limit of agreement was +/-82.8%.

CONCLUSIONS

The results show that EV is a safe, simple, noninvasive and cost-effective method for continuous trend monitoring of CO in piglets. The agreement of the EV-CO with TPTD-CO is not good enough to replace the standard method in our animal model. A correction factor for body habitus in piglets may be beneficial.

An investigation to show the effect of lung fluid on impedance cardiac output in the anaesthetized dog

BACKGROUND

Accumulation of lung fluid in the critically ill patient is believed to attenuate impedance cardiac output (CO(IC)) measurements. However, this phenomenon has never been shown experimentally.

METHODS

In eight anaesthetized and ventilated dogs (weight 15-22 kg) a high-precision flow probe was placed on the ascending aorta via a left thoracotomy incision and the direct cardiac output (CO(FP)) was measured. Simultaneous CO(IC) measurements were made using a RheoCardioMonitor (ACMA, Singapore). Lung oedema was induced by intravenous oleic acid 0.1 mg kg(-1). Lung fluid was assessed by the decrease in basal thoracic impedance (Z(b)). Percentage errors between the two methods (CO(IC)-CO(FP)) were calculated and compared as Z(b) decreased at 1 Omega intervals.

RESULTS

During the experiment mean Z(b) decreased from 35.9 (sd 5.2) to 27.8 (6.5) Omega (P=0.0037). This occurred over a period of 225 (range 112-338) min and Z(b) decreased by 1 Omega every 51 (22-68) min. The presence of excessive lung fluid was confirmed at post-mortem. Before lung oedema was induced, CO(IC) was 1.5 (0.6) litre min(-1) and the corresponding value of CO(FP) was 1.5 (0.7) litre min(-1) (data from eight dogs). As Z(b) decreased, and lung fluid accumulated, the error between CO(IC) and CO(FP) widened (P<0.0001, anova for repeated measures). Eventually, CO(IC) decreased to 0.7 (0.3) litre min(-1) and the corresponding value of CO(FP) was 1.2 (0.3) litre min(-1) (DeltaZ(b)=5 Omega, data from six dogs). Mean arterial pressure, central venous pressure and systemic vascular resistance were kept constant.

CONCLUSION

The presence of lung fluid attenuates CO(IC) measurements with respect to CO(FP).

The Effect of Peripheral Resistance on Impedance Cardiography Measurements in the Anesthetized Dog

In the vasodilated and septic patient, the impedance method of measuring cardiac output (CO) may underestimate the true value. In this study, we sought to determine whether impedance CO (COIC) measurements are influenced by total peripheral resistance (TPR). In eight anesthetized and ventilated dogs, a high-precision flowprobe was placed on the ascending aorta, and direct CO was measured (CO flowprobe (COFP)). Mean arterial blood pressure was measured from the femoral artery. Simultaneous COIC measurements were made. TPR (mean arterial blood pressure x 80/COFP) was varied over 1-2 h by using infusions of phenylephrine and adrenaline and inhaled halothane. The bias between methods of CO measurement (COIC-COFP) was calculated and compared with TPR by using correlation and regression analysis. A total of 547 pairs of CO measurements were collected from the 8 dogs as TPR was varied. COFP changed by a mean of 190% (range, 89%-425%), and TPR changed by a mean of 266% (range, 94%-580%) during the experiment. The impedance method underestimated CO when TPR was low and overestimated CO when TPR was high. There was a logarithmic relationship between the CO bias and TPR. Correlation coefficients (r) between the CO bias and TPR ranged from 0.46 to 0.89 (P < 0.0001). The bias changed by 0.62 +/- 1.8 L/min, or by 34%, every time TPR halved or doubled. This finding explains the poor agreement between COIC and other methods of CO measurement found in validation studies involving critically ill patients.

Measurement of cardiac output before and after cardiopulmonary bypass: Comparison among aortic transit-time ultrasound, thermodilution, and noninvasive partial CO2 rebreathing

OBJECTIVES

A noninvasive continuous cardiac output system (NICO) has been developed recently. NICO uses a ratio of the change in the end-tidal carbon dioxide partial pressure and carbon dioxide elimination in response to a brief period of partial rebreathing to measure CO. The aim of this study was to compare the agreement among NICO, bolus (TDCO), and continuous thermodilution (CCO), with transit-time flowmetry of the ascending aorta using an ultrasonic flow probe (UFP) before and after cardiopulmonary bypass (CPB).

DESIGN

Prospective, observational human study.

SETTING

Veterans Affairs Medical Center Hospital.

PARTICIPANTS

Sixty-eight patients.

METHODS

Matched sets of CO measurements between NICO, TDCO, CCO, and UFP were collected in 68 patients undergoing elective CABG at specific time periods before and after separation from CPB. After anesthetic induction, all patients had an NICO sensor attached between the endotracheal tube and the breathing circuit, a PAC floated into the pulmonary artery for TDCO and CCO monitoring, and a UFP positioned on the ascending aorta and used for the reference CO. Bland-Altman analysis was used to compare the agreement among the different methods.

MEASUREMENTS AND MAIN RESULTS

Bland-Altman analysis of CO measurements before CPB yielded a bias, precision, and percent error of 0.04 L/min +/- 1.07 L/min (44.8%) for NICO, 0.18 L/min +/- 1.01 L/min (41.7%) for TDCO, and 0.29 L/min +/- 1.40 L/min (57.5%) for CCO compared with simultaneous UFP CO measurements, respectively. After separation from CPB (average 29 mins), bias, precision, and percent error were -0.46 L/min +/- 1.06 L/min (37.3%) for NICO, 0.35 L/min +/- 1.39 L/min (46.1%) for TDCO, and 0.36 L/min +/- 1.96 L/min (64.7%) for CCO compared with UFP CO measurements, respectively.

CONCLUSIONS

Before initiation of CPB, the accuracy for all 3 techniques was similar. After separation from CPB, the tendency was for NICO to underestimate CO and for TDCO and CCO to overestimate it. NICO offers an alternative to invasive CO measurement.

Averaging improves the quality of impedance stroke volume measurements during the head up tilt test

OBJECTIVES

To assess the improvement in quality following averaging data from two or more tilts in the stroke volume (SV) response curve during a head-up tilt test.

METHODS

Ten adult male subjects were tilted head-up to 55 degrees four times. They were kept head-up for 3-minutes with intervening rests of 3-minutes. Impedance measurements were recorded by the RheoCardioMonitor every 10-seconds. The percentage changes in impedance variables with tilting and coefficients of variability (CV) were calculated. The SV data were divided into four single tilt response curves, which were used to assess the variability of the data when these tilts were analyzed separately, in pairs or in triplets. Confidence intervals reflecting this variability were estimated.

RESULTS

Head-up tilting resulted in a mean (SD) decrease in SV of 31 (11)%. Significant variability existed between SV readings (CV = 8 (2)%). Averaging improved the resolution of the wavelets and in most subjects (7 out of 10) allowed dynamic and static phases of the response to be identified. Upper confidence intervals (mean (range)) were reduced from 15 (10-23)% for single wavelets to 8 (3-10)% for pairs to 5 (3-8)% for triplets.

CONCLUSIONS

Impedance measurements can be very variable, making the assessment of SV changes during a head-up tilt test difficult. By averaging the data from several tilts one can improved the quality of the SV wavelet sufficiently to identify important postural changes.

A comparison of cardiac output derived from the arterial pressure wave against thermodilution in cardiac surgery patients

In three clinical centres, we compared a new method for measuring cardiac output with conventional thermodilution. The new method computes beat-to-beat cardiac output from radial artery pressure by simulating a three-element model of aortic input impedance, and includes non-linear aortic mechanical properties and a self-adapting systemic vascular resistance. We compared cardiac output by continuous model simulation (MF) with thermodilution cardiac output (TD) in 54 patients (18 female, 36 male) undergoing coronary artery bypass surgery. We made three or four conventional thermodilution estimates spread equally over the ventilatory cycle. In 490 series of measurements, thermodilution cardiac output ranged from 2.1 to 9.3, mean 5.0 litre min(-1). MF differed +0.32 (1.0) litre min(-1) on average with limits of agreement of -1.68 and +2.32 litre min(-1). Differences decreased when the first series of measurements in a patient was used to calibrate the model. In 436 remaining series, the mean difference became -0.13 (0.47) litre min(-1) with limits of agreement of -1.05 and +0.79 litre min(-1). When consecutive measurements were made, the change was greater than 0.5 litre min(-1), on 204 occasions. The direction of change was the same with both methods in 199. The difference between the methods remained near zero during surgery suggesting that a single calibration per patient was adequate. Aortic model simulation with radial artery pressure as input reliably monitors changes in cardiac output in cardiac surgery patients. Before calibration, the model cannot replace thermodilution, but after calibration the model method can quantitatively replace further thermodilution estimates.

Evaluation of a thoracic bioimpedance cardiac output monitor during cardiac catheterization

OBJECTIVE

To evaluate the accuracy and precision of an advanced thoracic bioimpedance cardiac output monitor by comparing it with conventional thermodilution.

DESIGN

Prospective data collected from 47 patients undergoing routine cardiac catheterization. The new bioimpedance system differs from its predecessors in electrode system configuration, advanced signal processing, use of a modified Kubicek equation, and a reliable estimate of left ventricular ejection time from the time derivative bioimpedance signals.

SETTING

A cardiac catheterization laboratory in a university affiliated teaching hospital.

PATIENTS

A series of 47 relatively homogenous patients undergoing routine cardiac catheterization for suspected cardiac disease.

MEASUREMENTS AND MAIN RESULTS

The data from the first 20 patients was used to determine optimal values for coefficients in the bioimpedance cardiac output equations. The coefficients found were used when the system was tested in the subsequent 27 patients. For the last 27 patients, a total of 80 simultaneous pairs of cardiac output measurements were made by conventional thermodilution and by thoracic bioimpedance. The mean difference between the two methods was -0.31 L/min and the standard deviation of the differences was (0.76 L/min). The correlation coefficient was r2 = .72 (p < .001).

CONCLUSIONS

The correlation between conventional thermodilution and thoracic bioimpedance cardiac output estimates was good and the standard deviation of the differences was lower than that reported for commercially available devices. The system can be used in the cardiac catheterization lab for reliable and continuous noninvasive measurement of cardiac output.