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

Noninvasive assessment of Cardiac Index using impedance cardiography during liver transplantation surgery: a comparison with pulmonary artery thermodilution

BACKGROUND

Liver transplantation (LT) is a high-risk surgery associated with significant hemodynamic changes requiring advanced hemodynamic monitoring. Pulmonary Artery Catheter (PAC) is still considered as a gold-standard for Cardiac Index (CI) measurement during LT despite association with an increased risk of complications. Noninvasive impedance cardiography (ICG) could be an interesting alternative tool for CI monitoring. The aim of this study was to compare the precision and trending ability of ICG versus PAC methods during LT.

METHODS

Patients undergoing LT were prospectively included. CI was measured with PAC and ICG at 4 time points (T1: before surgical incision, T2: during anhepatic phase, T3: after portal reperfusion, T4: during wound closure). Bias and percentage error (PE) between CI measured with PAC and ICG were analyzed with the Bland-Altman method for repeated measurements. Trending ability was studied with 4-quadrant and polar plots and correlation coefficient.

RESULTS

We included 43 patients with 156 measures. Mean bias was -0.95 L.min-1.m-2, SD±1.07, limits of agreement -3.73 to 1.83 L.min-1.m-2 and PE 58%. There was a significant increase in bias during LT (P<0.001). Assessment of trending ability displayed a concordance rate of 72% on the 4-quadrant plot and a mean angular bias of -8.4° (SD±28°) and radial limits of agreement ±55° on the polar plot.

CONCLUSIONS

CI measurements using ICG exhibited a low precision and a poor trending ability when compared to thermodilution method during LT. Consequently, ICG is not an adequate hemodynamic tool to monitor CI during LT.

Cardiac Output Measurements Based on the Pulse Wave Transit Time and Thoracic Impedance Exhibit Limited Agreement With Thermodilution Method During Orthotopic Liver Transplantation

BACKGROUND

Orthotopic liver transplantation (OLT) is characterized by significant intraoperative hemodynamic variability. Accurate and real-time cardiac output (CO) monitoring aids clinical decision making during OLT. The purpose of this study is to compare accuracy, precision, and trending ability of CO estimation obtained noninvasively using pulse wave transit time (estimated continuous cardiac output [esCCO; Nihon Kohden, Tokyo, Japan]) or thoracic bioimpedance (ICON; Osypka Medical GmbH, Berlin, Germany) to thermodilution cardiac output (TDCO) measured with a pulmonary artery catheter.

METHODS

Nineteen patients undergoing OLT were enrolled. CO measurements were collected with esCCO, ICON, and thermodilution at 5 time points: (T1) pulmonary artery catheter insertion; (T2) surgical incision; (T3) portal reperfusion; (T4) hepatic arterial reperfusion; and (T5) abdominal closure. The results were analyzed with Bland-Altman plot, percentage error (the percentage of the difference between the CO estimated with the noninvasive monitoring device and CO measured with the thermodilution technique), 4-quadrant plot with concordance rate (the percentage of the total number of points in the I and III quadrant of the 4-quadrant plot), and concordance correlation coefficient (a measure of how well the pairs of observations deviate from the 45-degree line of perfect agreement).

RESULTS

Although TDCO increased at T3-T5, both esCCO and ICON failed to track the changes of CO with sufficient accuracy and precision. The mean bias of esCCO and ICON compared to TDCO were -2.0 L/min (SD, ±2.7 L/min) and -3.3 L/min (SD, ±2.8 L/min), respectively. The percentage error was 69% for esCCO and 77% for ICON. The concordance correlation coefficient was 0.653 (95% confidence interval [CI], 0.283-0.853) for esCCO and 0.310 (95% CI, -0.167 to 0.669) for ICON. Nonetheless, esCCO and ICON exhibited reasonable trending ability of TDCO (concordance rate: 95% [95% CI, 88-100] and 100% [95% CI, 93-100]), respectively. The mean bias was correlated with systemic vascular resistance (SVR) and arterial elastance (Ea) for esCCO (SVR, r = 0.610, 95% CI, 0.216-0.833, P < .0001; Ea, r = 0.692, 95% CI, 0.347-0.872; P < .0001) and ICON (SVR, r = 0.573, 95% CI, 0.161-0.815, P < .0001; Ea, r = 0.612, 95% CI, 0.219-0.834, P < .0001).

CONCLUSIONS

The noninvasive CO estimation with esCCO and ICON exhibited limited accuracy and precision, despite with reasonable trending ability, when compared to TDCO, during OLT. The inaccuracy of esCCO and ICON is especially large when SVR and Ea were decreased during the neohepatic phase. Further refinement of the technology is desirable before noninvasive techniques can replace TDCO during OLT.

Bioreactance Is Not Interchangeable with Thermodilution for Measuring Cardiac Output during Adult Liver Transplantation

Background

Thermodilution technique using a pulmonary artery catheter is widely used for the assessment of cardiac output (CO) in patients undergoing liver transplantation. However, the unclearness of the risk-benefit ratio of this method has led to an interest in less invasive modalities. Thus, we evaluated whether noninvasive bioreactance CO monitoring is interchangeable with thermodilution technique.

Methods

Nineteen recipients undergoing adult-to-adult living donor liver transplantation were enrolled in this prospective observational study. COs were recorded automatically by the two devices and compared simultaneously at 3-minute intervals. The Bland–Altman plot was used to evaluate the agreement between bioreactance and thermodilution. Clinically acceptable agreement was defined as a percentage error of limits of agreement <30%. The four quadrant plot was used to evaluate concordance between bioreactance and thermodilution. Clinically acceptable concordance was defined as a concordance rate >92%.

Results

A total of 2640 datasets were collected. The mean CO difference between the two techniques was 0.9 l/min, and the 95% limits of agreement were -3.5 l/min and 5.4 l/min with a percentage error of 53.9%. The percentage errors in the dissection, anhepatic, and reperfusion phase were 50.6%, 56.1%, and 53.5%, respectively. The concordance rate between the two techniques was 54.8%.

Conclusion

Bioreactance and thermodilution failed to show acceptable interchangeability in terms of both estimating CO and tracking CO changes in patients undergoing liver transplantation. Thus, the use of bioreactance as an alternative CO monitoring to thermodilution, in spite of its noninvasiveness, would be hard to recommend in these surgical patients.

Accuracy of impedance cardiography for evaluating trends in cardiac output: a comparison with oesophageal Doppler

BACKGROUND

Impedance cardiography (ICG) enables continuous, beat-by-beat, non-invasive, operator-independent, and inexpensive cardiac output (CO) monitoring. We compared CO values and variations obtained by ICG (Niccomo™, Medis) and oesophageal Doppler monitoring (ODM) (CardioQ™, Deltex Medical) in surgical patients.

METHODS

This prospective, observational, single-centre study included 32 subjects undergoing surgery with general anaesthesia. CO was measured simultaneously with ICG and ODM before and after events likely to modify CO (vasopressor administration and volume expansion). One hundred and twenty pairs of CO measurements and 94 pairs of CO variation measurements were recorded.

RESULTS

The CO variations measured by ICG correlated with those measured by ODM [r=0.88 (0.82-0.94), P<0.001]. Trending ability was good for a four-quadrant plot analysis with exclusion of the central zone (<10%) [95% confidence interval (CI) for concordance (0.86; 1.00)]. Moderate to good trending ability was observed with a polar plot analysis (angular bias: -7.2°; 95% CI -12.3°; -2.5°; with radial limits of agreement -38°; 24°). After excluding subjects with chronic obstructive pulmonary disease, a Bland-Altman plot showed a mean bias of 0.47 litre min(-1), limits of agreements between -1.24 and 2.11 litre min(-1), and a percentage error of 35%.

CONCLUSION

ICG appears to be a reliable method for the non-invasive monitoring of CO in patients undergoing general surgery.

Bioreactance is not reliable for estimating cardiac output and the effects of passive leg raising in critically ill patients

BACKGROUND

Bioreactance estimates cardiac output in a non-invasive way. We evaluated the ability of a bioreactance device (NICOM®) to estimate cardiac index (CI) and to track relative changes induced by volume expansion.

METHODS

In 48 critically ill patients, we measured CI estimated by the NICOM® device (CINicom) and by transpulmonary thermodilution (CItd, PiCCO2™ device) before and after a 500 ml saline infusion. Before volume expansion, we performed a passive leg raising (PLR) test and measured the changes it induced in CINicom and in pulse contour analysis-derived CI.

RESULTS

Considering the values recorded before PLR and before and after volume expansion (n=144), the bias (lower and upper limits of agreement) between CItd and CINicom was 0.9 (-2.2 to 4.1) litre min(-1) m(-2). The percentage error was 82%. There was no significant correlation between the changes in CItd and CINicom induced by volume expansion (P=0.24). An increase in CI estimated by pulse contour analysis >9% during the PLR test predicted fluid responsiveness with a sensitivity of 84% (95% confidence interval 60-97%) and a specificity of 97% (95% confidence interval 82-100%). The area under the receiver operating characteristic curve constructed to test the ability of the PLR-induced changes in CINicom in predicting fluid responsiveness did not differ significantly from 0.5 (P=0.77).

CONCLUSIONS

The NICOM® device cannot accurately estimate the cardiac output in critically ill patients. Moreover, it could not predict fluid responsiveness through the PLR test.

Anterior-posterior impedance cardiography: a new approach to accurate, non-invasive monitoring of cardiac function

The conventional impedance cardiogram is a record of pulsatile changes in the electrical impedance of the chest with each heartbeat. The signal seems intuitively related to cardiac stroke volume. However doubts persist about the validity of stroke volume measurements based on electrical impedance. This paper presents a new electrical axis for impedance cardiography that is perpendicular to the conventional head-to-foot axis in an anterior-posterior direction. Dual chest and back electrodes are concentric, permitting tetrapolar technique. A relatively simple analytical model is developed, and this model is validated in a three-dimensional finite element model of current flow through the human chest. Three-dimensional simulations show predictable relationships between the fractional increase in anterior-posterior chest impedance and the ventricular ejection fraction (cardiac stroke volume/ventricular end-diastolic volume). Ejection fraction can be computed accurately with a roughly 30-fold increase in signal level compared to the conventional impedance cardiogram. Breathing causes only modest changes in the signal. When the axis of current flow is optimized, one can interpret the impedance changes during the cardiac cycle with greater confidence as noninvasive, beat-by-beat indicators of ventricular ejection fraction in a wide variety of clinical settings.

Evaluation of changes in cardiac output from the electrical impedance waveform in the forearm

We tested the validity of regional impedance cardiography (RIC) for measuring changes in both cardiac output and stroke volume by comparing the values with a 2D ultrasound technique in response to the breath-hold manipulation. Among 13 subjects, changes in the maximum amplitude of the regional impedance waveform from the forearm conformed to those in stroke volume (r = 0.86, p < 0.001) and cardiac output (r = 0.76, p < 0.003) measured with the ultrasound technique in baseline and immediately after a 30 s breath-hold maneuver. We also found that the per cent change in cardiac output (r = 0.73, p < 0.005) and the per cent change in stroke volume (r = 0.84, p < 0.0003) by the echocardiography were both positively correlated with the per cent change in the peak impedance amplitude. In addition, both the change and the per cent change in the mean area under the impedance curve were consistent with those in the stroke volume, respectively. Accordingly, the regional electrical impedance waveform from lower limbs may be helpful in providing a non-invasive and continuous assessment of left ventricular output, especially during cardiac procedures.

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).