Reliability of continuous cardiac output measurement during intra-abdominal hypertension relies on repeated calibrations: an experimental animal study

Abstract

Introduction: Monitoring cardiac output (CO) may allow early detection of haemodynamic instability, aiming to reduce morbidity and mortality in critically ill patients. Continuous cardiac output (CCO) monitoring is recommended in septic or postoperative patients with high incidences of intra-abdominal hypertension (IAH). The aim of the present study was to compare the agreement between three CCO methods and a bolus thermodilution CO technique during acute IAH and volume loading.

Methods: Ten pigs were anaesthetised and instrumented for haemodynamic measurements. Cardiac output was obtained using CCO by pulse power analysis (PulseCO; LiDCO monitor), using CCO by pulse contour analysis (PCCO; PiCCO monitor) and using CCO by pulmonary artery catheter thermodilution (CCOPAC), and was compared with bolus transcardiopulmonary thermodilution CO (COTCP) at baseline, after fluid loading, at IAH and after an additional fluid loading at IAH. Whereas PulseCO was only calibrated at baseline, PCCO was calibrated at each experimental step.

Results: PulseCO and PCCO underestimated CO, as the overall bias +/- standard deviation was 1.0 +/- 1.5 l/min and 1.0 +/- 1.1 l/min compared with COTCP. A clinically accepted agreement between all of the CCO methods and COTCP was observed only at baseline. Whereas IAH did not influence the CO, increased CO following fluid loading at IAH was only reflected by CCOPAC and COTCP, not by uncalibrated PulseCO and PCCO. After recalibration, PCCO was comparable with COTCP.

Conclusions: The CO obtained by uncalibrated PulseCO and PCCO failed to agree with COTCP during IAH and fluid loading. In the critically ill patient, recalibration of continuous arterial waveform CO methods should be performed after fluid loading or before a major change in therapy is initiated.

Figure 1

Experimental protocol. The methods used were continuous cardiac output by pulse contour analysis (PCCO; PiCCO system), continuous cardiac output by pulse power analysis (PulseCO; LiDCO system), continuous cardiac output by pulmonary artery catheter thermodilution (CCO_PAC), and bolus transcardiopulmonary thermodilution cardiac output (CO_TCP). PCCO was measured before recalibration (PCCO_pre) and after recalibration (PCCO_recal) by CO_TCP. Experimental steps: BL, baseline; + Fluid, fluid loading; IAH, intra-abdominal hypertension; IAH + Fluid, second fluid load at IAH. HES, hydroxyl-ethyl starch 6%; IAP, intra-abdominal pressure; n.a., not applicable.

Figure 2

Distribution and bias of cardiac output methods. (a) Cardiac output (CO) measured by the different CO methods at each experimental step. (b) Bias and precision (standard deviation (SD)) between bolus transcardiopulmonary thermodilution cardiac output (CO_TCP) and the different CO methods at each experimental step. PulseCO, continuous cardiac output by pulse power analysis (LiDCO system); PCCO, continuous cardiac output by pulse contour analysis (PiCCO system); CCO_PAC, continuous cardiac output by pulmonary artery catheter thermodilution. PCCO was measured before recalibration (PCCO_pre) and after recalibration (PCCO_recal) by CO_TCP. *P < 0.05 versus the previous experimental stage (PCCO_pre versus previous PCCO_recal). ^#Methods not interchangeable according to Critchley and Critchley [21]. Filled symbols, calibrated measures. Experimental steps: BL, baseline; + Fluid, fluid loading; IAH, intra-abdominal hypertension; IAH + Fluid, second fluid load at IAH. IAP, intra-abdominal pressure; na, not applicable; SEM, standard error of the mean.

Figure 3

Scatter plots and Bland–Altman plots of pooled data pairs. Scatter plots (left-hand side) and Bland–Altman plots (right-hand side) of pooled data pairs between (a) and (b) bolus transcardiopulmonary thermodilution cardiac output (CO_TCP) and continuous cardiac output by pulse power analysis (PulseCO; LiDCO system), (c) and (d) between CO_TCP and continuous cardiac output by pulse contour analysis before recalibration (PCCO_pre; PiCCO system), and (e) and (f) between CO_TCP and continuous cardiac output by pulmonary artery catheter thermodilution (CCO_PAC). (a), (c), (e) Scatter plots include line of identity (dotted line). (b), (d), (f) Bland–Altman plots include bias (solid lines) and limits of agreement (dotted lines).