Behaviour and stability of thermodilution signals in a closed extracorporeal circuit: a bench study

Abstract

Thermodilution is the gold standard for cardiac output measurement in critically ill patients. Its application in extracorporeal therapy is limited, as a portion of the thermal indicator is drawn into the extracorporeal circuit. The behaviour of thermodilution signals in extracorporeal circuits is unknown. We investigated thermodilution curves within a closed-circuit and assessed the impact of injection volume, flow and distance on the behaviour of the thermodilution signals and catheter constants. We injected 3, 5, 7 and 10 ml of thermal indicator into a heated closed circuit. Thermistors at distances of 40, 60, 80, and 100 cm from the injection port recorded the thermodilution signals (at flow settings of 0.5, 1, 1.5, and 2 L/min). Area under the curve (AUC), rise time, exponential decay and catheter constants were analysed. Linear mixed-effects models were used to evaluate the impact of circuit flow, distance and injection volume. Catheter positioning did not influence AUC (78 injections). Catheter constants were independent of flow, injection volume or distance to the injection port. The distance to the injection port increased peak temperature and rise time and decreased exponential time constant significantly. The distance to the injection port did not influence catheter constants, but the properties of the thermodilution signal itself. This may influence measurements that depend on the exponential decay of the thermodilution signal such as right ventricular ejection fraction.

Modified Thermodilution for Simultaneous Cardiac Output and Recirculation Assessment in Veno-venous Extracorporeal Membrane Oxygenation: A Prospective Diagnostic Accuracy Study

“…We and other groups have shown that the distance between injection port and the thermistor does not necessarily influence the AUC, but it does influence the downslope of thermal decay. 17,31 This has important implications for future use of adapted thermodilution techniques in ECMO therapy. While the varying decay may alter results for ejection fraction estimates for thermodilution curves, the constant AUC independent of catheter position allows for calibration of catheter constants in the in vivo system.…”

“…While the varying decay may alter results for ejection fraction estimates for thermodilution curves, the constant AUC independent of catheter position allows for calibration of catheter constants in the in vivo system. 10,17 The injection into either the ECMO IN or ECMO OUT with simultaneous measurement of the full thermodilution signal can be used to derive a catheter constant for each individual patient condition. The derived catheter constants are independent of injection volume, as demonstrated in this and previous studies.…”

“…The derived catheter constants are independent of injection volume, as demonstrated in this and previous studies. 17 For this project, we used an averaged catheter constant, but accuracy and precision may improve with more frequent calibration.…”

“…Phase 1 assessed the system’s catheter constant. 10,17 Phase 2 assessed recirculation and simulated native cardiac output using our novel thermodilution technique. Phase 1 consisted of five cold indicator injections (0.9% saline at room temperature, 22°C) at ECMO circuit flows of 1,000, 1,500, 2,000, and 2,500 ml/min and 4 different injection volumes (3, 5, 7, and 10 ml) summing up to 80 individual injections for all combinations of flow and injection volume.…”

“…C T and CC are dimensionless. 10,17,18 Circuit flow (cardiac output) is inversely proportional to the AUC. Although higher flows create a higher peak, the decay is faster, resulting in smaller AUC compared to low flows.…”