Influence of continuous renal replacement therapy on cardiac output measurement using thermodilution techniques

Acute renal failure (ARF) ranks among the most frequent complications in critically ill patients and continuous renal replacement therapy (CRRT) is a typical treatment regimen in intensive care patients. Contributing factors to ARF, such as septic shock and hemodynamic instability require extended hemodynamic monitoring, and the simultaneous use of CRRT and cardiac output measurement is common. In view of this, a systematic analysis of the interaction between CRRT and cardiac output measurements by thermodilution is warranted. Cardiac output (CO) is commonly measured with thermodilution-based methods in critically ill patients. The methods are accurate but the measurements are affected by inconstant indicator volumes or changes in blood temperature. Because continuous renal replacement therapy (CRRT) may alter blood volume and temperature, we investigated its effect on thermodilution-based CO measurement.

Thirty-two intensive care patients with both CRRT and CO monitoring were studied. Hemodynamic parameters were first measured in quintuple with bolus injections of cold saline during CRRT. Further five measurements were performed after CRRT had been shut off, and a final five measurements were performed after it had been restarted. Fifty measurement series were performed in patients with a pulmonary artery catheter and 25 in patients using a transpulmonary thermodilution method (PiCCO®).

The first measurements in each series after switching CRRT off or on deviated most markedly from the average. When these measurements were excluded, the averaged CO values with and without CRRT differed significantly but by <7% (P<0.05).

Substantial measurement error was only observed immediately after CRRT was switched off or on. Subsequent CO measurements did not depend on the CRRT status. Interrupting CRRT before measuring CO is not generally recommended, however, if interrupted, it is crucial to wait for blood temperature to reach a steady state before initiating the first measurements.