Arguments for the use of thermal indicators to measure whole body blood flow.

Measurement of cardiac output by thermodilution is the established standard method for the evaluation of whole body blood flow in critically ill patients. Its main advantages are practicality, feasibility for routine clinical use, ease of user training, and wellestablished sources of variability and error. Comparisons against the golden standard, the Fick method, indicates acceptable bias and precision and good correlation in detecting changes in trends (1). The negative bias (smaller values with the thermodilution) observed in several recent studies in the critically ill patients is at least in part due to the increased oxygen consumption of the lung in acute inflammation (2). The recently introduced continual thermodilution methods offer the additional advantage of automated, continual trending of cardiac output (3). Since cardiac output is a variable with both rapid and slower dynamic variability, it is conceivable that the measurement of thermodilution cardiac output is bound to have variability, which is a function of the dynamic variability of actual cardiac output and the error of he method (4). Large fluctuations in airway temperature in patients with acute respiratory distress may accentuate the variability due to unstable pulmonary artery temperature. These sources of variability can be satisfactorily controlled by performing the injections randomly over the respiratory cycle and accepting only those measurements with an appropriately shaped thermal dilution curve, and increasing the number of measurements, when the variability is high.