ERRORS IN THERMODILUTION CARDIAC-OUTPUT MEASUREMENTS CAUSED BY RAPID PULMONARY-ARTERY TEMPERATURE DECREASES AFTER CARDIOPULMONARY BYPASS

When systemic cooling and rewarming are performed during cardiopulmonary bypass (CPB), the pulmonary artery temperature typically decreases after CPB. This decrease may be rapid enough to cause substantial underestimation of cardiac output (CO) measured by thermodilution, due to changing baseline temperature during the thermodilution measurement. In 16 patients undergoing CPB for coronary artery grafts, digital recording of pulmonary artery temperature was done during room-temperature thermodilution CO (TDCO) injections. TDCO were computed with and without correction for baseline temperature decrease. Prior to CPB, the temperature change was 0.013-degrees-C/min, producing no significant effect on CO measurements; the coefficient of variation of CO measurements was 5.1%. One minute after CPB the temperature change was -0.144-degrees C/min, producing a CO measurement error of -0.57 +/- 0.52 l/min (SD), or about 11% of the average CO; the range of the error was 0.05 to -2.0 l/min. Ten minutes after CPB the temperature change was -0.063-degrees-C/min, and CO error was 0.31 +/- 0.36 (0.15 to -1.20) l/min. At 30 min the temperature change was 0.012-degrees-C/min (not significant), and CO error was 0.13 +/- 0.141/min. Duration of CPB was 104 +/- 30 min, with rewarming for 44 +/- 13 min; the average minimum bladder temperature was 25.1 +/- 2.3-degrees-C during cooling and 36.7 +/- 0.7-degrees-C at the end of CPB. Under these conditions TDCO measurements within the first 10 min after CPB often underestimate the true CO.