Comparison of iced and room temperature injectate for thermodilution cardiac output

Cardiac output estimation by thermodilution is carried out using room temperature or iced injectate, but the accuracy and variability of the two methods is not well documented. Room temperature and iced injectate were compared in 21 patients undergoing diagnostic cardiac catheterization. Dextrose injectate (10 ml) was administered in prefilled syringes left to stand either in iced water or in room air. Four injections were made sequentially with room temperature and iced injectate. Cardiac output by room temperature and iced injectate were not significantly different (4.70 +/- 1.22 for room temperature and 4.90 +/- 1.37 for iced injectate, n = 21, P = 0.155). There was a significant difference in the variance of the estimations by the two methods (room temperature = 0.296, iced = 0.120, P less than 0.005). From this variance the calculated number of measurements needed to estimate cardiac output to +/- 0.5 L/min with 95% confidence is seven for room temperature and four for iced injectate. For five patients with cardiac output less than 4.00 L/min with room temperature injectate, cardiac output with iced injectate was significantly higher (3.33 +/- 0.34 for room temperature vs. 3.69 +/- 0.49 for iced injectate, P = 0.05). Thus room temperature injectate generally gives a satisfactory cardiac output estimation but with significantly greater variability than iced injectate. Sample size for accurate cardiac output estimation must be greater with room temperature injectate. Iced injectate may over-estimate output when cardiac output is low.