OBJECTIVE: To determine the effect of 5-mL injectate on cardiac output measurements in critically ill patients with low ventricular ejection fraction (< 35%). METHODS: Thermodilution cardiac output measurements obtained with three 5-mL and three 10-mL (randomly ordered) iced injectates in 50 patients with low ejection fraction were averaged if the measurements were within 10% of the median. If the 3 measurements were not within those limits, additional measurements were obtained. RESULTS: Cardiac output measured with the 5-mL injectate (mean, 4.63 L/min) and cardiac output measured with the 10-mL injectate (mean, 4.52 L/min) were not significantly different (P = .64). Lower and upper limits of agreement were -1.7 L/min to +1.6 L/min. The bias (mean difference between 10- and 5-mL measurements) of all measurements was -0.09, and the precision was 1.43 L/min, with a 95% confidence limit (mean difference +/- 2 SD) of -1.7 to +1.6 L/min. An additional measurement was necessary in 77% of patients in the 5-mL group but in only 48% of the 10-mL group (P = .006). CONCLUSIONS: Cardiac outputs measured with 5- and 10-mL injectates do not differ significantly. The greater variability of measurements obtained with a 5-mL injectate suggests that more measurements, and thus more time, are needed to measure cardiac output accurately. Clinicians must weigh the benefit of minimizing fluid volume used against the potential decreased reliability of cardiac output measurements.
BACKGROUND: Nursing care of patients with advanced heart failure with low ejection fraction requires strict management of IV fluids. Measurement of mixed venous oxygen saturation offers advantages over measurement of cardiac output because no administration of fluid is required and data are obtained continuously. OBJECTIVES: To determine the relationship between mixed venous oxygen saturation and cardiac output in patients with advanced heart failure who have low ejection fraction and to determine if use of vasoactive medications alters the relationship between mixed venous oxygen saturation and cardiac output. METHODS: Simultaneously obtained measurements of mixed venous oxygen saturation and cardiac output were compared in 42 patients with advanced heart failure with ejection fractions of 30% or less (mean, 19.5%). RESULTS: Correlation between mixed venous oxygen saturation and cardiac output was r = 0.54 (P < .001). For subjects not receiving vasoactive medications (n = 28), r = 0.52 (P = .004); for those receiving vasoactive medications (n = 14), r = 0.57 (P = .03). CONCLUSIONS: Similar correlations in the groups receiving and not receiving vasoactive medications suggest that even with pharmacological support, changes in mixed venous oxygen saturation may not be reflected by concomitant changes in cardiac output. Measurement of mixed venous oxygen saturation should not replace measurement of cardiac output for clinical decision making in patients with advanced heart failure with low ejection fraction.