Comparison of Fick and thermodilution cardiac output determinations in standing horses

The Fick and thermodilution (TD) methods are two currently popular techniques for determination of cardiac output (CO) in adult horses. To our knowledge, a comparison of these two techniques has not been reported. Six healthy, resting, fit, adult horses of either sex and weighing 516.5 ± 33.2 kg (mean ± SD) were instrumented to enable measurement of cardiac output. Resting CO was determined by the Fick method and by thermodilution while the horses stood quietly in the stocks. Fick and thermodilution CO measurements were repeated under conditions of increased cardiac output achieved with the use of a dobutamine infusion (5 μg kg^-1 min^-1, IV), and again under conditions of decreased CO induced by administration of xylazine (0.5 mg/kg, IV). Fick and thermodilution cardiac outputs were compared using Bland-Altman analysis for repeated measures. The mean of the differences ± 1.96SD (bias and precision) between the two techniques was 1.88 ± 24.17 L/min. Variability between measurements with the two techniques was decreased to 3.41 ± 46.78 mL kg^-1 min^-1 when CO was normalized for body size by calculation of cardiac index.