Limitations of the metabolic rate meter for measuring oxygen consumption and cardiac output

Over the past few years, a metabolic rate meter has been introduced for easy measurement of oxygen consumption. However, its accuracy is unproved. In 40 patients (26 men, 14 women, ages 34 to 73 years), cardiac output was measured simultaneously by thermodilution and the Fick method using the metabolic rate meter to quantitate oxygen consumption. In comparison with thermodilution, the results using the Fick method were low (5.26 ± 1.18 vs 4.14 ± 0.99 liters/min, respectively, p <0.01). In 18 patients cardiac output also was measured by the Fick method using a Douglas bag to quantitate oxygen consumption. In these patients, oxygen consumption measured with the metaboic rate meter was lower than that obtained using the Douglas bag (168 ± 25 vs 216 ± 42 ml/ min, respectively, p <0.01). With the Douglas bag, the Fick and thermodilution cardiac output measurements were similar (4.68 ± 1.08 vs 4.87 ± 0.86 liters/min, respectively, difference not significant), and they differed by ≤ 10% in 15 patients. In contrast, with the metabolic rate meter, the results of thermdilution were higher than those with the Fick method (4.84 ± 0.95 vs 3.60 ± 0.71 liters/ min, respectively, p <0.01), and differed by ≤ 10% in only 1 patient (p <0.01). Thus, the values for oxygen consumption and cardiac output obtained with the metabolic rate meter are lower than actual values. This device is less accurate than the Douglas bag.