A hand-held device to measure oxygen uptake: Performance characteristics, patient selection and the propagation of its measurement error into fick cardiac output determinations

Measured oxygen consumption rates (VO2) are being used less frequently for direct Fick cardiac output determinations during cardiac catheterization because the instruments that measure VO2 are usually cumbersome, expensive and difficult to use. A small, inexpensive VO2 device recently became available. Therefore, the aims of this report are: (1) to delineate the advantages and limitations that would affect its use during cardiac catheterization; (2) to define the patient population in which its use would be safe and effective; and (3) to estimate the accuracy of cardiac output determinations based on its VO2 measurements. The MedGem® is a fist-size device that measures VO2 as a patient breathes through it using either a facemask or mouthpiece. We evaluated it by measuring the external deadspace it imposes on the patient, by measuring the time required for each measurement, by determining its precision from repeated measurements on healthy volunteers and animals, and by developing a statistical model that calculates cardiac output error as a function of the random errors in the measurements of arterial and venous oxygen concentrations and VO2. Our experience indicates the MedGem is easy to operate and can be used for a wide spectrum of patients at rest including children with the ventilatory capacity to compensate for the added deadspace. Cardiac output determinations made with the MedGem's measurements of oxygen uptake are clinically acceptable if the oximeter used to measure oxygen in arterial and mixed venous blood is also sufficiently accurate. Our statistical model illustrates strategies to minimize cardiac output error.