Vo2 CaO2 CvO2 x CO

where CaO2 is the arterial oxygen content, CvO2 is the mixed venous oxygen content, and CO is the cardiac output. Because the oxygen content is calculated at STPD, Vo2 is expressed as the STPD volume.

The Fick method does not allow for continuous measurement, and as the different components are measured at different times, it is actually a composite rather than a real-time measurement (Weissman^^^^ Also, the determination depends on the accuracy of thermodilution cardiac output which is usually around ±15

per cent. Under ideal conditions, the values for Vo2 obtained by the Fick method and the gas exchange methods discussed below are similar, but VCO2 and the respiratory quotient (RQ) cannot be accurately determined by the Fick method. When Vo2 measured by gas exchange methods exceeds Vo2 obtained using the Fick method, this may be due to the contribution of the metabolic activity of the lung.

The specific advantages and difficulties of the different types of Vo2 measurement should be used to decide which method is appropriate for particular applications. A

closed-circuit system is best suited for a patient on control or assist-control modes of mechanical ventilation and requiring elevated FiO 2 (above 0.6) (Branson 1990).

Open-circuit methods are suitable for any mechanically ventilated patient with FiO 2 < 0.6 and spontaneously breathing patients since the system can be adapted to interface with the patient through a canopy hood instead of via a mask or direct airway access. The Fick method is reserved for patients with major air leaks.

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