Metabolic measurements in the critically ill.

Measurements of VO2 and VCO2 can be used to calculate REE, which can be used to determine the caloric requirements and metabolic state of critically ill patients. These measurements are made using the gas exchange method--measuring the minute ventilation and the differences between the inspired and expired concentrations of oxygen and carbon dioxide. Mechanical ventilation provides a challenging environment in which to make these measurements because of elevated oxygen concentrations, fluctuating airway pressures, and humidity. Careful attention must be paid to details to ensure accurate measurements under these conditions.