Oxygen consumption during spontaneous ventilation with continuous positive airway pressure: assessment in normal volunteers and patients with acute respiratory failure.
Continuous positive airway pressures (CPAP) of 0.49 kPa and 0.98 kPa were applied to ten healthy volunteers and nine critically ill patients with acute respiratory failure. A modified Godart-Statham NV 16003 spirometer was used to measure respiratory frequency (f), tidal volume (VT), oxygen consumption (VO2), and changes in functional residual capacity (delta FRC). During CPAP of 0.49 kPa, volunteers had a decrease in f, and increased VT and minute volume (MV). At 0.98 kPa CPAP, f did not change but VT and MV significantly increased. VO2 did not change at either pressure. The volume of delta FRC increased with an increased level of CPAP. The entire volunteer group was comfortable throughout the whole study. When CPAP was applied to acutely ill patients, f decreased. VT and MV increased at both CPAP pressures. delta FRC was similar to the volunteers. VO2 in the patient group rose significantly at 0.49 and 0.98 kPa CPAP. Some of the patients were uncomfortable with 0.49 kPa pressure, while all the patients were distressed at 0.98 kPa CPAP. The effects of increased oxygen consumption and patients discomfort should be considered in critically ill patients receiving CPAP therapy.