Abstract
Continuous positive airway pressure (CPAP) is known to improve oxygenation and although the exact mechanism is uncertain, the literature suggests that the positive pressure may have a significant role. There may be other mechanisms. In this study, a lung model was used to investigate the mean fractional inspired oxygen at different respiratory rates delivered by CPAP, a venturi mask with 15 L/min flow and a Hudson mask at 6 L/min flow. In each, the fractional inspired oxygen was set at 0.6. An observational study was then undertaken to determine the effect of CPAP at pressures of 0 cm H2O, 5 cm H2O and 10 cm H2O on oxygenation in patients with acute respiratory failure. The model demonstrated that while the CPAP delivered a fractional inspired oxygen of 0.6 at all respiratory rates and at all pressures, the venturi and Hudson masks both become less efficient as the respiratory rate rose. In patients with acute respiratory failure, there is an initial and immediate rise in the partial pressure of oxygen when the patient changes from a venturi mask to a CPAP system at a pressure of 0 cm H2O but there is no further increase in oxygenation at 5 or even 10 cm H2O. The respiratory rate does, however, decrease with time. The initial effects of CPAP on oxygenation are immediate but do not appear to be significantly influenced by pressure, at least within the first two hours.