The application of positive pressure in a closed breathing system through spontaneous breath cycles is known as CPAP; it increases mean airway pressure and may decrease the work of breathing by moving tidal ventilation to a more compliant part of the volume-pressure curve ( Fig 1). CPAP acts as a pneumatic splint, promoting alveolar re-expansion in areas of atelectasis, and may prevent the need for endotracheal intubation and conventional ventilation in certain patients. The hemodynamic effects of CPAP are dependent upon myocardial filling pressures and underlying cardiac function. Cardiac index improves in patients with left ventricular failure by reducing pre- and afterload, but falls in hypovolemic states. Mild lung injury may respond to CPAP, but tracheal intubation and mechanical ventilation are usually required.
Fig. 1 Pulmonary pressure-volume relationship. Lung compliance (the change in lung volume per unit alteration in pressure) is improved with increasing lung volume. CPAP increases functional residual capacity to the compliant part of the curve, thus reducing the work of breathing.
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