Continuous Positive-Pressure Ventilation: Effects on Systemic Oxygen Transport and Tissue Oxygenation

In this study we examined the effects of continuous positive-pressure (CPP) ventilation at 0, 5, and 10 cm H2O end-expiratory pressure on systemic oxygen transport (cardiac index times arterial oxygen content) and peripheral tissue oxygenation. We studied 19 patients, divided into 3 groups, who required mechanically assisted ventilation: 5 had normal lungs; 10 had decreased lung or chest wall compliance, or both; and 4 had chronic obstructive pulmonary disease. The alveolar-arterial oxygen tension difference narrowed, and hence arterial oxygen content tended to improve in most patients as expiratory pressure increased. Systemic oxygen transport fell in all groups, however, owing to a significant reduction in cardiac index. Despite the fall in oxygen transport, no significant changes occurred in oxygen consumption, respiratory quotient, pH, or mixed venous oxygen tension. We concluded that CPP ventilation significantly decreases cardiac output and oxygen delivery but that total body tissue oxygenation was probably not impaired.