THE EFFECTS OF HEMORRHAGE ON PULMONARY CIRCULATION AND RESPIRATORY GAS EXCHANGE*†

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Abstract
In normal lung function there exists an intimate relationship between ventilation and perfusion so that primary alterations in one process are often difficult to distinguish from secondary changes occurring in the other. This study reports the effects in dogs of changes in perfusion induced by hemorrhage and blood replacement while maintaining ventilation constant in rate and tidal volume. Following hemorrhage there was a marked increase in the physiological dead space accompanied by a significant CO2 tension difference between arterial blood and end-tidal gas. At the same time there was only a moderate decrease in the pulmonary arterial blood pressure despite marked reduction in cardiac output systemic blood pressure. The total lung-thorax compliance also was diminished. Microscopic examination of the lungs revealed avascular aerated regions at the periphery and areas of atelectasis near the hilum. These studies suggest that, under conditions of intermittent positive pressure ventilation, as pulmonary blood flow is reduced portions of the pulmonary vascular bed close completely, and there is some delay following blood flow restoration before these vessels again reopen.