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THE EFFECTS of breathing low oxygen (acute hypoxia) on the pulmonary cireulation in normal subjects and in patients with various cardiopulmonary diseases have been reported by many workers.'-8 In general, during the period of acute hypoxia the pulmonary arterial pressure and the pulmonary blood flow usually rise, while the pulmonary wedge or left atrial pressure is unchanged or slightly increased. In most instances there is an increase in the calculated pulmonary vascular resistance, since the magnitude of rise in the pulmonary arterial to left atrial pressure gradient usually is proportionately higher than that in the pulmonary blood flow. Based upon the augmentation in the pulmonary vascular resistance during acute hypoxia, it has been generally assumed that decreased alveolar oxygen tension invokes active constriction of the pulmonary vessels. Changes in the vascular resistanees, however, can result simply from passive distention or narrowing associated with a proportional increase or decrease in both the volume of blood and distending pressure within the vessel. Therefore, the measurement of pulmonary vascular resistance alone may not necessarily reflect a change of caliber of the vessels.9' 10 Active vasoconstriction of the pulmonary vascular bed in man during acute hypoxia would be more reliably demonstrated if one could

Effects of Acute Hypoxia on the Pulmonary Vascular Bed of Patients with Acquired Heart Disease