Some Circulatory Effects of Thoracotomy and Intermittent Positive Pressure Respiration in Dogs

In the anesthetized dog, thoracotomy and intermittent positive-atmospheric pressure (I.P.-A.P.) respiration produce a marked arid sustained fall in cardiac output. Evidence suggests that most of the fall is due to thora cotomy, and the I.P.-A.P. respiration at levels sufficient to maintain adequate ventilation plays a relatively minor part. Thoracotorny also results in considerable reduction of all compartments of the central blood volume, presumably due to loss of the distending ef fect of negative intrapleural pressure on the heart aiid great vessels. There was a consistent rise in pulmonary vascular resistance which was not significantly affected by the adminis tration of hexamethonium or phentolamine. It is suggested that humoral or neurogenic factors play a minor role in producing the in creased resistance.