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

IT HAS been shown by many workers thatpositive pressure respiration may cause a fall in cardiac output, and work in this field has been the subject of several excellent re-views.1"3 In intermittent positive pressure respira-tion, the fall in output has been proved to oc-cur during the positive pressure phase in ani-mals with both closed and open chest.4"0 The mechanisms most commonly cited as produc-ing1 this effect are diminished venous return to the thorax consequent upon increased in-trathoracic pressure,3 ' " • 7 and increased pul-monary vascular resistance.1 ' * • 7> 8 Conversely, in the animal with closed chest, a negative pressure phase of respiration has been demon-strated to produce increased venous return to the thorax.3 ' 9 The purpose of this study was to investigate the effect of thoracotomy and of intermittent positive pressure respiration, sufficient to maintain adequate ventilation, on cardiac output, systemic blood pressure, pul-monary vascular resistance, and central blood volume (CBV) in dogs, since it is under these conditions that much experimental work on circulator} ' dynamics must occur. Methods Twenty-five mongrel dogs were kept lightly anesthetized by separated small intravenous injec