Some circulatory effects of thoracotomy and intermittent positive pressure respiration in dogs.

I T HAS been shown by many workers that positive pressure respiration may cause a fall in cardiac output, and work in this field has been the subject of several excellent reviews. 1 " 3 In intermittent positive pressure respiration , the fall in output has been proved to occur during the positive pressure phase in animals with both closed and open chest. 4 " 0 The mechanisms most commonly cited as producing 1 this effect are diminished venous return to the thorax consequent upon increased in-trathoracic pressure, 3 ' " • 7 and increased pulmonary vascular resistance. 1 ' * • 7 > 8 Conversely, in the animal with closed chest, a negative pressure phase of respiration has been demonstrated 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, pulmonary 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 injections of sodium thiopental. Measurements of cardiac output were not done in the periods immediately following injection. By this technique, it was hoped to avoid the effects of significant respiratory and circulatory depression on our results. Polyethylene catheters were introduced into the superior vena cava (SVC) and into the lower abdominal aorta via the femoral artery. Cardiac output, systemic blood pressure, and CBV were measured (a) with chest closed and on spontaneous respiration, and (b) with chest open and on intermittent positive pressure respiration. In three dogs, the effects of varying positive pressure on the cardiac output, systemic blood pressure, and CBV were assessed under the above conditions (a) and (b). In a further series of dogs, pulmonary artery pressures were measured by right heart catheterization. The left ventricular end-diastolic pressure, obtained by direct needle puncture , was assumed to be equal to the left atrial mean pressure in the absence of mitral valvular disease, so that the pressure gradient across the pulmonary vasculature could be obtained. Using values of cardiac output obtained as described below, total systemic, total pulmonary, and pulmonary vascular resistances before and after tho-racotomy were calculated, and the effects of varying levels of positive pressure respiratory, of hexa-methonium, …