Some Respiratory and Circulatory Effects of Mechanical Respirators

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Abstract
A comparative study of the effects of several mechanical respirators on pulmonary ventilation and systemic arterial pressure was carried out in anesthetized human subjects. All the respirators studied were capable of ventilating the avg. apneic subject adequately, but in obese individuals the Pneolator occasionally did not produce a sufficient tidal vol. when operated in the same mask pressure range employed with the other respirators. At any given level of mean mask pressure the intermittent positive pressure devices (Bennett and M. S. A. exptl. model) and the positive-negative pressure devices produced approx. equal tidal and min. vols. of ventilation, but the cyclic positive pressure machine studied (Pneolator) produced less than either of the other 2 types. Some subjects who had slight ability to respire apparently tolerated the intermittent positive pressure devices and the cyclic positive pressure valve better than the positive-negative pressure machines. Adequate ventilation could be produced without a decrease in arterial pressure by any of the devices studied in subjects who showed "moderate" vasomotor reactivity. In subjects whose vasomotor reactivity had been greatly reduced by the admn. of spinal anesthesia, ganglionic blocking agents, or large amts. of Na thiopental the level of arterial pressure decreased during mechanical ventilation of the lungs at all pressures greater than atmospheric, and the amt. of this decrease was related both to the prevailing level of mean mask pressure and to the time the respirator remained in operation. Apneic indivi-duals with greatly reduced vasomotor reactivity who were ventilated at mean mask pressures less than atmospheric usually showed an increase in arterial pressure during this procedure.