PHYSIOLOGICAL STUDIES OF THE EFFECTS OF INTERMITTENT POSITIVE PRESSURE BREATHING ON CARDIAC OUTPUT IN MAN

Abstract

Three types of positive pressure breathing have been differentiated : a) continuous positive pressure breathing where a pressure above atmospheric is maintained in the lungs throughout the respiratory cycle, b) expiratory positive pressure breathing in which a pressure above atmospheric is present during the expiratory phase of breathing, and c) intermittent positive pressure breathing (IPPB), provided by automatic respirators, which consists in an active inflation of the lungs under an increasing positive (above atmospheric) pressure; while after cycling of the respirator, deflation of the lungs occurs almost to or to atmospheric pressure as a passive process engendered by the elasticity of the lungs and chest-wall structures. Only the latter type of pressure breathing (IPPB) is suitable for maintaining artificial respiration in the apneic subject. Moreover, IPPB as produced by automatic respirators is more comfortable and less exhausting fur the conscious subjects when compared to continuous or expiratory positive pressure l In a number of recent studies measurements of cardiac output were made on the same subjects during normal breathing at ambient pressure and during positive pressure breathing. Several observers (1, 2, 3,) have reported a reduction in cardiac output during positive pressure breathing in man, more or less proportional to the mean mask pressure used. Previous studies from this laboratory (4, 5) have shown that the changes in cardiac output are quite variable depending upon the types of intermittent positive pressure-breathing respirators used. In the same individual the cardiac output was decreased when on one type of respirator and increased when on another type. It has been suggested that these differences are related to the shape of the mask pressure intermittent positive pressure-breathing respirators (5).