LOCAL DIFFERENCES IN INTRATHORACIC PRESSURE AND THEIR RELATION TO CARDIAC FILLING PRESSURE IN THE DOG

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Abstract
Anew modification of the glass-membrane manometer for optical measurement of differential pressures is described. Optical pressure measurements in supine anesthetized dogs indicate that extrapericardial pressure is higher than pressure in a small closed pneumothorax; that the difference becomes greater when the lungs are inflated either by normal respiration or by positive intrapulmonary pressure; and that the difference becomes less when the lungs are deflated by enlarging the pneumothorax. Pressure within a balloon which produces distortion of the lung is shown to behave in a similar fashion. in the dog, local elevation of intrathoracic pressure occurs in the region of the heart because of the distortion of the lung produced by the heart. The influence of local variations in intrathoracic pressure upon measurements of effective filling pressure is described and discussed. It is concluded that effective filling pressure can be measured most accurately as the difference between intracardiac and extra-cardiac pressures.