EFFECT OF NORMAL AND ABNORMAL CHANGES OF INTRATHORACIC PRESSURE ON EFFECTIVE RIGHT AND LEFT ATRIAL PRESSURES

Abstract

Simultaneously recorded intrathoracic pressure and right and left atrial pressures were obtained in closed-chest dogs. Instantaneous effective right and left atrial pressures were calculated during normal respiration in states where there were abnormally great fluctuations of intrathoracic pressure. During the apneic phase of normal respiration effective left atrial pressure exceeds effective right atrial pressure. A decrease of intrathoracic pressure (inspiration) during normal respiration increases effective right atrial pressure, but has no significant effect on effective left atrial pressure. Greatly decreased intrathoracic pressure (Muller''s experiment) results in an increase of both right and left effective atrial pressure, but the former increases more promptly and to a greater extent than the latter. Increased intrathoracic pressure occasioned by mild positive pressure lung inflation, gentle abdominal pressure and dyspnea from sciatic nerve stimulation generally decreased both right and left effective atrial pressures, the decrease of effective right atrial pressure predominating. In one case where sciatic stimulation produced a maintained active expiratory attempt (with trachea occluded), effective right atrial pressure exceeded effective left atrial pressure. It is concluded that physiol. changes of intrathoracic pressure result in slight but predictable changes of effective atrial pressure. The magnitude of the left to right inter-atrial pressure gradient reflects these changes of effective right and left atrial pressures, the gradient becoming less with decreasing intrathoracic pressure and vice versa. However, a decrease of intrathoracic pressure seldom reverses the direction of the gradient except in the presence of pathological complications, which tend to equalize right and left atrial pressures.