I : F 1 Ects or Isoproti : Renol on Peripheral Venous Tone and Transmural Right Atrial Pressure in Man

The increase in cardiac output during' isoproterenol administration has been attributed to a direct action on the heart causing it to extract blood more effectively from the central venous reservoir (1). Another suggestion is that isoproterenol constricts peripheral veins and causes blood to be pushed into the heart (2). The observation that right atrial pressure falls during infusion of the drug (3) would seem to dispose of the latter explanation. Right atrial pressure, measured with respect to atmospheric pressure, however, is not the force which distends the atrium.' The pressure which actually distends the atrium, the transmural pressure, is the difference between the pressure within the chamber and the intrapleural pressure which surrounds it. Transmural atrial pressure, then, may be influenced by changes in intrapleural pressure. In previous studies (4) we found that hyperventilation causes transmural atrial pressure to rise even though atrial pressure falls. This occurs because overbreathing is associated with a greater reduction in intrapleural pressure than in right atrial pressure. Isoproterenol, like epinephrine, appears to cause central excitation (5). It is known that epinephrine causes hyperventilation (6), probably by a central excitatory action. If isoproterenol has a similar effect on breathing, the fall it produces in right atrial pressure could be a ventilatory effect. If this were true the fall in atrial pressure might not indicate a coincident fall in transmural pressure. If isoproterenol causes blood to be pushed centrally, peripheral venous constriction would have to provide the "pushing" force. To our knowl-