Studies on Digitalis. Ii. Extracardiac Effects on Venous Return and on the Capacity of the Peripheral Vascular Bed

The decline in cardiac output following the ad-ministration of digitalis to experimental animals (1-5) and to normal human subjects (6-10) would appear to be inconsistent with the well known positive inotropic action of this drug. The latter has been further confirmed by direct meas-urement of myocardial contractile force in both man and dog in the absence of heart failure ( 5, 11, 12). Peripheral pooling of blood leading to a decreased return of venous blood to the heart has been suggested by several investigators as a pos-sible explanation for this phenomenon (13-15). Tainter and Dock (16) observed an increase in portal venous pressure following digitalis adminis-tration in the dog and postulated that hepatic vein constriction resulted in pooling of blood in the splanchnic bed. It is now clear that digitalis gly-cosides act directly on the peripheral circulation of both dog and man (12, 17). To define further the peripheral actions of these drugs, a prepara-tion was developed which permitted characteriza-tion of the effects of digitalis on the capacity of the peripheral vascular bed and on venous return. METHODS Twenty-four dogs, averaging 15.0 kg in weight, were studied. Anesthesia was administered with morphine, chloralose and urethan; complete bypass of the heart and lungs was established, and pressure and flow measure-ments carried out as previously described (17). Per-fusion rates were kept constant throughout each ex-periment and varied from 83 to 108, averaging 97 ml