Alternative mechanism of cardiac acceleration in Bainbridge's infusion experiments

Abstract

Intravenous infusions on intact dog hearts both in closed- and open-chest experiments indicate that a rise in the intra-atrial pressure is accompanied by either a positive, a negative or a no chronotropic effect, irrespective of the integrity of vagi. Positive chronotropic effects of similar intensity were also obtained even after bilateral vagotomy, topical application of cocaine or parenteral administration of atropine, tetraethylammonium or hexamethonium compounds. Moreover, the rise in the intra-atrial pressure or the degree of visible distention of the right atrium was not necessarily associated with a parallel rise in the heart rate. Thus, the infusion acceleration response obtained with some infusions only, fails to fulfill the requirements of a reflex action. There is, therefore, no justification for labeling it as a "reflex" effect, as advocated by Bainbridge. The chronotropic effects of the infusions seem to be due to stretch-induced alterations in impulse generation as a result of changes in the intra-atrial pressure and possibly depend upon the initial conditions at the pacemaker.