Hemodynamic studies of nitroglycerin in dogs using an open loop method.

Nitroglycerin is considered to decrease both preload and afterload. To elucidate the actions of nitroglycerin on afterload, venous return was kept constant in this experiment. Three different doses of nitroglycerin, 0.3 microgram/kg/min, 1.0 microgram/kg/min, 3.0 micrograms/kg/min were administered into the ascending aorta of 6 mongrel dogs weighing over 20 kg. These doses of nitroglycerin were randomized in each of six procedures. The plasma concentration of nitroglycerin increased during 10 min of infusion, then it decreased exponentially. Heart rate, cardiac output and left atrial pressure did not change significantly at any dose of nitroglycerin. Systolic blood pressure and systemic vascular resistance (as indices of afterload) decreased significantly during nitroglycerin infusion, especially with the 3 micrograms/kg/min dose. These data suggest that nitroglycerin decreases the afterload. The effects appeared rapidly with administration and disappeared rapidly after cessation of the infusion.