[Effect of sodium nitroprusside and nitroglycerin on the resistance and capacitance system in cardiac surgery...

The whole-body arterial and venous dilating properties of nitroglycerin (NTG) and sodium nitroprusside (SNP) were investigated during extracorporeal circulation and normal circulation in patients undergoing coronary bypass surgery. SNP was found to lower preload and afterload for the time of drug administration. During extracorporeal circulation and normal circulation, an average volume of 340 ml was stored in the capacitance vessels when the mean arterial pressure was lowered for 20 mm Hg. When the reduction in preload was compensated by a volume load (blood transfusions), this resulted in a pure impedance reduction by SNP. NTG showed more complex effects. Even with the continuous infusion of NTG only a short-termed, self-limiting afterload reduction occurred during extracorporeal circulation. During normal circulation, the reduction in preload was more extensive with NTG than with SNP, an average volume of 600 ml was stored in the capacitance vessels when the mean arterial pressure was lowered for 20 mm Hg by NTG. While the volume storage by SNP was reversible after termination of the infusion, this was not the case with NTG, where the volume was stored for 1-2 hours. The volume storage by NTG became smaller with an increased filling of the capacitance system. Additionally, the venous resistance of the cardiopulmonary bypass showed different effects on the volume storage by SNP and NTG. In conclusion, the different effects of NTG and SNP cannot be explained by a single, homogenous compartment model of the peripheral circulation but by affecting two different compartments of the capacitance system.