INFLUENCE OF NITROGLYCERIN ON SPLANCHNIC CAPACITY AND SPLANCHNIC CAPACITY CARDIAC-OUTPUT RELATIONSHIP

It has been postulated, but not tested directly, that nitroglycerin's venodilatory effects attenuate cardiac output. Thus, the present study examined the importance of changes in splanchnic capacity, as assesse d by scintigraphy, in the regulation of cardiac output during nitrogly cerin administration in 16 anesthetized pigs under conditions of carot id sinus denervation and cervical vagotomy. With nitroglycerin adminis tration (0.5 mg/min iv) for 5 min, systemic arterial pressure decrease d from 115 +/- 7 to 95 +/- 7 mmHg (P < 0.0001), portal vein pressure d ecreased from 9.0 +/- 0.5 to 8.5 +/- 0.5 mmHg (P < 0.0001), portal flo w increased from 637 +/- 49 to 668 +/- 60 ml/min (P = 0.09), and trans hepatic resistance decreased from 7.5 +/- 1.5 to 6.5 +/- 1.0 mmHg.min. l(-1) (P < 0.01), but cardiac output was unchanged (1,929 +/- 126 to 1 ,890 +/- 138 ml/min). Total splanchnic intravascular volume (VI) incre ased 1.6 +/- 1.0% (P < 0.05, 14 +/- 10 ml). This increase was due to a n increment in extrahepatosplenic (mesenteric) VI (12.9 +/- 1.9%, P < 0.0001), since splenic VI decreased (9.6 +/- 2.8%, P < 0.0001) and hep atic VI did not change. After splenectomy, nitroglycerin infusions at doses of 0.5 and 2 mg/min were associated with increases in total spla nchnic VI of 3.7 +/- 1.2% (P < 0.0001, 30 +/- 10 ml) and 7.6 +/- 1.7% (P < 0.001, 59 +/- 10 ml) due entirely to increases in mesenteric volu me of 9.9 +/- 2.7% (P < 0.0001) and 16.5 +/- 1.9% (P < 0.0001), respec tively, but cardiac output was unchanged at the end of infusion at eit her dose. Subsequent evisceration did not alter the cardiac output res ponse at either dose. Thus, nitroglycerin administration is associated with an increment in total splanchnic VI due to active mesenteric ven odilation and an increase in portal flow. The splanchnic VI changes do not affect cardiac output.