Haemodynamic responses to glyceryl trinitrate: influence of rate and duration of delivery

Glyceryl trinitrate (GTN) is known to be a potent venodilator, but the effects might differ depending on the regime of administration. Using a radionuclide blood pool method we examined the changes in peripheral (and pulmonary) venous volume induced by GTN administered in three ways: short (4 min) intravenous infusions at 75 micrograms.min-1 (n = 7), prolonged intravenous infusion at 50 micrograms.min-1 (n = 7) or 500 micrograms sublingual tablet (n = 5). Sublingual GTN and the prolonged infusion caused similar decreases in systolic blood pressure (-23 +/- 12, -24 +/- 10 mmHg) and increases in calf venous volume (12 +/- 4%, 12 +/- 5%). For both, the peak effect occurred around 20 min after starting GTN administration (blood pressure 18 +/- 12 vs 25 +/- 12 min, venous volume 21 +/- 1 vs 22 +/- 7 min). Only the prolonged infusion caused a significant fall in diastolic pressure (-14 +/- 6 mmHg). The short infusion had a similar effect on systolic pressure (-21 +/- 16 mmHg) to the other two regimes, but caused a significantly smaller increase in venous volume (6 +/- 2%), which occurred much earlier (5 +/- 2 min). Anterior lung blood volume decreased in response to intravenous GTN and the posterior lung blood volume did not change, suggesting this alteration was secondary to peripheral venodilatation. In conclusion, GTN causes venous dilatation at low levels of delivery with the maximum response occurring after 20 min. In contrast, arterial effects are seen more rapidly and at higher rates of delivery.