Nitroglycerin and premature ventricular complexes in myocardial infarction

Effects of isosorbide-5-mononitrate and isosorbide-2-mononitrate on the contractile and electrical activity and on the content of cyclic nucleotides in isolated heart muscles of the guinea-pig and dog

Isosorbide-5-mononitrate and isosorbide-2-mononitrate, the metabolites of isosorbide dinitrate, were studied for their effects on the contractile and electrical activity and on the content of cyclic nucleotides in isolated heart muscles of guinea-pig and dog. Isosorbide-5-mononitrate at all concentrations tested and isosorbide-2-mononitrate at low concentrations did not produce significant changes in the mechanical activity of guinea-pig heart preparations. Isosorbide-2-mononitrate in concentrations above 10(-4) M inhibited the contractility and shortened the action potential of guinea-pig and dog heart muscle. Neither mononitrate had an effect on cAMP concentration but both induced a significant elevation of cGMP content in guinea-pig heart tissue, the effect of isosorbide-2-mononitrate being greater.

Nitrates in acute myocardial infarction

Nitrates relieve symptoms and improve left ventricular haemodynamics during acute myocardial ischaemia. The ability to increase cardiac output appears to be greatest in those patients with the most severe left ventricular failure. In humans and in animal experiments it has been demonstrated that indices of infarct evolution are reduced by early (less than 6 hours) administration of nitrate. Glyceryl trinitrate reduces myocardial oxygen consumption and improves delivery of oxygen to the ischaemic subendocardium. In addition, nitrates dilate epicardial coronary vessels with improvement of collateral flow Glyceryl trinitrate appears to have a more favourable effect on coronary collateral flow, pulmonary artery pressure, myocardial oxygen consumption and lactate production than other vasodilators. The effect of glyceryl trinitrate is dose dependent; doses larger than 100 micrograms/min may cause a paradoxical increase in the ischaemic condition. Hypovolaemic patients or patients with right heart infarction seem especially susceptible to hypotension. Nitrate-induced hypotension occurs in 15 to 20% of all patients with acute myocardial infarction. Administration of glyceryl trinitrate appears to offer most benefit in acute myocardial infarction complicated by significant left ventricular dysfunction. There is evidence that morbidity and mortality are reduced by early administration of nitrates; however, a properly conducted randomised double-blind trial remains to be performed.

Intravenous nitroglycerin in acute myocardial infarction

Until recently, the use of nitroglycerin in patients with acute myocardial infarction has been contraindicated primarily because of possible hypotension or tachycardia which, with sublingual or oral preparations, would be difficult to control. The introduction of intravenous nitroglycerin and its use during experimentally induced infarction paved the way for its use in patients with acute myocardial ischemia. It was shown that nitroglycerin, administered intravenously during experimentally induced coronary artery occlusion, redistributed flow to the subendocardium of the ischemic area while it maintained flow to the normal areas and was associated with decreased S-T segment elevation. Preliminary studies in patients with acute infarction have confirmed the beneficial effect of intravenous nitroglycerin on S-T segment elevation, and a subsequent study in 31 patients with acute infarction showed a decrease in infarct size with improved hemodynamics. In a more recent randomized prospective study in 85 patients with acute myocardial infarction, we found that intravenous nitroglycerin was associated with a decrease in infarct size in patients with inferior infarction (36 percent reduction, p less than 0.05) but that it had no effect on infarct size in patients with anterior infarction. Whether or not nitroglycerin should be used routinely for reduction of infarct size remains to be determined, but results indicate that it is effective and safe for the treatment of cardiac failure in patients with acute myocardial infarction.

Reduction of infarct size in patients with inferior infarction with intravenous glyceryl trinitrate. A randomised study

Glyceryl trinitrate was previously said to be contraindicated in patients with acute myocardial infarction. Its intravenous administration during acute infarction, however, was associated with a beneficial effect as determined by ST segment mapping. Most recently in a selected group of patients with acute infarction and abnormal haemodynamics, intravenous glyceryl trinitrate was shown to reduce infarct size estimated by enzymes. The present study was performed to verify the safety of intravenous glyceryl trinitrate in patients with infarction under conventional clinical conditions without invasive monitoring and to determine its effect on infarct size in a prospective randomised trial involving 85 patients with infarction (43 treated and 42 control). Treated patients received glyceryl trinitrate within 10 hours of the onset of symptoms (mean 6.0 hours), and the dose was titrated to preset limits for changes in heart rate and blood pressure. In patients with inferior infarction, infarct size estimated by enzymes in the treated was only 12.2 +/- 1.8 versus 19.1 +/- 3.6 CK gram equivalents per metre squared in the placebo group. A similar but statistically insignificant trend was observed for subendocardial infarction but no difference was observed for anterior infarction. Ventricular arrhythmias determined from 24 hour tapes were more frequent in treated patients though this was not statistically significant. Lignocaine requirements in treated and control (1692 +/- 250 vs 1512 +/- 232 mg/24 h) were similar, as were the requirements for morphine (11.4 +/- 1.8 vs 12.2 +/- 2.2 mg/24 h). Results indicate that intravenous glyceryl trinitrate can be administered safely during evolving infarction without invasive monitoring and reduces infarct size in patients with inferior infarction.

Relation of time course of plasma nitroglycerin levels to echocardiographic, arterial pressure and heart rate changes after sublingual administration of nitroglycerin

Despite the widespread use of nitroglycerin, a relation between plasma nitroglycerin concentrations and the associated cardiovascular effects has not been well established. It was hypothesized that nitroglycerin levels may help predict the hemodynamic responses. With use of a recently developed nitroglycerin assay technique, the relation between the time course of plasma nitroglycerin levels and echocardiographic changes after sublingual administration of 0.6 mg of the drug was evaluated in 12 normal volunteers. Mean plasma nitroglycerin levels were maximal at 2 (1.1 +/- 0.3 ng/ml) and 5 (1.4 +/- 0.6 ng/ml) minutes, when the changes in mean heart rate (+17 +/- 7 and +12 +/- 3 min-1) and decreases in echocardiographic left ventricular diastolic (-4.2 +/- 0.8 mm at 5 minutes) and systolic (-3.1 +/- 0.6 mm at 5 minutes) dimensions were also maximal. Parallel changes were noted in left atrial dimension, ventricular velocity of circumferential fiber shortening and systolic blood pressure, but not in diastolic blood pressure. These findings demonstrate the existence of a close relation between plasma nitroglycerin levels and variables that reflect the known responses to this drug.

Quantitative determination of trinitroglycerin in human plasma

We developed a simplified method for quantitative measurement of trinitroglycerin in human plasma using hexane extraction and analysis by gas-liquid chromatography with electron-capture detection. This assay was linear from 0.5-60 ng/ml. Sensitivity and reproducibility were +/- 0.5 ng/ml. We used this assay to evaluate the pharmacodynamics of trinitroglycerin in 14 patients. Maximum plasma levels were similar with trinitroglycerin given by constant intravenous infusion (1.6 +/- 0.4 ng/ml (SEM)), transcutaneously (2.3 +/- 0.6 ng/ml), or sublingually (1.6 +/- 0.6 ng/ml). Despite similar levels and hemodynamic responses after intravenous trinitroglycerin, the dose range was wide (37.5-175 microg/min, n = 5), emphasizing the need to individualize therapy. In normal volunteers on no other drugs, the plasma level time course followed changes in heart rate better than blood pressure changes. Use of the trinitroglycerin assay may enhance optimization of trinitroglycerin therapy when administered by different methods.