Methaemoglobin levels during nitroglycerin infusion for the intraoperative induction of controlled hypotension.

The methaemoglobin formation resulting from the intravenous infusion of nitroglycerin, was investigated, intraoperatively, in 46 patients in whom the substance was used to achieve controlled hypotension. The mean infusion rate was 3.50 +/- 0.38 micrograms/kg/min, and the mean duration of the infusion 87 +/- 7.0 min (mean +/- SEM). The concentrations of methaemoglobin were measured prior to, during, and also 60 min after, the infusion of nitroglycerin, using an IL CO Oximeter 282. The control level of the methaemoglobin prior to the start of infusion was 0.6 +/- 0.1%. Nitroglycerin infusion resulted in a slight increase in the level of methaemoglobin of about 1% maximum, which showed no correlation with the total dose of nitroglycerin or the rate of infusion. The mean maximum methaemoglobin level was 0.9 +/- 0.2%. The results suggest that, at average infusion rates of up to 10 micrograms/kg/min, nitroglycerin does not give rise to any clinically significant methaemoblobinaemia.