Haemodynamic effects of induced hypotension with a nitroprusside-trimetaphan mixture

The haemodynamic effects of a 10:1 mixture of trimetaphan and nitroprusside have been studied before and during the course of surgery, in 12 patients placed in 5 degrees reverse Trendelenburg position. Six patients breathed spontaneously and in six the lungs were ventilated artificially. The mixture had a potent hypotensive action that was almost as rapid in onset and recovery as that produced by nitroprusside alone, but the required dose of each drug was decreased considerably. In patients breathing spontaneously, cardiac output remained unchanged on the induction of hypotension, but it decreased significantly in the IPPV group. Heart rate increased, and stroke volume, peripheral resistance and central venous pressure decreased in both groups. The start of surgery was associated with a need to increase the dose of the hypotensive mixture, and with a further increase in heart rate and decrease in stroke volume. Levelling the operating table produced an increase in cardiac output and discontinuation of the mixture resulted in a rapid return of all measurements towards control values.