Haemodynamic effects of procainamide and phenytoin at apparent therapeutic plasma levels.

The haemodynamic effects of procainamide and phenytoin after intravenous administration to 19 patients with suspected congenital heart lesions were studied. The drugs were injected into the pulmonary artery in a dose of 5 mg per kg bodyweight, at an infusion rate of 50 mg per minute, thus giving plasma concentrations within the suggested therapeutic ranges. The diastolic pressures in the pulmonary artery and the right ventricle were reduced by procainamide and the converse effect was noted after phenytoin. The heart rate was significantly increased by both drugs but with phenytoin this effect was only transitory. The latter drug also caused a small and transient fall in cardiac output and mean arterial pressure. It was concluded that of the two drugs phenytoin seemed to have the most marked myocardial depressant effect.