GUANETHIDINE AND METHYLDOPA: A HAeMODYNAMIC STUDY

Abstract

The blood pressure, heart rate, and cardiac output were measured in 6 patients before and after treatment with oral guanethidine, and in 10 patients before and after treatment with oral methyldopa. Observations were made with the patients supine, standing, and during treadmill walking. Patients were therefore studied under conditions as relevant as possible to the ordinary treatment of hypertension. Both drugs produced their hypo-tensive response principally by lowering peripheral resistance; their effect was greater when patients were upright because the increase in peripheral resistance that compensates for the normal fall in cardiac output in this position was reduced or abolished. Heart rate was reduced by both drugs, and in this respect guanethidine was more potent than methyldopa. The slowing of the heart rate was usually compensated by an increased stroke volume, and in many cases cardiac output was greater on treatment especially during exercise. Of the 8 patients in whom the supine output fell, 7 had radiological evidence of cardiac enlargement; it is postulated that in these patients the stroke volume may be unable to increase sufficiently to compensate for the slower heart rate. The biggest falls in blood pressure were obtained on exercise, and this was due to a greater than normal decrease in peripheral resistance: it occurred despite the fact that cardiac output was raised to more than pre-treatment levels in most cases. There was little difference in the haemodynamic effects of the 2 drugs, except that guanethidine caused more slowing of the heart rate and therefore greater increases in stroke volume.