Comparative haemodynamic effects of labetalol, timolol, prazosin and the combination of tolamolol and prazosin.

Four groups of patients with previously untreated essential hypertension in WHO stage I were treated either with timolol ( = 16), or prazosin ( = 13) or prazosin plus tolamolol ( = 12), or labetalol ( = 15). Oxygen consumption, heart rate, cardiac output (Cardiogreen) and intraarterial brachial BP were recorded at rest in the supine and sitting position and during steady-state work at 50, 100 and 150 W before treatment and after 1 yr on drug therapy. All regimes induced significant decrease in arterial BP at rest as well as during exercise. BP reduction was achieved through different haemodynamic mechanisms. In the timolol group BP reduction was associated with a marked decrease in heart rate and cardiac output but no decrease in total peripheral resistance. In the prazosin group there was a significant decrease in total peripheral resistance at rest as well as during exercise. During exercise the cardiac index was higher than before treatment. In the groups treated with prazosin plus tolamolol or labetalol alone the changes were rather similar. There was a significant decrease in total peripheral resistance at rest, supine and during exercise. Heart rate was decreased, but much less than by the use of a pure β-blocker alone. Due to a compensatory increase in stroke volume, particularly during muscular exercise, the cardiac index was reduced much less than in the group treated with timolol. The results indicate that the haemodynamic long-term effects of labetalol differ from those seen after long-term therapy on prazosin or β-adrenoceptor blockers and resemble those seen after combined treatment with both α- and β-adrenoceptor blockers. The clinical significance of these differences is briefly discussed.