Haemodynamic effects and plasma concentrations of labetalol during long-term treatment of essential hypertension.

Fifteen men with untreated essential hypertension in WHO stage 1 were studied on an outpatient basis to evaluate the haemodynamic long-term effect of a new α- and β-adrenergic receptor blocker, labetalol. Oxygen consumption, heart rate, cardiac output (Cardiogreen) and intraarterial brachial pressure were recorded at rest in a supine and sitting position, and during steady state work at 300, 600 and 900 kpm/min. The subjects were treated with labetalol (dose 200-800 mg/day) as the sole drug for 1 year. The haemodynamic study was then repeated and the concentration of labetalol in plasma 2-2.5 h after the morning dose was measured. Mean arterial blood pressure was reduced approximately 23% at rest and 21% during exercise. The heart rate was decreased 15% at rest and 16% during exercise. There was a compensatory increase in the stroke volume and consequently the cardiac index was reduced less than the heart rate, 7% at rest supine and 10% during exercise. There was a significant decrease in total peripheral resistance at rest supine (16%) and during exercise (12%). No serious side-effects were seen, but two subjects almost syncopated in the sitting position after the 900 kpm/min work load at the restudy. There was no correlation between the plasma concentration and the effect of labetalol. The haemodynamic changes differ from those seen after long-term therapy with drugs possessing only β-adrenoceptor blocking properties, and agree well with what should be expected with a drug which possesses both α- and β-adrenoceptor blocking properties.