Short-Term systemic hemodynamic adaptation to beta-adrenergic inhibition with atenolol in hypertensive patients

Early systemic hemodynamic adjustments to antihypertensire therapy with the cardioselective beta inhibitor, atenolol, were investigated in 12 hospitalized men, mean age 52 years, with uncomplicated mildto-moderate essential hypertension. Twice daily measurements of cardiac output (CO) by CO₂ rebreathing, blood pressure by cuff, and heart rate were performed in all subjects for 3 days before and S days after initiation of oral atenolol therapy (50 or 100 mg daily). Cardiac output by CO, rebreathing was checked with dye diluion just before, and 4 hours and 4 days after the start of therapy. Plasma volume (radioiodinated albumin) was measured before therapy and on Day 5 of therapy. The CO results obtained with the two methods were not significantly different (r = 0.88, p < 0.01, n = 12). A reduction in heart rate, 18 ± 2 beats/min (mean ± SE), occurred in all patients while taking atenolol. By 4 hours after the first dose of atenolol, CO fell from 5.49 ± 0.30 to 4.24 ± 0.21 liters/min (p < 0.01), while the control mean arterial pressure (MAP) of 108 ± 4 mm Hg was not significantly changed, 110 ± 4 mm Hg. At 24 hours, CO returned near baseline (5.10 ± 0.21 liters/min) but MAP was reduced (95 ± 3 mm Hg, p < 0.001) and remained so thereafter. CO remained at baseline at 48 hours (5.50 ± 0.29 liters/min) but fell again (p < 0.01) to 4.81 ± 0.11 on Day 4 and to 4.68 ± 0.25 liters/min on Day 5 of atenolol therapy. Plasma volume, 3110 ± 100 ml before therapy, was reduced to 2850 ± 100 by Day 5 of atenolol therapy (p < 0.01). The findings indicate a delayed onset of the antihypertensive action of atenolol. The transient return to baseline of CO on Day 2 and 3 of atenolol therapy suggests a reverse autoregulatory adjustment to the initial fall in CO.