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Relation of cardiovascular response to the hypotensive effect of metoprolol
Insights
Metoprolol lowers blood pressure in hypertensive patients by reducing heart rate and cardiac output. The blood pressure reduction is linked to changes in systemic resistance, not solely to decreased cardiac output.
Area of Science:
- Cardiology
- Pharmacology
Background:
- Essential hypertension is a common condition affecting systemic hemodynamics.
- Beta-blockers, like metoprolol, are frequently prescribed for hypertension.
- Understanding the precise mechanisms of beta-blocker action is crucial for optimizing treatment.
Purpose of the Study:
- To investigate the influence of cardioselective beta blockade with metoprolol on systemic hemodynamics and cardiac performance in patients with essential hypertension.
- To identify the mechanisms underlying blood pressure reduction by metoprolol.
Main Methods:
- 17 patients with essential hypertension received metoprolol (50-100 mg twice daily) for 1 month.
- Systemic hemodynamics and cardiac performance were assessed, including mean arterial pressure, heart rate, cardiac output, mean transit time, ejection fraction, and total peripheral resistance.
- Plasma renin activity and plasma catecholamines were also measured.
Main Results:
- Metoprolol significantly reduced mean arterial pressure in 11 of 17 patients.
- Heart rate and cardiac output decreased, but these changes were not significantly different between responders and non-responders.
- A significant correlation was found between changes in mean arterial pressure and changes in total peripheral resistance (r=0.68, p<0.01).
- Plasma renin activity significantly decreased in responders but not in non-responders.
Conclusions:
- Metoprolol reduces cardiac output by slowing heart rate, without impairing cardiac performance.
- The primary mechanism for blood pressure reduction appears to be the modulation of systemic vascular resistance in response to decreased cardiac output, rather than the decrease in cardiac output itself.
Abstract:
The influence of cardioselective beta blockade on systemic hemodynamics and cardiac performance was assessed in 17 essential hypertensive patients treated with metoprolol (50 to 100 mg twice daily) for 1 month. Eleven patients had a significant reduction of mean arterial pressure (less than 10 mm Hg); the other six patients did not show significant blood pressure change. There was no difference in response of heart rate or cardiac output between the two groups [-14 +/- 1.6 (SE) vs. -9.5 +/- 2.08 bpm, NS, and -0.55 +/- 0.39 vs. 0.80 +/- 0.35 L/min, NS, respectively]. Mean transit time was significantly prolonged in both groups (1.9 +/- 0.56 vs. 1 +/- 0.53 seconds, NS), and changes in ejection fraction in the two groups were not significant. The only significant difference between the two groups was in the response of total peripheral resistance (-1.4 +/- 3.04 vs. +10.2 +/- 3.77 U . m2, p less than 0.05). The reduction in heart rate correlated significantly with control heart rate (r = -0.71, p less than 0.001) and the decreased in cardiac output correlated significantly (r = -0.77, p less than 0.001) and the decrease in cardiac output correlated significantly (r = -0.77, p less than 0.001) with the initial cardiac output. The changes in mean arterial pressure did not correlate with changes in cardiac output (r = 0.116). On the other hand, there was a significant correlation of 0.68 (p less than 0.01) between the changes in mean arterial pressure and the changes in total peripheral resistance. Changes in plasma renin activity (PRA) were not significant in the nonresponders (1.8 +/- 0.9 to 0.6 +/- 0.2 ng/ml, NS), and the reduction of plasma renin activity was significant in the group of responders (5.5 +/- 2.4 to 1.7 +/- 0.8 ng/ml, p less than 0.05). However, there was no correlation between the changes of mean arterial pressure and either initial PRA or the change in PRA. As regards plasma catecholamines (measured in nine patients), there was a tendency to reduction in both responders and nonresponders (411 +/- 84 to 319 +/- 67 ng/L and 562 +/- 141 to 388 +/- 166, respectively); but changes did not reach statistical significance in both groups. It was concluded that metoprolol reduces cardiac output by slowing heart rate; this reduction of cardiac output was not related to decreased cardiac performance. The mechanism of blood pressure reduction seems to be related to the response of systemic resistance to the diminution in cardiac output and not to the decrease in cardiac output per se, inasmuch as the latter occurred in both responders and nonresponders.