Which mechanism best explains how beta-1 blockade reduces myocardial oxygen demand?

• A. Decreased heart rate and contractility
• B. Increased afterload
• C. Decreased preload
• D. Increased heart rate

Answer: (A)

Beta-1 blockade lowers the heart’s workload by reducing both how fast and how hard it beats. Blocking beta-1 receptors diminishes sympathetic stimulation of the SA node and the ventricle, leading to a slower heart rate (negative chronotropy) and weaker contractions (negative inotropy). With fewer beats per minute and less force per beat, the heart uses less oxygen—the distance between oxygen supply and demand narrows because myocardial oxygen demand tracks heart rate and contractility (and thus the rate-pressure product). In practice, this means less wall stress and mechanical work of the heart, which translates to reduced oxygen consumption.

Other choices don’t explain the primary effect: increasing afterload would raise oxygen demand; decreasing preload or increasing heart rate doesn’t align with the mechanism of beta-1 blockade and would not produce the same reduction in oxygen use.