What is a major cause of premature ventricular beats?

What is a major cause of premature ventricular beats?

• A. Excessive exercise.
• B. Low levels of oxygen sensed by automaticity foci.
• C. High levels of adrenergic substances.
• D. Increased parasympathetic activity.

Answer: (B)

The correct choice highlights that low levels of oxygen can trigger premature ventricular beats due to the mechanisms of automaticity foci within the heart. These foci are groups of cardiac cells capable of initiating impulses outside the normal conduction pathway, typically taking over when there is inadequate oxygen supply to the heart muscle.

When the heart experiences hypoxia, the myocardium may struggle to function optimally, leading to a shift in the electrophysiological stability of the cardiac cells. This disruption increases the likelihood of ectopic beats, such as premature ventricular contractions (PVCs), as the heart tries to compensate for insufficient oxygen. Such conditions often arise in scenarios like ischemia, where the blood supply — and consequently, oxygen levels — are compromised.

While the other options can influence heart rhythm, they don't specifically implicate the direct relationship with premature ventricular beats in the way that oxygen levels do. Excessive exercise, for instance, can increase overall myocardial oxygen demand but is not a direct cause of PVCs. High levels of adrenergic substances can indeed lead to increased heart rate and potential arrhythmias due to heightened sympathetic activity, but they are not the primary driver in the context of oxygen deprivation. Finally, increased parasympathetic activity typically has a calming effect on heart