Among the listed factors, which is not a major contributor to the 'perfect squeeze'?

• A. AV Timing
• B. Chronotropy
• C. The cardiac conduction system
• D. Heart rate variability

Answer: (D)

The moment-to-moment strength of the ventricular squeeze comes from keeping the timing in sync and maximizing preload: AV timing ensures the atrial contraction adds to ventricular filling, chronotropy tunes how long the heart spends in diastole and how hard it contracts, and the cardiac conduction system keeps the sequence of contractions coordinated so the ventricles eject together. Heart rate variability, on the other hand, reflects beat-to-beat fluctuations in heart rhythm due to autonomic tone over time. It does not directly control the instantaneous mechanics of one beat’s squeeze. So, heart rate variability is not a major contributor to the perfect squeeze.