During tachyarrhythmias, how are cardiac output and filling time affected?

• A. Ventricular filling time decreases as rate increases
• B. Atrial contribution becomes negligible only at very high rates
• C. The heart doesn't have time to fill and squeeze after the optimal rate point
• D. Cardiac output increases with rate indefinitely

Answer: (C)

At faster heart rates, diastolic filling time shrinks, so ventricular preload and stroke volume drop. Cardiac output (rate × stroke volume) can rise with rate only up to a point; beyond an optimal rate, the reduced filling and shortened ejection time prevent effective contraction, so the heart doesn’t have time to fill and squeeze. This is why cardiac output does not increase indefinitely with rate—the flow is limited by insufficient filling time.