Duration of one cardiac cycle is?

Appeared in: VSSC Nursing Officer - 2021

Explanation

The duration of a single cardiac cycle is conventionally calculated based on a normal resting heart rate of 75 beats per minute (bpm).
To find the duration of one beat (cycle), you divide the total seconds in a minute by the heart rate: 60 seconds / 75 bpm = 0.8 seconds.
This 0.8-second cycle is composed of specific phases: atrial systole (0.1s), ventricular systole (0.3s), and complete cardiac diastole (0.4s), which sum up to 0.8 seconds.

Option A: 8 seconds is far too long for a single cardiac cycle. This would correspond to an extremely slow heart rate of 7.5 beats per minute (60/8), which is incompatible with life.
Option C: 08 seconds is too short for a full cardiac cycle. It would imply an impossibly high heart rate of 750 beats per minute (60/0.08). This duration is closer to the time of specific electrical events within the cycle, like the P-R interval, but not the entire cycle.
Option D: 4 seconds represents only a part of the cardiac cycle, specifically the duration of complete cardiac diastole (when both atria and ventricles are relaxed). It does not include the systolic phases.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Physiology of the Cardiac Cycle as background academic context rather than a clinical decision trigger.
Understanding the 0.8-second cardiac cycle is fundamental for interpreting heart rates and rhythms. Deviations indicate conditions like bradycardia (slower rate, longer cycle) or tachycardia (faster rate, shorter cycle).
During tachycardia, the diastolic period shortens significantly more than the systolic period. This is clinically important because it reduces the time for ventricular filling, potentially lowering cardiac output, and decreases the time for coronary artery perfusion, which primarily occurs during diastole.
What if the patient's heart rate is 150 bpm? The cardiac cycle duration would be halved to 0.4 seconds (60/150). This rapid rate severely compromises diastolic filling time, which can lead to symptoms of poor perfusion like dizziness or syncope.

How to Approach the Question

Identify the core concept: The question asks for the standard duration of one cardiac cycle.
Recall the standard resting heart rate used for physiological calculations, which is typically 72 or 75 beats per minute.
Apply the formula to calculate the duration of a single beat: Time (seconds) = 60 seconds / Heart Rate (beats per minute).
Calculate: 60 / 75 = 0.8 seconds.
Review the options to find the one that matches your calculation.
Mentally check the other options to confirm they are incorrect (e.g., 0.4s is diastole only, the others are physiologically improbable).

Concept Tested & Keywords

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E6 Physiology Ganong 27e Vol 2 Part 1 pp. 87-89, 88-90

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