Which equation is correct for stroke volume (SV)?

MP NHM CHO-2024 (Shift-2)

Applied Physiology

Easy

Appeared in: MP NHM CHO-2024 (Shift-2)

Explanation

Stroke Volume (SV) is the volume of blood the heart ejects from a ventricle with each beat.
It is calculated as the difference between the volume of blood in the ventricle at the end of filling (End-Diastolic Volume or EDV) and the volume remaining at the end of contraction (End-Systolic Volume or ESV).
The formula SV = EDV - ESV correctly represents the volume of blood that was actually pumped out into circulation during one heartbeat.

Option A: This formula incorrectly uses Heart Rate (HR), which is the number of beats per minute. Adding a frequency (HR) to a volume (ESV) is dimensionally and physiologically incorrect.
Option B: This formula incorrectly adds the starting (EDV) and ending (ESV) ventricular volumes. Stroke volume represents the amount of blood ejected, which is a difference, not a sum.
Option D: This formula incorrectly uses Heart Rate (HR). Subtracting a volume (ESV) from a frequency (HR) is dimensionally and physiologically incorrect.

Visual 1: Diagram: Cardiac Cycle Pressure-Volume Loop. This visual helps to show that Stroke Volume is the width of the loop, representing the change in volume from End-Diastolic Volume to End-Systolic Volume.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Formula for calculating Stroke Volume (SV) as background academic context rather than a clinical decision trigger.
Assessing stroke volume is crucial for evaluating a patient's cardiac function and hemodynamic status.
A low stroke volume can indicate heart failure, hypovolemia (e.g., from hemorrhage or dehydration), or impaired myocardial contractility, leading to decreased cardiac output and poor tissue perfusion.
What if? If a patient's heart rate increases to compensate for a low stroke volume, their cardiac output might initially be maintained. However, a sustained high heart rate reduces diastolic filling time, which can further decrease stroke volume and eventually lead to cardiac decompensation.

How to Approach the Question

First, identify the core concept being tested: the formula for Stroke Volume (SV).
Recall the definition of SV: the amount of blood pumped out of a ventricle during one contraction.
Think logically about how this amount is calculated. It must be the volume you start with (a full ventricle) minus the volume that is left over after contraction.
Translate this logic into the provided physiological terms: The starting volume is the End-Diastolic Volume (EDV), and the leftover volume is the End-Systolic Volume (ESV).
Construct the formula based on this logic: SV = EDV - ESV.
Scan the options and eliminate any that include Heart Rate (HR), as HR relates to the frequency of beats (rate), not the volume per beat.

Concept Tested & Keywords

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