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What is stroke volume

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Here is a thorough explanation of stroke volume, drawn from authoritative physiology and anesthesiology textbooks.

Stroke Volume

Stroke volume (SV) is the volume of blood ejected by the left ventricle during a single contraction (systole). It is simply the difference between two ventricular volumes:
SV = End-Diastolic Volume (EDV) - End-Systolic Volume (ESV)
  • End-Diastolic Volume (EDV): the amount of blood in the ventricle just before it contracts (~120 mL)
  • End-Systolic Volume (ESV): the amount of blood remaining after contraction (~50 mL)
  • Typical SV at rest: ~70 mL
(Costanzo Physiology 7th Edition, p. 155)

Ejection Fraction

A related measure is the ejection fraction (EF), which expresses how efficiently the ventricle ejects its contents:
EF = SV / EDV
Normally ~55-65%. A falling ejection fraction signals reduced contractility (as seen in heart failure).

Cardiac Output

Stroke volume is multiplied by heart rate to give cardiac output:
Cardiac Output = SV × Heart Rate
At rest in a 70-kg man: 70 mL × 72 beats/min ≈ 5,000 mL/min (5 L/min)

The Three Major Determinants of Stroke Volume

According to Morgan and Mikhail's Clinical Anesthesiology, stroke volume is governed by three factors:
FactorDefinitionEffect on SV
PreloadVentricular end-diastolic volume (degree of fiber stretch before contraction)More preload → more SV (up to a limit)
AfterloadResistance the ventricle must overcome to eject blood (aortic impedance / wall stress)More afterload → less SV
ContractilityIntrinsic force of myocardial contraction, independent of preload and afterloadMore contractility → more SV
Additional factors that can affect SV include wall motion abnormalities and valvular dysfunction.

Preload and the Frank-Starling Law

Preload is primarily determined by venous return. As end-diastolic volume increases, cardiac muscle fibers are stretched, which increases the force of contraction and therefore SV. This is the Frank-Starling Law of the Heart - the strength of contraction depends on initial fiber length.
The graph below (from Starling's original work, as reproduced in Morgan & Mikhail) shows how cardiac output varies with ventricular end-diastolic volume under different contractile states:
Frank-Starling Curves showing cardiac output vs. ventricular end-diastolic volume under different conditions including exercise, rest, heart failure, and cardiogenic shock
  • The upper curves (exercise, normal at rest) reflect higher contractility
  • The lower curves (heart failure, cardiogenic shock) reflect depressed contractility
  • Each curve peaks and can decline at very high EDV (ventricular overdistension)

Factors Affecting Preload

(Morgan & Mikhail's Clinical Anesthesiology, p. 658)
  • Blood volume
  • Distribution of blood volume (posture, venous tone)
  • Intrathoracic pressure (e.g., positive-pressure ventilation reduces venous return)
  • Pericardial pressure
  • Heart rate and rhythm (atrial contraction contributes 20-30% of filling)

Quick Reference: Sample Calculation

A patient with EDV = 140 mL, ESV = 70 mL, HR = 75 beats/min:
  • SV = 140 - 70 = 70 mL
  • Cardiac Output = 70 × 75 = 5,250 mL/min
  • Ejection Fraction = 70/140 = 50%
(Costanzo Physiology 7th Edition, p. 155)
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