Cardiac Output and Stroke Volume

Stroke volume is the amount of blood ejected by a ventricle in a single beat, and cardiac output is that volume multiplied by the heart rate - the total volume of blood the heart pumps per minute. Together they quantify the heart's performance as a pump and describe how delivery of blood is matched to the body's metabolic demands.

Definition

Stroke volume is the volume of blood ejected by a ventricle per beat (end-diastolic minus end-systolic volume); cardiac output is the volume of blood pumped per minute, equal to stroke volume times heart rate.

Scope

The topic covers the definitions and determinants of stroke volume (preload, afterload, contractility) and cardiac output (stroke volume and heart rate), and the principles by which they are measured. It treats these as whole-organ output measures; the cellular basis of force is covered under cardiac muscle contraction and chamber performance under ventricular function.

Core questions

• What is the difference between stroke volume and cardiac output?
• How do preload, afterload, and contractility each affect stroke volume?
• How does heart rate interact with stroke volume to determine output?
• How are stroke volume and cardiac output measured?
• How is cardiac output matched to changing metabolic demand?

Key concepts

• Stroke volume = end-diastolic volume minus end-systolic volume
• Cardiac output = stroke volume x heart rate
• Preload, afterload, and contractility
• Cardiac index (output normalised to body surface area)
• Ejection fraction
• Fick principle and thermodilution measurement

Key theories

Frank-Starling mechanism

Stroke volume rises with increased end-diastolic filling because greater stretch of the myocardium increases the force of contraction, allowing the heart to adjust output to venous return as captured by ventricular function curves.

Mechanisms

Stroke volume is set by three factors: preload (the degree of ventricular filling, which raises force through the Frank-Starling mechanism described by Sarnoff), afterload (the load against which the ventricle ejects, which opposes shortening), and contractility (the intrinsic strength of contraction independent of loading). Multiplying stroke volume by heart rate gives cardiac output, the body's principal measure of circulatory delivery, which can be expressed relative to body size as the cardiac index. Output is measured non-invasively by echocardiographic estimation of chamber volumes - standardised by the ASE/EACVI chamber-quantification recommendations summarised by Lang - or invasively using the Fick principle or thermodilution.

Clinical relevance

Cardiac output and stroke volume are core measures of circulatory adequacy and underlie the assessment of shock, heart failure, and exercise capacity, while ejection fraction derived from chamber volumes is a routine clinical index of ventricular performance. This entry explains the physiology and measurement principles for reference and is not a basis for individual diagnosis or treatment decisions.

Evidence & guidelines

The determinants of stroke volume rest on classic physiology (Sarnoff's ventricular function curves) and standard textbooks, while contemporary measurement of chamber volumes and ejection fraction follows the ASE/EACVI chamber-quantification recommendations (Lang, 2015).

History

Adolf Fick proposed in the nineteenth century the principle that still bears his name for calculating cardiac output from oxygen uptake and arteriovenous oxygen difference. The dependence of stroke volume on filling was formalised in the Frank-Starling tradition and made quantitative by Sarnoff's ventricular function curves, and modern imaging consensus documents such as the ASE/EACVI recommendations standardised the volumetric measurements used today.

Key figures

• Ernest Starling
• Adolf Fick
• Stanley Sarnoff

Seminal works

• sarnoff-1955
• lang-2015

Frequently asked questions

How are stroke volume and cardiac output related?

Cardiac output equals stroke volume multiplied by heart rate. Stroke volume is the blood ejected per beat, so multiplying it by the number of beats per minute gives the volume pumped each minute.

What three factors determine stroke volume?

Preload (how much the ventricle fills), afterload (the resistance it must eject against), and contractility (the intrinsic strength of the contraction). Increasing preload or contractility raises stroke volume, whereas increasing afterload tends to reduce it.