What is Stroke Volume?
Stroke volume (SV) is the volume of blood ejected from the left ventricle of the heart with each heartbeat. It is one of the most fundamental parameters in cardiovascular physiology, as it directly determines how much blood the heart delivers to the body with every contraction.
Together with heart rate, stroke volume determines cardiac output (CO = SV × HR), which is the total volume of blood pumped by the heart per minute. A normal resting cardiac output is approximately 4–8 liters per minute. During exercise, cardiac output can increase 4–6 fold, primarily through increases in both heart rate and stroke volume.
Stroke volume is clinically important because it reflects the pumping efficiency of the heart. Reduced stroke volume is a hallmark of heart failure and other cardiovascular diseases. Indexed to body surface area, the stroke volume index (SVI) allows comparison between individuals of different body sizes.
Formulas
Stroke Volume from Cardiac Output
Stroke Volume from Ventricular Volumes
Ejection Fraction
Body Surface Area (Du Bois Formula)
Indexed Values
Normal Values
| Parameter | Normal Range | Unit |
|---|---|---|
| Stroke Volume (SV) | 60–100 | mL |
| Ejection Fraction (EF) | 55–70 | % |
| Stroke Volume Index (SVI) | 33–47 | mL/m² |
| Cardiac Output (CO) | 4.0–8.0 | L/min |
| Cardiac Index (CI) | 2.5–4.0 | L/min/m² |
| End-Diastolic Volume (EDV) | 65–240 | mL |
| End-Systolic Volume (ESV) | 16–143 | mL |
| Heart Rate (resting) | 60–100 | bpm |
Ejection Fraction
Ejection fraction (EF) is the percentage of blood that is pumped out of the ventricle with each heartbeat. It is the most commonly used clinical measure of ventricular systolic function and is typically assessed using echocardiography, cardiac MRI, or nuclear imaging.
| EF Range | Classification | Clinical Significance |
|---|---|---|
| ≥ 55% | Normal (HFpEF if symptomatic) | Normal systolic function |
| 41–54% | Mildly reduced (HFmrEF) | Borderline; may indicate early disease |
| 30–40% | Moderately reduced (HFrEF) | Significant systolic dysfunction |
| < 30% | Severely reduced (HFrEF) | Severe heart failure; ICD may be indicated |
It is important to note that a normal ejection fraction does not necessarily mean normal cardiac function. Heart failure with preserved ejection fraction (HFpEF) is a condition in which the heart pumps normally in terms of percentage but the ventricle is stiff and does not fill adequately, resulting in reduced stroke volume and symptoms of heart failure.
Frank-Starling Mechanism
The Frank-Starling mechanism (also known as Starling's Law of the Heart) is a fundamental principle of cardiac physiology. It states that the heart's stroke volume increases in response to an increase in the volume of blood filling the heart (end-diastolic volume) when all other factors remain constant.
The mechanism works because stretching of cardiac muscle fibers (myocytes) increases the sensitivity of the contractile machinery to calcium, resulting in a more forceful contraction. This allows the heart to automatically adjust its output to match venous return without requiring neuronal or hormonal input.
Key implications of the Frank-Starling mechanism:
- Matching output to input: If more blood returns to the heart (increased preload), the heart automatically pumps more blood out
- Left-right balance: It ensures that the output of the left and right ventricles remains equal over time
- Exercise response: During exercise, increased venous return stretches the ventricles, contributing to increased cardiac output
- Heart failure: In a failing heart, the Frank-Starling curve is flattened — increasing preload no longer effectively increases stroke volume, and further volume loading can worsen congestion
Determinants of Stroke Volume
Stroke volume is determined by three primary factors:
1. Preload
Preload is the degree of stretch of the ventricular myocardium at the end of diastole. It is related to the end-diastolic volume (EDV). Higher preload generally leads to higher stroke volume (Frank-Starling mechanism). Preload is influenced by venous return, blood volume, venous tone, body position, and intrathoracic pressure.
2. Afterload
Afterload is the resistance against which the ventricle must pump to eject blood. It is primarily determined by systemic vascular resistance (for the left ventricle) and pulmonary vascular resistance (for the right ventricle). Increased afterload (e.g., from hypertension or aortic stenosis) reduces stroke volume if contractility remains constant.
3. Contractility (Inotropy)
Contractility is the intrinsic ability of the cardiac muscle to generate force at a given preload and afterload. It is enhanced by sympathetic nervous system activation (norepinephrine), catecholamines, and positive inotropic drugs (digoxin, dobutamine). It is reduced in heart failure, myocardial infarction, and by negative inotropic agents (beta-blockers, calcium channel blockers).
Heart Failure
Heart failure occurs when the heart cannot pump enough blood to meet the body's metabolic demands, or can only do so at elevated filling pressures. It is classified by ejection fraction:
- HFrEF (EF < 40%): Heart failure with reduced ejection fraction. The heart muscle is weak and cannot contract forcefully enough. Also called systolic heart failure. Treated with ACE inhibitors/ARBs, beta-blockers, mineralocorticoid receptor antagonists, SGLT2 inhibitors, and in some cases devices (ICD, CRT).
- HFmrEF (EF 41–49%): Heart failure with mildly reduced ejection fraction. An intermediate category with features of both HFrEF and HFpEF.
- HFpEF (EF ≥ 50%): Heart failure with preserved ejection fraction. The heart contracts normally but is stiff and does not relax or fill properly. Also called diastolic heart failure. More common in older adults, women, and those with hypertension, obesity, or diabetes. Treatment focuses on managing symptoms and comorbidities; SGLT2 inhibitors have shown benefit.
Common causes of heart failure include coronary artery disease, hypertension, cardiomyopathy, valvular heart disease, and myocarditis. Symptoms include dyspnea (shortness of breath), fatigue, exercise intolerance, fluid retention (edema), and orthopnea (difficulty breathing when lying flat).
Frequently Asked Questions
What is a normal stroke volume?
A normal resting stroke volume is approximately 60–100 mL per beat. When indexed to body surface area (stroke volume index), the normal range is 33–47 mL/m². Stroke volume can increase significantly during exercise, reaching 150–200 mL in trained athletes.
What is the difference between stroke volume and cardiac output?
Stroke volume is the amount of blood pumped per heartbeat, while cardiac output is the total amount pumped per minute. Cardiac output = Stroke volume × Heart rate. For example, a stroke volume of 70 mL at a heart rate of 72 bpm gives a cardiac output of 5,040 mL/min (approximately 5 L/min).
Why is body surface area used for indexing?
Body surface area (BSA) is used to normalize cardiac parameters because larger people naturally have larger hearts and higher cardiac output. Indexing to BSA allows meaningful comparison between individuals of different body sizes. The Du Bois formula is the most commonly used method for estimating BSA from height and weight.
What does a low ejection fraction mean?
A low ejection fraction (< 40%) indicates that the heart is not pumping effectively and is a hallmark of systolic heart failure (HFrEF). It is associated with increased risk of arrhythmias, hospitalization, and mortality. An EF below 35% is generally the threshold for considering an implantable cardioverter-defibrillator (ICD) to prevent sudden cardiac death.
Can ejection fraction improve?
Yes. With appropriate treatment (medications, lifestyle changes, and sometimes procedures), ejection fraction can improve. This is particularly true when heart failure is caused by a reversible condition (e.g., tachycardia-induced cardiomyopathy, alcoholic cardiomyopathy, or stress cardiomyopathy). Guideline-directed medical therapy for HFrEF can improve EF by 5–15% or more in many patients.
How is stroke volume measured clinically?
Stroke volume can be measured by several methods: echocardiography (most common, using the biplane method of discs or LVOT method), cardiac MRI (most accurate), cardiac catheterization (thermodilution or Fick method), and non-invasive cardiac output monitoring devices. Each method has its own advantages and limitations in terms of accuracy, invasiveness, and availability.