Stroke volume is the blood volume the heart’s left ventricle ejects with each beat. It is fundamental to understanding circulatory efficiency and heart pumping effectiveness.
Understanding Key Volumes
Directly calculating stroke volume requires understanding two primary volumes: end-diastolic volume and end-systolic volume. They reflect ventricular blood volume at different cardiac cycle stages.
End-diastolic volume (EDV) is the blood volume in the left ventricle at the end of diastole, the heart’s filling phase. It is the maximum blood volume the ventricle holds before contraction.
Conversely, end-systolic volume (ESV) is the remaining blood volume in the left ventricle at the end of systole, the heart’s ejection phase. This is the minimum blood volume left after pumping.
Direct Calculation Method
Stroke volume is directly calculated using these two ventricular volumes. Stroke volume (SV) is determined by subtracting the end-systolic volume (ESV) from the end-diastolic volume (EDV): SV = EDV – ESV. For example, if the left ventricle has 120 milliliters (mL) of blood at the end of diastole (EDV) and 50 mL remains post-systole (ESV), the stroke volume would be 70 mL (120 mL – 50 mL = 70 mL).
In clinical settings, EDV and ESV are measured using cardiac imaging techniques. Echocardiography, a common non-invasive method, uses sound waves. Magnetic resonance imaging (MRI) and computed tomography (CT) scans also provide ventricular volume measurements. These technologies allow healthcare professionals to assess heart performance by observing blood held and ejected.
Indirect Calculation Method
Stroke volume can also be determined using cardiac output and heart rate. Cardiac output (CO) is the volume of blood pumped per minute. Heart rate (HR) is the beats per minute. The relationship is: Stroke Volume (SV) = Cardiac Output (CO) / Heart Rate (HR).
This formula is derived from CO = SV x HR. For instance, if cardiac output is 5 liters per minute (L/min), or 5000 mL/min, and heart rate is 70 beats per minute (bpm), stroke volume would be approximately 71.4 mL per beat (5000 mL/min / 70 bpm ≈ 71.4 mL/beat). This method is useful when cardiac output and heart rate are measurable, as with continuous monitoring.
Why Stroke Volume Matters
Stroke volume is a cardiovascular health indicator. Deviations from a normal stroke volume can signal underlying health issues. Normal stroke volume values vary from 48.2–114.3 mL for individuals aged 18 to 29 years.
An abnormally low stroke volume can indicate the heart is insufficiently pumping blood to meet body demands, characteristic of conditions like heart failure. Conversely, an unusually high stroke volume might suggest issues such as hypertension or increased blood volume. Monitoring stroke volume helps healthcare professionals evaluate heart pumping efficiency and circulatory function, aiding in diagnosis and management of cardiac conditions.