Stroke volume (SV) is the volume of blood ejected by the heart’s ventricle, typically the left, with each beat. It is calculated as the difference between the end-diastolic volume (EDV) and the end-systolic volume (ESV). Measuring SV is a fundamental practice in medicine because it provides a direct assessment of the heart’s pumping efficiency. Understanding a patient’s SV is essential for evaluating cardiac function, determining fluid status, and guiding therapeutic interventions for conditions such as heart failure or shock.
Measuring Stroke Volume Using Foundational Principles
Established methods for determining blood flow, from which stroke volume is derived, rely on principles of mass balance and dilution. The Indicator Dilution technique involves introducing a known quantity of a substance into the bloodstream and observing its concentration change downstream. This concept is most commonly applied through thermodilution, where cold saline is injected into the central circulation via a catheter.
A sensor in the pulmonary or peripheral artery detects the temperature change, creating a concentration-time curve. The indicator’s dilution is inversely proportional to the blood flow, allowing calculation of cardiac output using the Stewart-Hamilton equation. Stroke volume is then calculated by dividing the cardiac output by the heart rate. Although highly accurate and considered a reference standard, this technique requires invasive catheters and carries inherent risks.
The Fick Principle is another basis for measuring cardiac output. It states that the total uptake or release of a substance by an organ is the product of blood flow and the arteriovenous concentration difference of the substance. Applied to the entire body, the substance used is oxygen. Cardiac output is calculated by dividing the body’s total oxygen consumption by the difference in oxygen content between arterial and mixed venous blood.
Once cardiac output is determined, stroke volume is calculated by dividing the output by the heart rate. Although the Fick method provides accurate measurements, the technical challenges and time required to measure oxygen consumption and obtain mixed venous blood samples limit its use in routine clinical monitoring. This mass balance equation remains a gold standard for validating newer and less invasive technologies.
Non-Invasive Measurement Through Imaging
Echocardiography, which uses ultrasound technology, is the most widely used non-invasive tool for assessing stroke volume. The volume method involves two-dimensional (2D) imaging to directly measure the volume of the left ventricle at its fullest (EDV) and its emptiest (ESV). SV is calculated as the difference between these two volumes. This method offers a direct anatomical assessment of the heart’s pumping action and is frequently used for diagnosis.
A more common echocardiographic method uses Doppler technology to measure the velocity of blood flow across the aortic valve. This technique requires measuring the diameter of the left ventricular outflow tract (LVOT) using 2D imaging to calculate its cross-sectional area. The device then measures the Velocity-Time Integral (VTI), which is the distance the column of blood travels with each beat.
The VTI represents the average distance blood travels per cardiac cycle. Multiplying the LVOT cross-sectional area by the VTI calculates the volume of blood ejected per beat, which is the stroke volume. This Doppler-based approach is often preferred because it is less susceptible to geometric assumptions about the ventricle’s shape than the direct volume method.
Magnetic Resonance Imaging (MRI) offers a highly accurate, non-invasive method, often used as a reference standard in research. Specialized phase-contrast MRI sequences directly measure the velocity and volume of blood flow through large vessels, such as the ascending aorta. The technique captures a cross-sectional image and quantifies the flow volume over the cardiac cycle. The total volume of blood passing through the cross-section per beat is the stroke volume. While precise, the high cost and limited accessibility of MRI make it unsuitable for continuous bedside monitoring.
Continuous Hemodynamic Monitoring Techniques
Continuous monitoring of stroke volume is essential for real-time guidance of fluid and drug administration in critical care patients. Pulse contour analysis uses an existing arterial blood pressure waveform to estimate stroke volume on a beat-by-beat basis. Devices analyze the shape of the arterial pressure curve, relying on the principle that the contour reflects the volume of blood ejected by the heart.
Early pulse contour devices required initial calibration against an invasive reference method, such as transpulmonary thermodilution. Newer, uncalibrated systems use advanced algorithms to model the relationship between pressure and flow, providing continuous stroke volume data. This method is favored for its rapid, continuous output but requires an invasive arterial line for pressure sensing.
Thoracic bioimpedance and bioreactance offer non-invasive ways to estimate stroke volume using surface electrodes placed on the chest. Bioimpedance devices measure the change in electrical conductivity across the thorax during the cardiac cycle. As the heart ejects blood into the aorta, the change in the volume of conductive blood alters the electrical impedance, which is used to calculate the stroke volume.
Bioreactance is a distinct technique that measures the phase shift of an oscillating electrical current as it traverses the chest, rather than the amplitude change. Both bioimpedance and bioreactance track the movement of blood volume within the chest, allowing for continuous, non-invasive monitoring. These technologies are simpler to use than imaging modalities and provide immediate data for guiding fluid management.