Understanding Stroke Volume: Calculation Methods Explained

Stroke volume is a crucial indicator of health and efficiency, referring to the volume of blood pumped by the heart with each contraction. It is an important determinant of cardiac output and is used to calculate ejection fraction. Calculating stroke volume requires knowledge of various physiological parameters, including heart rate, end-diastolic volume, and end-systolic volume. This can be done through medical imaging techniques such as echocardiography or invasive procedures like thermodilution. The stroke volume formula is given as SV = EDV - ESV, where SV represents stroke volume, EDV is end-diastolic volume, and ESV is end-systolic volume.

End-diastolic volume

EDV is typically measured using imaging techniques such as echocardiography or cardiac catheterization. Echocardiography is a non-invasive procedure where ultrasound technology is used to create detailed images of a person's heart. Cardiac catheterization, on the other hand, is an invasive procedure where a thin, flexible tube called a catheter is threaded through a large blood vessel and into the heart. This allows doctors to take blood samples and measure the pressure and oxygen content in the four chambers of the heart.

EDV is an important measurement for evaluating the condition of a person's heart and their general health. It is used to estimate the heart's preload volume, which is how much the cardiac fibres of the ventricle stretch before contraction. Preload cannot be directly measured, so EDV is used as a close estimate.

EDV is also used to calculate stroke volume, which is the volume of blood pumped from the ventricle per beat. The formula for calculating stroke volume is:

> Stroke volume = EDV – End-Systolic Volume (ESV)

Here, ESV refers to the volume of blood remaining in the ventricle at the end of systole, after the heart has contracted.

The normal range for stroke volume in a healthy 70-kg man is approximately 90 mL, with an EDV of 140 mL and an ESV of 50 mL. However, it's important to note that stroke volume can vary based on factors such as age, gender, body size, and cardiac health.

In summary, end-diastolic volume is a critical measurement in assessing cardiac function and plays a vital role in calculating stroke volume, which is essential for understanding the health and efficiency of the heart.

End-systolic volume

ESV is a crucial parameter used for the assessment of cardiac function and the calculation of stroke volume and ejection fraction. It is determined by the sum of the computed left/right ventricular cross-sectional areas determined by the endocardial contours at the end of the systolic ejection phase. Clinically, ESV can be measured using two-dimensional echocardiography, MRI (magnetic resonance tomography), cardiac CT (computed tomography) or SPECT (single-photon emission computed tomography).

The end-systolic volume index (ESVI) is the ESV corrected for the body surface area (BSA). It is calculated as ESVI = ESV/BSA [mL/m2]. Normal values differ for the left and right ventricles depending on gender and age and the imaging modality used. For example, for women aged 16-83 years, the ESVI range is 10-38 mL/m2.

The difference between end-diastolic volume (EDV) and ESV represents the volume of blood ejected in one beat (stroke volume). The stroke volume formula is:

SV = EDV – ESV

This calculation is essential for healthcare professionals in diagnosing cardiac conditions, monitoring patient health, and guiding treatment strategies.

Echocardiography

The parasternal long axis view is used to measure the LVOT diameter. The PW line is then positioned in the apical 5-chamber or 3-chamber view to obtain the PW waveform. The PW gate should be 2-4mm and aligned as closely as possible to the aortic valve without entering the area of flow acceleration. The PW waveform is obtained, and the sample volume is moved towards the aortic valve until flow accelerates, and then slightly away from the valve until laminar flow returns. The area under the curve (Velocity Time Integral – VTI) is then traced, and this value is multiplied by the LVOT area to obtain the stroke volume.

The stroke volume is then multiplied by the heart rate to obtain the cardiac output. This can be further divided by the body surface area to obtain the cardiac index.

Cardiac catheterization

SV = EDV - ESV

This formula calculates the stroke volume by finding the difference between the volume of blood in the ventricles at the end of diastole (EDV) and the volume of blood remaining in the ventricles after contraction (ESV).

The procedure is often used in conjunction with other techniques, such as echocardiography, to comprehensively assess cardiac function and guide clinical decision-making. It is important to note that cardiac catheterization is an invasive procedure and may not be suitable for all patients. However, its direct measurement capabilities make it a valuable tool in specific clinical scenarios.

Thermodilution technique

The thermodilution technique is an invasive procedure used to calculate stroke volume and cardiac output. It involves injecting a known quantity of a cold solution, usually saline, into a central vein, typically the right atrium. The temperature change as the cold solution mixes with the blood in the heart is then measured, usually in the pulmonary artery. This method can be used repeatedly without the accumulation of indicators in the blood.

The thermodilution technique is based on the indicator dilution principle, which states that when an indicator substance is added to circulating blood, the rate of blood flow is inversely proportional to the indicator concentration change over time. The cold solution is injected into the right atrium via the proximal port of a pulmonary artery catheter. The temperature change is then measured by a thermistor in the pulmonary artery, which is a variable resistor where resistance is proportional to temperature. As the resistance changes, a change in voltage occurs, and this generates a temperature curve. The cardiac output can be derived from the modified Stewart-Hamilton conservation of heat equation.

The thermodilution technique is considered the gold standard for cardiac output measurement. It is advantageous because it is intuitive and easy to understand. It allows for instantaneous calculation, integrating all influencing factors and confounders, such as myocardial contractility, characteristics of the vasculature, and ventilatory settings. However, it has some disadvantages, including a time-consuming setup and the requirement for an invasive procedure.

Frequently asked questions