Stroke volume equation
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Stroke Volume Equation: A Comprehensive Overview
Introduction to Stroke Volume Calculation
Stroke volume (SV) is a critical parameter in cardiovascular physiology, representing the volume of blood pumped by the heart with each beat. Accurate measurement of SV is essential for diagnosing and managing various cardiac conditions. Several methods and equations have been developed to estimate SV, each with its own theoretical basis and practical applications.
Thoracic Electrical Bioimpedance Method
A novel stroke volume equation for thoracic electrical bioimpedance has been proposed, which models the thoracic volume conductor as a frustum (truncated cone) rather than a cylinder. This new equation eliminates the specific resistivity of blood as an independent variable and uses a percentage of overall body height instead of a measured thoracic length. Additionally, a new term is introduced to scale the equation for deviations from ideal body weight.
Starr's Equation and Its Limitations
Starr's formula, which derives stroke volume from pulse pressure, diastolic pressure, and age, has been experimentally tested. However, the measured stroke volume often differed significantly from the values calculated using this formula. The study highlighted the theoretical limitations of applying multiple regression equations to this material, indicating that Starr's equation may not be reliable for clinical use.
Echocardiographic Methods
Echocardiography, particularly M-mode and two-dimensional echocardiography, is widely used for estimating left ventricular (LV) chamber and stroke volume. A study developed regression equations to improve SV estimation from linear LV dimensions measured by M-mode echocardiography. The new method showed better performance compared to the Teichholz formula, with a simplified calculation process that can be performed using a pocket calculator.
Pulse Contour Methods
Pulse contour methods estimate stroke volume from the systolic area of the central aortic pressure curve. One study tested this method in anesthetized dogs and found a high correlation between SV measured by the pulse contour method and an electromagnetic flowmeter. However, the need for frequent recalibration and the variability in calibration constants limit its clinical applicability .
Oxygen Pulse During Exercise
Another approach involves estimating stroke volume from oxygen pulse (OP) during exercise. This method uses a model of arterio-venous oxygen difference (a-vO2D) estimation and has shown reasonable agreement with impedance cardiography measurements. This model may be applicable, particularly in healthy populations.
Impedance Cardiography
Impedance cardiography (ICG) is another non-invasive method for estimating stroke volume. A new equation implementing a square root transformation for dZ/dtmax/Z0 and a mass-based volume conductor has shown superior performance compared to existing transthoracic impedance techniques. This new equation provided better agreement with thermodilution cardiac output measurements.
Conclusion
Various methods and equations for estimating stroke volume have been developed, each with its own advantages and limitations. The choice of method depends on the clinical context, the need for accuracy, and the available resources. While thoracic electrical bioimpedance and echocardiographic methods offer non-invasive and relatively accurate measurements, pulse contour methods and oxygen pulse estimation provide alternative approaches that may be useful in specific scenarios. Further research and refinement of these methods are essential to improve their clinical applicability and accuracy.
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