The Simpson Method of Stroke Volume Estimation

Measurement of systolic and diastolic volumes of the left ventricle

 
The Simpson method of cardiac output measurement
The Simpson method of cardiac output measurement
The Simpson method of cardiac output measurement
The biplane Simpson method divides the 2D myocardial outlines into slices, and then assumes that each slice width represents the diameter of an ellipse. This way, an estimate of the left ventricular volume is made.


Obviously, the left ventricle is not elliptical in crossection, and thus an error is introduced into this calculation of volume.

    • Obviously, (end diastolic volume-systolic volume) = stroke volume.
    • This is also how we arrive at the ejection fraction.
    • Stroke volume and heart rate will give you cardiac output.
    • The devil is in the measuring of the volumes.
    • It is difficult to estimate volume from 2D echo, because the algorithms for measuring it are inaccurate.  The Simpson method is one such beast, and it is frequently wrong.  
    • You need good pictures to estimate the endocardial border, and you need to be sure you have the left ventricle in its widest crossection, both in systole and diastole.
    • This method does not agree well with thermodilution methods. Probably because its crap.

The next step is to measure the same volumes in systole, and subtract it from the end-diastolic volume. This gives you the stroke volume of one heartbeat. Hopefully, that was a representative heartbeat. Multiplying this stroke volume by the heart rate gives you the cardiac output.

References

From Bersten and Soni’s” Oh's Intensive Care Manual”, 6th Edition;

The echo stuff comes from Scott D. Solomon’s Essential Echocardiography