Measurement of systolic and diastolic volumes of the left ventricle
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.