Editing Cardiac output (section)

====Echocardiography====
[[Echocardiography]] is a non-invasive method of quantifying cardiac output using ultrasound. Two-dimensional (2D) ultrasound and Doppler measurements are used together to calculate cardiac output. 2D measurement of the diameter (d) of the aortic annulus allows calculation of the flow cross-sectional area (CSA), which is then multiplied by the VTI of the Doppler flow profile across the aortic valve to determine the flow volume per beat ([[Stroke Volume|stroke volume]], SV). The result is then multiplied by the heart rate (HR) to obtain cardiac output. Although used in clinical medicine, it has a wide test-retest variability.<ref>{{cite journal | vauthors = Finegold JA, Manisty CH, Cecaro F, Sutaria N, Mayet J, Francis DP | title = Choosing between velocity-time-integral ratio and peak velocity ratio for calculation of the dimensionless index (or aortic valve area) in serial follow-up of aortic stenosis | journal = International Journal of Cardiology | volume = 167 | issue = 4 | pages = 1524–31 | date = August 2013 | pmid = 22575631 | doi = 10.1016/j.ijcard.2012.04.105 }}</ref> It is said to require extensive training and skill, but the exact steps needed to achieve clinically adequate precision have never been disclosed. 2D measurement of the aortic valve diameter is one source of noise; others are beat-to-beat variation in stroke volume and subtle differences in probe position. An alternative that is not necessarily more reproducible is the measurement of the pulmonary valve to calculate right-sided CO. Although it is in wide general use, the technique is time-consuming and is limited by the reproducibility of its component elements. In the manner used in clinical practice, precision of SV and CO is of the order of ±20%.{{citation needed|date=October 2014}}