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Real-time images of LV filling and ejection permit qualitative,
immediate assessment of marked changes in cardiac output. However, with PW and CW
Doppler, TEE can quantify cardiac output. A Doppler measurement of blood flow velocity
is combined with a two-dimensional measurement of cross-sectional area:
Cardiac output = VTI · CSA · cos theta
· Heart rate
where VTI is the velocity time integral (the area under the Doppler-derived velocity-versus-time
curve per systole), CSA is the cross-sectional area through which the velocity passes,
and cos theta is the cosine of the angle between the ultrasound beam and blood
flow (usually, the angle is assumed to be zero degrees so that this factor equals
unity and can be disregarded). An initial study of flow in the main pulmonary artery
and across the mitral valve versus thermodilution yielded somewhat disappointing
results.[47]
Subsequent studies excluding patients
with tricuspid regurgitation and using other TEE cross sections demonstrated that
TEE can estimate cardiac output reliably.[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
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