Though  it has bees suggested that Doppler echocardiography may be useful for non-invasive determinations of  stroke volume and cardiac output, it  should be recognized that th4re are limitations and  sources of error in  the technique. Because of these limitations, Doppler cardiac output calculations have been accepted especiall$ for detecting relative changes in  flow volume. Doppler cardiac oQtput  calculations are based  on  a series of  assumptions about the geometry of the aorta (or other site of velocity measurement with  in the heart)  and about the flow velocity profile in that vessel. Two primary assumptions abouti the hemodynamics of flow are:

1)  Flow  is occurring in a rigid, circular tube

2)  There is a unidorm velocity profile across that tube The greatest souroe of error in this technique is the measurement of the LVOT
area.

Difficulties associated with accurate aortic  flow are measurements and calculations

The aorta is elastic and changes size over the course of a cardiac cycle LVOT  area increases with  increased flow volume The diameter and cross sectional area may vary at different measurement sites

Any error  in  a diameter measurement will be magnified when  it  is squared for an area calculation. As the diameter of  the aorta decreased a measurement error will  increase the degree of error  in  the area calculation If  the vessel area  is planimetered from a two-dimensional view, error may occur due  to obliquity of  the ultrasound beam

The blood flow velocity measurement is the second component of  the cardiac output calculation. Since flow velocities are not uniform across the vessel, which spectral velocities should be used to represent aortic velocity: peak velocity, mean  velocity, mode velocity? The most  important aspect of the velocity measurement is  thorough sampling technique, taking care to orient the ultrasound beam as parallel to  flow as possible  (as indicated by  the strength of  the flow  signal and the peak velocity measurement).