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).