4.2.7 The use of Doppler echocardiography
In a clinical setting, DE is primarily used for confirming a diagnosis of valvular incompetence based on auscultatory findings, for mapping the area of the jet or regurgitant blood in order to estimate the severity of the condition, and for measuring the velocity of blood shunting through a VSD in order to determine the pressure gradient between the ventricles.
In a research setting it is a very useful, non-invasive method of estimating stroke volume and cardiac output. Further information on the use of DE for haemodynamic investigations is found in specialist texts and references given in the list of further reading.
DE allows identification of the timing of blood flow during the cardiac cycle if an ECG is recorded simultaneously. It enables the echocardiographer to deterthe direction of blood flow, provided that aliasing does not occur (see section 4.2.1). This is usually sufficient information to confirm the diagnosis. Using pulsed-wave Doppler echocardiography this can be combined with spatial information from the 2DE image so that the location of blood flow within the heart can be documented.
The duration of the jet may not appear to last throughout the period during which regurgitation takes place. This may be because the jet moves out of the line of the ultrasound beam during the cardiac cycle. In addition, the heart swings towards the apex during systole so the position of the sample volume relative to the valve and chamber alters during the cardiac cycle.
Pulsed-wave Doppler mapping
Mapping of the area of the jet within the receiving chamber is a time consuming and laborious process, which must be thoroughly performed if it is to be of value. DE in the horse is hampered by the limited number of views which can be obtained. Satisfactory 2DE and M-mode views can be obtained from parasternal views because the chambers lie perpendicular to the line of the beam and therefore return strong signals. When using DE to measure blood flow, the line of the beam should be as close to parallel with blood flow as possible, and this is seldom achievable. A second problem is that many jets of regurgitant blood flow, particularly mitral regurgitation, do not flow in predictable patterns and can be difficult to detect. With experience, auscultation is probably just as accurate a method of identifying regurgitant blood flow. Nevertheless, PWD mapping is a useful method of assessing the severity of valvular disease.
For evaluation of MR, a left parasternal long-axis view is obtained, and the transducer is then moved slightly ventral and angled steeply dorsal so that the LA is in the far-field and the MV is as near perpendicular to the valve as possible. An angle of around 45o to the valve is usually achievable. The sample volume is then placed behind the valve and moved along the atrial side of the valve until a regurgitant jet is detected. The area of the origin of the jet is mapped out if possible, and then the sample volume is moved gradually further and further into the atrium. The process should be repeated with the image plane slightly altered, remembering that the jet and the atrium are three-dimensional. Using colour-flow Doppler, the process is far less time-consuming, but multiple image planes should be examined. Jets of MR often run up the walls of the atrium, or somerun straight into it. The echocardiographic features of MR are summarised in Table 4.9.
For examination of tricuspid regurgitation (TR), a right parasternal long-axis view optimised for the LV outflow tract is obtained. The transducer is then tilted slightly dorsal and the sample volume is placed on the atrial side on the tricuspid valve. Jets usually run down the atrial wall where it borders the aorta. The echocardiographic features of TR are summarised in Table 4.10.
Mapping of jets of AR is less helpful than for AV valve regurgitation. It is difficult to get a good angle on the valve and the whole of the LV at the same time. A tilted left parasternal long-axis view with the transducer aimed cranially and dorsally is best. The echocardiographic features of AR are summarised in Table 4.11.
One of the problems in interpreting DE is that most valves leak to a degree in normal horses. Jets of regurgitant flow which are detected at or close to the valve in early systole (AV valves) or diastole (semilunar valves) can be regarded as normal. There is no clear cut-off point between normality and abnormality, particularly in the case of tricuspid regurgitation. As a general rule, significant jets are much more extensive and are more easily detected. They may be graded from mild to severe on the grounds of the extent of the jet (Figure 4.30). This is an estimate rather than a quantitative method because other factors may influthe size of the jet, including the control settings of the ultrasound machine and the quality of the Doppler unit.
Estimation of pressure gradients
Measurement of the velocity of a jet allows the pressure gradient between the two chambers between which the jet is flowing to be estimated. For this calcua simplified version of the Bernoulli equation is used (Table 4.4). If the pressure within one chamber is known, the pressure within another can be estimated if a regurgitant jet of blood flows between them. From a practical standpoint, this is seldom helpful in the case of assessment of the severity of valvular regurgitation. However, it is very helpful in evaluating the haemodyeffects of VSDs (see section 5.5.5).