Mid and high end veterinary ultrasound machines invariably come with Colour Doppler and Pulsed Wave (PW) Doppler. They frequently come with Continuous Wave Doppler capability as well, although this is usually only active on phased array probes. Here, we look at the main differences between pulsed and continuous wave dopplers, as well as their different and complimentary uses.
Pulsed Wave Doppler:
When you enter into Pulsed Wave (PW) mode, you will be able to move the sampling area (usually a square or rectangular box along a single line) over the region you wish to interrogate. Pressing ‘update’ or your PW activation button again will usually begin the sampling process. The machine will then display a waveform (either alone or below a miniaturised B-mode image, known as duplex scanning), which is a plot of the blood flow velocities present within your sample region.
It is good practice to adjust your baseline (there will be a ‘baseline’ control somewhere on your keyboard or touch screen) and scale (sometimes referred to as “PRF” for “pulse repetition frequency”) to make the most of your area of interest and avoid aliasing. If you are looking at flow towards the probe (coded red in Colour Doppler mode) – mitral inflow, for example – you will want to bring your baseline to the bottom of the scale, shown below:
If you are looking at flow away from the probe (coded blue in Colour Doppler mode) – for example, through the left ventricular outflow tract in the 4 chamber view – you will want your baseline towards the top of the scale so that the waveform can fill up the scale below it, shown below:
The scale should be adjusted so that the waveform fills the area almost completely.
In other words, don’t have your baseline in the middle of the scale, with a diminutive waveform from which very little can be seen, let alone measured.
As the name suggests, pulsed wave Doppler involves sending a receiving a pulse. It is therefore limited in the velocities it can accurately sample by immutable physical principles, primarily, the speed of sound in soft tissue. You can maximise the highest velocities you are able to measure by optimising your scale or PRF (pulse repetition frequency) and adjusting your baseline, but there will still be an upper velocity limit above which pulsed wave Doppler cannot sample.
Continuous Wave Doppler
Continuous Wave (CW) Doppler should be used when velocities are likely to exceed the limitations of PW Doppler. For example, it is common to take a Pulsed Wave sample just before the aortic and pulmonary valves, and then align a Continuous Wave through the valve. For a normal valve, the two measurements will not be dramatically different, but in a stenotic valve, the velocity through the valve will clearly be a lot higher than the sample taken just before it.
Another very useful application of CW Doppler is in measuring the gradient of the tricuspid regurgitation (TR). It can be difficult to obtain a complete TR trace in healthy hearts, but in volume or pressure-loaded right ventricles, it is fortunately very simple to obtain as there is invariably tricuspid annular dilatation and therefore significant secondary regurgitation. Secondary pulmonary hypertension is common in dogs with degenerative mitral valve disease.
From Yuill & McGrady (1991): Doppler-derived Velocity of Blood Flow across the Cardiac Valves in the Normal Dog. Canadian Journal of Veterinary Research, 55(185-192).
Tricuspid regurgitation can be found from two views in the dog – the apical four chamber, and the right parasternal short axis (shown above). It is good practice to use Colour Doppler to help you to visualise the regurgitant jet, and therefore to align your Doppler beam through it.
Tip: Look for the area of flow convergence on the ventricular side of the valve. If you align your Doppler beam through this region, rather than looking for the jet on the atrial side of the valve, you are more likely to obtain a full trace. Remember you may need to go off-axis in order to find the optimal alignment.
Example:
In the above CW trace, the peak velocity of the TR is 2.4m/s (240cm/s). We can convert the peak velocity into a pressure gradient using the simplified Bernoulli equation, 4V² (where V is velocity). Therefore, 2.4m/s corresponds to a pressure gradient of 23mmHg. By adding on an estimated right atrial pressure (estimate 5mmHg for a normal animal, up to 15mmHg for an animal in right heart failure), it is possible to estimate pulmonary artery systolic pressure. In this example, assuming a right atrial pressure of 5mmHg, estimated PASP is 27mmHg.
Doppler-derived estimations of right ventricular systolic pressure / pulmonary artery systolic pressure are non-invasive and accurate. Estimating right atrial pressure can be one source of error, given its qualitative nature, and some veterinarians prefer to simply work off the principle that a peak TR velocity below 3m/s is invariably within the normal range for a dog, with anything above indicating elevated pulmonary pressures.
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