Pedoff Transducer Assessment of Aortic Stenosis
©Walter Rasmussen, R.D.C.S.
Using the Pedoff Transducer for Aortic Valve Disease:
The Pedoff probe can be helpful for obtaining CW Doppler because of its small footprint and its highly specialized design particularly when a velocity profile is not clearly defined with the imaging probe. Since the Pedoff probe does not provide a reference image, novice sonographers need a considerable amount of practice in order to develop sufficient skill and confidence in its use.
For example, if the imaging probe does not produce the full spectral Doppler envelope of tricuspid regurgitation, instead of moving immediately to using saline contrast, the sonographer should first try the Pedoff probe because it might give the signal enough of a boost to avoid subjecting the patient the discomfort of inserting an intravenous line and injection.
The Pedoff probe is required to be used as part of a thorough evaluation of aortic stenosis or an aortic prosthesis.
The best placement of the Pedoff probe is most easily found by first locating the area of interest with the two-dimensional probe, noting the transducer position and angle, and then carefully exchanging it for the Pedoff probe.
Guiding the Pedoff transducer requires mental visualization of where the cardiac anatomy is in relationship to where the probe is placed on the chest wall.
More information on using the Pedoff probe can be found in the, Using the Pedoff Probe section of this website.
While using the Pedoff probe from the apical window, identify the Doppler pattern which first comes into view and what its pattern is typical of.
If the spectral Doppler pattern is typical of the mitral valve, angle slightly medially and anteriorly to find the aortic valve.
From the aortic valve, the sonographer can angle the probe medially and anteriorly again to obtain the tricuspid valve or posteriorly and laterally for the mitral valve.
Because the tricuspid and mitral valve patterns look similar, their identity should be confirmed by checking their physical relationship to the aortic valve as well as their peak velocities. The mitral valve will usually have higher diastolic and systolic velocities.
The Suprasternal Notch:
The primary use of the Pedoff probe in the suprasternal notch position is for recording the velocity profile and pressure gradient in aortic stenosis and prosthetic aortic valves. Accurate assessment of stenosis is dependent upon obtaining the best possible alignment of the ultrasound beam with the jet, which can emerge from the valve at an unpredictable angle and so needs to be interrogated from more than one perspective. Aortic Stenosis in one patient from different transducer positions.
The example above demonstrates that it is not uncommon to see significant differences in velocity from different sites on the chest on an individual patient and therefore, aortic stenosis and prosthetic valves cannot be assumed to be fully assessed if examined from the apical window exclusively.
Using the Pedoff Probe is perhaps the most difficult skill to master for the Cardiac Sonographer. Additionally, each patient is unique in window location and jet direction. In many patients, not all Pedoff windows will yield a signal and the Sonographer must learn when a site has been thoroughly investigated and it’s time to move on. ASE guidelines require the Pedoff probe be used for a complete evaluation of aortic stenosis.
Technique: Placing the probe at the suprasternal notch, mentally visualize the position of the ascending aorta, usually posterior to the sternum, directing the face of the probe steeply straight downward. While observing the CW spectral display and listening to the blood flow, try to adjust the angle of the transducer to find the triangular velocity profile typical of the aortic valve. Attempt to make the signal become stronger and the velocity higher by tilting in various directions and/or turning the transducer minutely clockwise and counter-clockwise. The spectral display will become the main focus and the audio, somewhat less important as the stenotic jet is localized.
As stated earlier in this book, listening to the audio signal is a very useful component to producing top quality Doppler recordings, particularly while investigating aortic stenosis. To the skilled Sonographer, the audio signal actually provides a guide to the positioning and angulation of the transducer and often times, with careful listening; a higher velocity can be obtained using this feature than by relying-upon the spectral Doppler display alone. For this reason, Doppler audio should never be disabled during an echocardiography exam, much as an automobile driver would be at a disadvantage if there were no audio clues to aid in driving the car through heavy traffic.
The strength of the signal is conveyed through the loudness of the sound and confirmed by the brightness of the spectral display, helping to position the CW cursor within the center of the aortic jet. Experience really counts in obtaining quality spectral Doppler recordings of aortic stenosis because it is easy to obtain a blunted peak velocity, which may result in underestimation of peak and mean pressure gradients.
Because the valve is at a distance of five to ten centimeters or more from the transducer, once the aortic valve Doppler signal is located, only a tiny amount of adjustment is necessary for refinement. As the exam proceeds, the signal will become very strong, with the spectral display showing a well-defined edge with minimal adjustment of gain and dynamic range. If the signal remains very faint, the gain may have to be adjusted very high but this means that there could be a better position for the transducer and further searching is necessary. Once the best signal is obtained, the gain and dynamic range or compression should be adjusted so that faint speckling from background noise is recorded in order to assure that the edge of the velocity profile is not being dialed-out.
It is advisable that the novice Sonographer practice the use of the Pedoff probe as much as possible. It is not too much more effort to spend a little extra time on patients to gain competency or stay in practice with the Pedoff probe however, it is important to know that normal aortic valves are much more difficult to record from the suprasternal notch and right parasternal positions than those that are stenotic.
Right Supra-Clavicular Fossa Window:
The right supraclavicular fossa window is found at the junction of the superior margin of the clavicle and the neck, just to the right of the suprasternal notch.
There are instances when the right supra-clavicular fossa will provide the strongest signal and the highest velocity when assessing severity of aortic stenosis and therefore, it should be made a routine part of the exam. This is particularly helpful if the suprasternal notch did not yield a good velocity profile because it often yields good Doppler data.
At this site, other pulsatile and non-pulsatile blood flow from arteries and veins supplying the head and right arm will be encountered and the sonographer needs to know how to distinguish them from aortic valve blood flow. Prosthetic valves will typically have lower velocities than aortic stenosis; therefore, look for characteristics of valvular motion such as opening and closing spikes and other mechanical noises on the spectral Doppler display.
Right Parasternal Pedoff Window:
The right parasternal area, from the clavicle to the 6th intercostal space provides one of the best sites for evaluation of aortic stenosis however, it can be one of the most difficult to locate.
Although the right parasternal aortic stenosis Doppler can be found with the patient lying supine, the best position is with the patient lying on their right side. It is recommended that the sonographer stand for this part of the exam in order to avoid undue stress on the shoulder, hand and back.
The key aspects to right parasternal Doppler of the aortic valve are:
- The patient is positioned steeply on their right side, supporting their back against a pillow in order to help relax the chest. The sonographer can brace the pillow by leaning in to it while standing behind to the patient.
- Move the patient’s arm out of the way by positioning it upward, resting on the table with the elbow bent and hand slipped under the pillow. This flattens the pectoralis muscle, making beam penetration easier.
- Start by placing the transducer at the 3rd or 4th right intercostal space. Angle and rotate the transducer toward the left, exploring shallow and steep angles while mentally visualizing where the aorta lies in the chest. Explore each intercostal space moving up the chest after sliding laterally along each intercostal space until the Doppler signal is found with the Doppler audio and spectral display.
- Expand the search to lower than the 4th intercostal space if the previous attempts were unsuccessful. Turning the patient more steeply on their side can sometimes make a huge difference in finding the aortic signal.
- Exhalation can sometimes improve the quality of the Doppler signals.
- If no Doppler signal is found from the above techniques, it is possible to briefly use the imaging transducer to locate the aortic valve and then, noting the transducer position and angle, exchanging it for the Pedoff transducer, directed to the same area.
- A notation should be made if there was an unsuccessful result to the right sternal border view.