Applications of POCUS for the carotid artery and an introduction to spectral Doppler
The carotid artery is the first cardiovascular chapter because it is an ideal way to begin understanding spectral Doppler. However, this exam has utility for cardiovascular risk assessment and for estimating volume responsiveness in patients.
Screening for asymptomatic carotid stenosis is not routinely done in our clinic. However, carotid atherosclerotic plaque is easily detected. Here is a depiction of the carotid artery anatomy in the neck. The internal carotid usually remains larger and more posterior than the external carotid.
A bilateral, longitudinal carotid survey up through the bulb is a reasonable and efficient approach. Plaque is hyperechoic and usually most severe within 2 cm of the carotid bulb. This survey is done using a linear probe with settings optimized for the carotid. In some patients, plaque can be easily seen using a magnified, transverse approach. Below is a modest plaque in a longitudinal carotid just before the bulb.
The presence of carotid plaque confirms non-coronary atherosclerotic disease which should put a patient in a higher risk category, usually requiring maximum secondary prevention therapy. Thus, patients at intermediate risk for atherosclerotic vascular disease could have a carotid survey first, instead of a coronary CT study. If plaque is seen, the coronary CT might not be needed for risk stratification. If plaque is not seen, the CT could still be done. In this situation, ultrasound is specific but not completely sensitive.
AN EXERCISE IN SPECTRAL DOPPLER
Spectral Doppler includes Color Flow Doppler (CF), Power Doppler Imaging (PDI), Pulse Wave Doppler (PW), Continuous Wave Doppler (CW), and Tissue Doppler Imaging (TDI). CF and PW Doppler will be explored in this chapter and these are key for many types of cardiovascular exams.
Image the right carotid transversely just above the clavicle, optimizing the carotid image for the machine. Then, rotate the probe so the indicator is cephalad and optimize the longitudinal view of the carotid, several cm below the bulb. The carotid in this location will be horizontal on the screen.
Activate CF: Make the sector box optimum size. The greater the width of the box the lower the frame rate for the image. The height of the box has minimal effect on the frame rate with an optimized B-mode image. So, make the box clearly taller than the carotid and then cover only as much width as you need. Optimizing the size of the CF sector box applies to any use of CF. The CF sector box needs to be tilted +20 to enhance the appearance of red flow coming from the heart on the right and the preset of some machines may do this automatically. Adjust the GAIN so color flow fills the vessel without any speckling out beyond the vessel. The little areas of blue that may appear in the red is “aliasing” from higher velocity flow. CF aliasing will be important in subsequent cardiac chapters.
Activate PW: PW is usually used to measure the velocity of blood flow normally seen in the body (< 1.5 m/sec). The cursor/gate needs to be as parallel as possible to the flow or velocity is underestimated. Use the machine controls to get the gate in the middle of the vessel, parallel to flow.
Activate the PW tracing and be sure the complexes are fully on the screen (not aliased). The tracing below was aliased so the baseline needed to be brought towards the bottom of the screen.
If the carotid complexes are too small or too large, the scale needs to be adjusted. Next is an optimized tracing of carotid complexes with PW.
Changing the sweep speed: The sweep speed is the recording speed. A fast sweep speed spreads things out, showing fewer complexes but allowing greater detail of individual complexes to be seen. A slow sweep speed bunches complexes up so we can see many of them at one time. The carotid flow time discussed next needs a medium or fast sweep speed, but s slow sweep speed is needed to assess carotid velocity variation with respiration in suspected pericardial tamponade.
CAROTID FLOW TIME (CFT)
The carotid flow time is a measure of LV ejection time, a classical measure of LV function that is dependent on heart rate, preload, afterload, and contractile state. It is optimal to capture this measure at end expiration. The complexes in a tracing need to show good dicrotic notches, which indicates aortic valve closure. When these are seen, freeze at end-expiration. The middle complex in the tracing above is good.
Activate a caliper that measures time and put a first caliper at the start of the carotid upstroke and then an ending caliper at the beginning of the next carotid upstroke. This is the CYCLE TIME (CT). In this next example, the CT was 0.815 seconds. The heart rate (HR) of this beat is 60/0.815 = 74 bpm and some machines calculate this automatically.
Then, measure the interval from the start of the carotid upstroke to the bottom of the dicrotic notch, which is the raw CFT. The example raw CFT of 305 msec is then corrected to a heart rate of 60 bpm using the formula:
Corrected carotid flow time (CFTc) = raw CFT + [1.29 x (HR – 60)].
The example patient’s CFTc = 305 + [1.29 x (74 – 60)] = 323. Some machines perform this calculation automatically.
One series of 142 normal volunteers showed a 95% normal range of about 280 – 360 for the CFTc. Importantly, a 45 degree passive leg raise for 1 minute caused a mean change of about 2% in the CFTc in these normal adults, although a few showed greater changes because the CFTc is sensitive to changes in volume status. Normal individuals at the COC clinic have also shown good response to passive leg raise when they have been less than fully hydrated.
The main proposed use of the CFTc is to detect volume responsiveness with the use of passive leg raise, which requires a helper. It is key that the patient does not help lift the legs but relaxes the abdomen and breathes normally. An induced Valsalva maneuver will blunt the effect of any autobolus. As the increase in the CFTc with leg raise is progressively greater than 5%, the evidence for volume responsiveness strengthens. When a supine CFTc is at the low end of normal or clearly low and the change with passive leg raise is 15% or greater, there is strong evidence for low left atrial pressure that is volume responsive.
If a helper is not available or a passive leg raise is physically difficult for a patient to perform, we have anecdotally noticed that the comparison of a sitting/legs dangling CFTc with a supine CFTc after 1 minute may also give an indication of volume responsiveness.
The published work with LV ejection time would predict that severe aortic stenosis could prolong the CFTc (pulsus tardus). In addition, HFREF patients can develop shorter LV ejection times while HFPEF patients may have longer ejection times and the CFTc might mirror these effects. More importantly, heart failure patients being diuresed could have CFTc measurements to help identify when they become volume responsive and not in need of further diuresis.