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.
Carotid stenosis quantification is occasionally needed in clinic (see section at the end of this chapter). However, detecting carotid atherosclerotic plaque is routine and important. Here is a depiction of the carotid artery anatomy in the neck. The internal carotid remains larger and more posterior than the external carotid.
For a carotid-specific exam, a patient can be examined standing, which is faster and easier for the patient than supine positioning. However, if the heart will also be examined, the carotid exam is performed with the patient supine. A bilateral, longitudinal carotid survey with the linear probe up through the bulb is a reasonable and efficient approach. Scanning from a more lateral position (often through the internal jugular vein) lets the probe slide further cephalad before being blocked by the mandible. This allows a better view of the internal carotid artery. Plaque is hyperechoic and usually most severe within 2 cm of the carotid bulb. In some patients, plaque can be easily seen using just a transverse survey. Below is a moderate plaque in a longitudinal carotid just at the beginning of the internal carotid. Stenosis was minimal with this plaque.
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
Ultrasound Doppler includes Color Flow Doppler (Color), Power Doppler Imaging (PDI), Pulse Wave Doppler (PW), Continuous Wave Doppler (CW), and Tissue Doppler Imaging (TDI). Color and PW will be explored in this chapter and are key for some cardiovascular exams. PDI, CW, and TDI will be introduced in subsequent chapters.
Image the right carotid transversely just above the clavicle, optimizing the carotid image. 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 Color: The default Color sector box width for the carotid setting on Venue is appropriate for most carotids. The greater the width of the box the lower the frame rate for the Color image. The height of the box has minimal effect on the frame rate with an optimized B-mode image. Optimizing the width of the Color sector box applies to any use of Color. The default carotid Color sector box is tilted +20 to enhance the display of red flow coming from the heart on the right. Adjust the GAIN so Color fills the vessel without any speckling out beyond the vessel. Here is a clip of optimized Color in a longitudinal common carotid. The flash of yellow with each beat is “aliasing”, which will be discussed in future chapters.
Activate PW: PW is used to measure the velocity of blood flow normally seen in the body (< 2.0 m/sec). The gate line needs to be as parallel as possible to the flow or velocity is underestimated. Move the gate and use angle correction to get the gate in the middle of the vessel, parallel to flow. Here is an optimized PW tracing showing appropriate gate placement in the upper B-mode image.
Make sure the PW complexes occupy most of the graph. The Baseline can be moved with a finger and the Scale adjusted to change the size of the complexes. The tracing above is excellent. The peak velocity of a complex could have been measured and would have been about 1.0 m/sec.
Changing the sweep speed: The sweep speed is the recording speed. A fast sweep speed spreads things out, showing fewer complexes but greater detail of individual complexes. A slow sweep speed bunches complexes up so we can see many of them at one time, but without detail. The carotid flow time discussed next needs a medium or fast sweep speed, but slow sweep speed is needed to assess carotid velocity variation with respiration as discussed in the Pericardium chapter.
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 when left heart volume and function are maximal. 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 second complex in the tracing above is excellent for measurement.
Measure the Cycle Time: 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 the following tracing, the CT was 0.99 seconds. The heart rate (HR) of this beat is 60/0.99 = 60.6 bpm and Venue calculated this, but rounded down.
Measure the Carotid Flow Time (CFT): Put the first caliper again at the start of the carotid upstroke and measure to the central depth of the dicrotic notch. This is the raw CFT and was 321 msec in this example.
The raw CFT is corrected to a heart rate of 60 bpm using an empirically derived formula that Venue uses:
Corrected CFT (CFTc) = raw CFT + [1.29 x (HR – 60)]
This patient’s CFTc was 322, virtually unchanged from the raw CFT because the heart rate was barely over 60 bpm..
A series of 142 normal volunteers showed a 95% normal range of about 280 – 360 for the supine 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 a one-minute passive leg raise, which requires a helper. The head of the table should be flat for the leg raise. 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.
The published work with LV ejection time would predict that severe aortic stenosis could prolong the CFTc (the equivalent of 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.CAROTID STENOSIS: For the Advanced IMBUS physician, determining the degree of carotid stenosis is similar to determining the severity of aortic stenosis. The peak PW velocity in the stenosis is measured and compared to the velocity in the mid common carotid. The pre- and post-stenosis velocity ratio is interpreted exactly the same as the dimensionless index (DI) for aortic stenosis, except that the numerator and denominator are reversed for carotid stenosis. Peak velocity through a stenosis that is less than 1.25 m/sec with a ratio less than 2.0 when compared to the common carotid indicates stenosis less than 50%. Peak velocity of 1.25 – 2.3 m/sec and a ratio of 2.0 – 4.0 is stenosis from 50 - 70%. Peak velocity greater than 2.3 m/sec with a ratio greater than 4.0 indicates stenosis greater than 70%. We can measure these higher velocities because of the PW angle correction function of Venue. Nearly occluded carotids are probably a visual diagnosis because only very narrow residual flow may be seen and there is difficulty getting the PW gate in the small residual jet.