Applications of POCUS for the carotid artery and an introduction to spectral Doppler
The carotid artery is the first cardiovascular chapter mostly because it is an ideal way to develop expertise with spectral Doppler. However, this exam also has utility for cardiovascular risk assessment and for estimating volume responsiveness in patients.
Screening for carotid stenosis (usually of the internal carotid) is beyond the equipment and experience in the clinic. However, carotid atherosclerotic plaque is easily detected because it is echogenic. Here is a depiction of the carotid artery anatomy in the neck. The internal carotid remains larger and more posterior than the external carotid.
A longitudinal bilateral carotid survey up through the bulb is a reasonable and efficient approach. Plaque is often most severe within 2 cm of the carotid bulb. This survey is done using the same VENOUS exam type used for the carotid flow time described below and the JVP exam discussed in the next chapter. Below is a modest plaque in the carotid just before the bulb.
The presence of carotid plaque confirms non-coronary atherosclerotic disease and should put a patient in the high-risk category, 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. 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 Doppler (CD), Pulse Wave Doppler (PWD), Continuous Wave Doppler (CWD), and Tissue Doppler Imaging (TDI). The latter two varieties of spectral Doppler will be covered in future chapters. The technique below is difficult to understand just from reading, so a learner needs to be shown carefully how to do it and then reading about it can reinforce things.
Image the right carotid and internal jugular vein transversely just above the clavicle with the L38 linear probe. Adjust the depth to put the carotid in the middle of the screen. 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 in most patients.
Activate CD: Make the CD sector only large enough to cover the carotid for a modest length. Restricting the size of the CD sector box applies to any use of CD. Adjust the CD sector size with SELECT and then horizontal and vertical movements on the track pad. Push SELECT again when done. Change the insonation angle to + 20 on the Edge using a button at the top right of the keyboard. The angle of insonation is the angle between the US beam and the direction of flow. By angling the CD sector box +20 degrees towards the chest, the strength of the Doppler signal coming towards the probe (red) is strengthened. Finally, use the GAIN button to fill the vessel with flow signal without any speckling out beyond the vessel. The little areas of blue that appear are higher velocity flow that “aliases” the color. CD aliasing will be important in subsequent cardiac chapters.
Activate PWD: PWD measures the velocity of blood flow normally seen in the body (< 1.5 m/sec). The cursor/gate needs to be less than 30 degrees off parallel to the flow or we will underestimate the velocity. Make sure the angle of insonation buttons are to +60 and +20 and move the gate into the middle of the lumen where CD is strong. Make final adjustments to the gate line by pushing the SELECT button and using horizontal track pad movements.
Push the Doppler button again to activate the tracing. In Default mode, the baseline is usually near the top of the screen and the PWD tracing is “Aliased”, which means it goes off the top and then appears at the bottom, as in the tracing below.
Push the LINE button at the top left of the keyboard to bring the baseline down to the bottom of the screen since only positive waves are being evaluated. If the tracing is upside down, the indicator of the probe is reversed OR someone switched the INVERT button. Check the indicator and if it is correct, then push INVERT to bring the tracing positive. Finally, if the baseline is at the bottom and the tracing is positive but still running off the screen at the top, use the SCALE button. Toggling up will increase the scale and shrink the size of the waves. An occasional patient may need the gain increased while in PWD to make the tracing more echogenic. The image below shows an optimized PWD tracing.
Changing the sweep speed: The sweep speed is the recording speed. A fast sweep speed spreads things out, 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 fast sweep speed, but there are a few situations when slow sweep speed is better (e.g. carotid velocity variation with respiration in assessing cardiac tamponade). With the PWD tracing running, push the Page 1/2 button at the upper right to get the second page; if the sweep speed button is on “Fast”; change it to “Slow” and see the difference. Then, change back to “Fast” and return to page 1.
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. Activate the tracing at the beginning of expiration and watch the optimized PWD tracing until you see complexes with good dicrotic notches and freeze at end-expiration. The dicrotic notch indicates aortic valve closure. Scroll back to be sure you have the best end-expiratory complex. The middle complex in the tracing above is good.
Activate CURSOR and then push the TIME button at the top of the keyboard to create a first timing caliper because TIME, not velocity, is being measured. Put the first caliper at the very start of the carotid upstroke and then push SELECT to get an ending caliper to place at the beginning of the next carotid upstroke. This is the CYCLE TIME (CT) and it will show as “A” at the bottom. Push TIME again to get the caliper to place at the start of a second time interval directly over the first interval start line so that it turns purple. Push SELECT to get the second time interval ending caliper and move this to the exact bottom of the dicrotic notch. This interval is the raw CFT and it will be “B” at the bottom.
In this example, the CT of 815 msec is 0.815 seconds. The heart rate (HR) of this beat is 60/0.815 = 74 bpm.
The raw CFT of 305 msec needs to be corrected to a heart rate of 60 bpm. A very old formula by Bazett (1920) performed this correction by dividing various cardiac times by the square root of the CT. This method has been used for the CFT by several authors, but in the largest series of normal patients (Hossein-Nejad, J. Critical Care, 2017), this correction did not completely control for HR variation. Another method by Wodey (J Clin Monit comput, 2000) better controls for heart rate and is what will be used in clinic. The Wodey formula is: Corrected carotid flow time (CFTc) = raw CFT + [1.29 x (HR – 60)]. Using the Wodey method, this patient’s CFTc = 305 + [1.29 x (74 – 60)] = 323.
The Hossein-Nejad series of 142 normal volunteers showed a 95% normal range of about 280 – 360 for the CFTc. Importantly, they showed that a 45 degree passive leg raise for 1 minute caused a mean change of about 2% in the CFTc, although some normals showed greater changes because the CFTc is sensitive to small changes in volume status. Normal individuals at the COC clinic have also shown strong responses to passive leg raise when they have been less than optimally 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 block the effects 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 and the change with passive leg raise is 15% or greater, there is strong evidence for low LAP 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 CFTc with the 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. 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. Heart failure patients being diuresed could have serial CFTc measurements to identify when a patient starts to become volume responsive and not in need of further diuresis. At a minimum, CFTc change with passive leg raise should be part of volume assessment in the clinic.