Hepatic vein Doppler is a key component of both VExUS and eVExUS assessments. Getting the S and D waves right in the hepatic vein Doppler waveform can tell you a lot about hemodynamics and cardiac function, not just the VExUS grade. See below figure.

To make this work, it’s essential to have an electrocardiogram (EKG/ECG) trace alongside the Doppler waveform. The EKG helps you line up the phases of the cardiac cycle with the Doppler waves. The S wave follows the R wave on the EKG, the D wave follows the T wave, and the A wave follows the P wave. A recent study has shown that EKG-enhanced VExUS is a reliable and reproducible method, making it interpretable by clinicians from various backgrounds. Most cart-based ultrasound machines come with a dedicated port for connecting a 3-lead EKG module, which shows the EKG right on the ultrasound screen. While some doctors may try to compare the timing of the S and D waves using a separate telemetry monitor that the patient is hooked up to, I find this method to be unreliable and impractical.

The figure below shows how not having an EKG can lead to misinterpretation – what you might think is normal could be mild congestion, or you could mistakenly categorize severe congestion as normal. It really turns the whole interpretation upside down.

Without the EKG, it’s nearly impossible to accurately differentiate between the S and D waves, no matter how classic the Doppler trace looks. Take a look at these two tracings – without the EKG, many would likely categorize both as S>D (normal). But with the EKG, the interpretation completely flips to S<D.

Here’s another tricky waveform: at first glance, most would call it S > D without an EKG trace. But once you add the EKG, it tells a different story: an S-wave reversal, indicating severe congestion.

Even though I stress this point regularly, people still ask if there are any cheat codes or tricks to accurately identify the S and D waves without an EKG. There are a few methods, but again, they’re not infallible and don’t always work. Here’s what I have for you:

The Calipers Method: This method involves comparing the duration of the cardiac cycle with any arterial waveform, typically from the renal artery. If there’s only one wave below the baseline in each cardiac cycle on the hepatic vein Doppler, this suggests absent S-wave or S-wave reversal (except in rare situations like tamponade, where the D-wave may be absent or reversed). However, this technique can’t distinguish between S>D and S<D. Also, it only works if the Doppler traces you’re comparing are obtained at the same sweep speed (the scrolling pace of the Doppler spectrum, usually shown as mm/s under the trace). I recommend 50-75 mm/s for VExUS (but 100 mm/s if you’re doing cardiac Doppler per guidelines). I like to carry around an EKG caliper to show off, but you don’t have to have it!

The Renal Rescue Method: Renal Doppler typically displays both arterial and venous waveforms, and the arterial waveform works like a ‘built-in EKG’. If the renal vein shows S<D, it’s likely that the hepatic vein is also S<D, and vice versa. However, don’t get too confident with this assumption. While we generally expect renal and hepatic veins to correlate, the renal vein is farther from the heart, and its waveforms can be blunted, making it tricky to identify the relative S-D amplitude. Additionally, the intrarenal (renal parenchymal) waveform doesn’t always match the main renal vein waveform, particularly due to interstitial edema. This is why we use the intrarenal vein waveform for VExUS scoring (tells us how the renal parenchyma is feeling) and on the other hand, main renal vein tends to align better with the hepatic vein for comparison purposes. And a quick reminder: don’t play with the Doppler sweep speed while doing this. Below is a good example of hepatic S<D, which many would incorrectly categorize as S>D without EKG, assuming that the A wave is always larger than the V wave (but this isn’t always true in pathological states!). However, by comparing with the renal (and portal) waveforms, the mystery is solved.

Below is another case. Right off the bat, the tracing looks a bit too monomorphic, with a small notch in the upslope of the below-the-baseline wave—something doesn’t quite add up. When you compare it with the renal arterial trace and the faint hepatic artery trace in the background of the portal vein, you can see that each below-the-baseline wave lasts longer than just systole or diastole alone. Assuming the patient isn’t in atrial fibrillation (Doppler rhythm is regular here) and that the above-the-baseline bump is the A-wave, this has to be S-D fusion. This usually happens in tachycardia and, without an EKG, can be mistaken for S-reversal, especially if the A-wave is prominent.

The CVP Method: The hepatic vein Doppler waveform is essentially a reflection of the CVP waveform for all practical purposes. So, if the patient has a central venous catheter (CVC), comparing the Doppler trace with the CVP waveform can help identify the S wave (which correlates with the X descent) and the D wave (which correlates with the Y descent). In the example below, notice the deep Y descent (which corresponds to diastole based on the EKG), indicating a D-only pattern or severe congestion (A-S-V above the baseline). However, most nephrology patients don’t have an indwelling central venous catheter. Additionally, non-cardiac ICUs seem to be moving away from using CVCs, as pressors can now be administered through midline catheters.

The bottom line is, if you have access to EKG gating, use it to accurately interpret hepatic vein Doppler. It only takes an extra 2 minutes but can save you a lot of frustration during interpretation. While there are cheat codes, they’re not foolproof. Don’t feel judged, but if your department can invest in an ultrasound machine, they can spend a few hundred dollars on an EKG cable as well. It just takes a little nudge from leadership. So, next time, advocate for it! (and keep my name out of your arguments 😊)