Day-2 Post-Transplant Duplex: Is the Hepatic Artery Really Occluded?

Work out what the machine is actually able to detect.

Expected Doppler shift. A normal hepatic artery peak systolic velocity is roughly 50–60 cm/s. At 2.5 MHz and 78°:

f_d = (2 × f₀ × v × cosθ) / c = (2 × 2.5×10⁶ × 0.55 × 0.208) / 1540 ≈ 370 Hz

So only about a fifth of the available shift is being generated, because cos 78° = 0.21. At a steep angle the spectrum is broad, angle-corrected velocity is unreliable, and the displayed waveform is gain-dependent.

Wall filter. A 200 Hz high-pass filter at this angle removes everything below:

v = (200 × 1540) / (2 × 2.5×10⁶ × 0.208) ≈ 0.30 m/s (30 cm/s)

Hepatic artery end-diastolic velocity is typically 15–25 cm/s, and a transplanted artery with oedema or low cardiac output may barely reach 30 cm/s in systole. The filter alone can erase the entire waveform.

Nyquist limit. PRF 2000 Hz gives a Nyquist of 1000 Hz, which at 78° corresponds to about 1.5 m/s - aliasing is not the problem here.

Conclusion. The absence of signal is not interpretable until the wall filter is reduced to ~50 Hz, the color scale to ±15–20 cm/s, the angle brought to ≤60° (where the Nyquist limit is still ~0.6 m/s), and the scan repeated intercostally with a lower-frequency or phased-array probe and a small color box.

This is a tardus-parvus waveform: slow upstroke, low peak velocity, low resistance. It indicates that the artery is receiving blood through a significant resistance proximal to the sample volume, or that the distal bed is vasodilated and the artery itself is compromised.

Differential in a day-2 graft:

• Hepatic artery stenosis or kink at the anastomosis (the classic cause; look for a focal colour aliasing jet at the anastomosis with a ≥2–3× velocity step-up).
• Hepatic artery thrombosis with reconstitution - flow seen may be collateral.
• Splenic artery steal / low-flow state from sepsis, vasopressors or poor graft function.
• External compression by haematoma, drain or oedema.

Next action: do not simply report "flow present, normal." Document the waveform, velocities, acceleration time and the presence or absence of a visible anastomotic jet, then escalate as a critical finding to the transplant surgeon the same hour. Ask for contrast-enhanced ultrasound (microbubble contrast is highly sensitive for hepatic artery thrombosis) or CT angiography if CEUS is unavailable or equivocal. The rising transaminases and lactate make the pre-test probability of vascular compromise high, so a technically limited study must be reported as indeterminate, not normal.

The ALARA principle applies to every patient, not only to obstetric scanning.

• Thermal. The on-screen thermal index (TIS for soft tissue, TIB if bone is in the beam path at depth) should be watched; keep TI ≤1.0 where achievable and reduce dwell time when the acoustic window is poor. Deep abdominal imaging through a large habitus forces high output and long dwell, which is exactly the situation where heating accumulates.
• Mechanical. In B-mode and spectral Doppler, mechanical index is generally low. It becomes relevant if you add contrast: CEUS uses a very low MI (roughly 0.05–0.2) precisely to avoid destroying microbubbles. Note local licensing and consent for off-label or specific-indication use.
• Justification. The exam is clinically indicated and time-critical; the small, well-justified increase in output needed to answer a binary question (patent versus thrombosed artery) is defensible, whereas leaving the output at maximum for a long, non-diagnostic sweep is not.
• Practical steps. Shorten the colour box and depth to raise frame rate, use the lowest filter and scale that still show flow, freeze and review rather than scanning continuously, and record the TI/MI in the report if they are at or above 1.0.