Perfusion and Function
What to know
The liver has a dual supply: the portal vein delivers roughly 75% of flow, but the hepatic artery supplies about half the oxygen. The hepatic arterial buffer response compensates for falling portal flow by dilating the hepatic artery. The normal hepatic artery waveform is low-resistance with continuous diastolic forward flow; the portal vein is gently undulating and hepatopetal.
Renal perfusion is assessed with spectral Doppler of the main renal artery and intrarenal vessels. The resistive index, RI = (PSV − EDV)/PSV, normally stays below about 0.70 in adults, though age, heart rate, and parenchymal disease shift it. The aorta is triphasic peripherally; the superior mesenteric artery is high-resistance in the fasting state and becomes low-resistance postprandially.
Traps
Angle correction must be 60 degrees or less, and a small sample volume should sit mid-vessel. Do not call a portal vein abnormal just because it is phasic with respiration-respiratory variation is normal. An RI is meaningless when end-diastolic velocity is absent. Do not diagnose stenosis from velocity alone without considering angle and collaterals.
Rule of thumb
A tardus-parvus intrarenal waveform-slow upstroke, low peak, delayed acceleration-points to an upstream stenosis even when the stenosis itself is not seen.
Worked example
Peak systolic velocity 100 cm/s, end-diastolic 25 cm/s: RI = (100 − 25)/100 = 0.75, mildly elevated and worth correlating with clinical and laboratory findings.