Indications
Recent-onset hypertension in a young or elderly patient, hypertension resistant to treatment, deterioration of renal function after starting an ACE inhibitor or ARB, asymmetric reduction in kidney size, sudden pulmonary oedema ("flash pulmonary oedema") unexplained by a cardiac cause, and follow-up after renal angioplasty or stenting.
Patient preparation and equipment
- Fasting for at least 6–8 hours, as for the abdominal aorta: bowel gas is the main limiting factor in this territory too.
- Curvilinear transducer 2–5 MHz.
- An anterior approach for the origin of the renal arteries (patient supine), and a lateral or posterior approach (contralateral decubitus) for the intraparenchymal study, using the acoustic window of the flank.
- A long examination that requires patience: it is among the technically most demanding territories in vascular ultrasound, because of depth, respiratory motion and anatomical variability.
Direct assessment of the renal artery origin
From the aorta, in the transverse plane at the level of the superior mesenteric artery origin (about 1–2 cm below), the origins of the renal arteries are identified: the right arises from the posterolateral aortic wall and runs behind the inferior vena cava; the left arises more laterally and takes a more direct course towards the kidney. The vessel is followed with pulsed-wave Doppler from its origin to the renal hilum, sampling at an angle ≤ 60° at several points, since ostial stenosis (the commonest, particularly in atherosclerotic disease) may be very focal.
Renal-aortic ratio (RAR)
The aortic PSV is measured at the level of the renal artery origins and compared with the maximum renal PSV: a RAR ≥ 3.5, together with a renal PSV ≥ 180 cm/s, is the most widely used combined criterion for haemodynamically significant stenosis (conventionally ≥ 60%). The RAR corrects the estimate in patients whose hyperdynamic or hypodynamic circulation alters absolute velocities.
Intrarenal Doppler and the parvus-tardus pattern
When the origin cannot be visualised directly (obesity, bowel gas, anatomical variability), Doppler assessment of the intraparenchymal segmental and interlobar arteries gives indirect but useful information: distal to a severe stenosis the waveform takes on the characteristic "parvus et tardus" pattern — reduced systolic amplitude (parvus) and prolonged systolic acceleration time (tardus), with loss of the early, sharp systolic peak of the normal kidney.
The acceleration time (from the onset of systole to the peak) and the acceleration index (systolic slope) are measured: an acceleration time > 0.07 s suggests significant proximal stenosis.
Contents di resistenza intraparenchimale
The resistive index (RI = [PSV − end-diastolic velocity] / PSV), measured in the interlobar arteries at several poles of the kidney, mainly reflects the resistance of the downstream renal parenchyma (nephrosclerosis, parenchymal nephropathy) rather than stenosis of the main artery. A raised RI that is symmetrical in both kidneys points to diffuse parenchymal disease rather than haemodynamically significant arterial stenosis, and carries prognostic value for the response to revascularisation.
What to measure and document
- Bipolar renal length on both sides: a kidney smaller than the contralateral one (a difference > 1.5–2 cm) is an important indirect finding.
- PSV at the origin and along the course of the renal artery, with the RAR.
- Intrarenal acceleration time and index where the direct study is inconclusive.
- Intraparenchymal resistive index on both sides.
- The number of renal arteries on each side: multiple or accessory renal arteries are a common variant and should always be sought and documented.