Renal Artery Doppler

Renal artery Doppler (renal artery duplex ultrasound) examines the main renal arteries from their origin at the aorta to the hilum, and the intrarenal (segmental and interlobar) arteries, using grey scale imaging with colour and spectral Doppler. A 2 to 5 MHz curvilinear probe is used through an anterior abdominal window (with a lateral or posterior window as a supplement).

The patient fasts for 8 to 12 hours to reduce bowel gas. Direct measurements are peak systolic velocity (PSV) in the aorta and in the renal artery, the renal to aortic PSV ratio (RAR), and end diastolic velocity. Indirect intrarenal measurements are the resistive index (RI), the acceleration time and the acceleration index in the segmental arteries.

The examination takes 30 to 60 minutes, with no radiation or nephrotoxic contrast, so it is useful in renal impairment.

Renovascular disease causes about 1 to 5% of hypertension in the general hypertensive population and up to 30% in selected high risk groups (severe or resistant hypertension, hypertension with unexplained renal impairment, flash pulmonary oedema, hypertension that starts before 30 or after 55 years).

Atherosclerotic stenosis accounts for about 90% of cases (ostial or proximal third), fibromuscular dysplasia for about 10% (mid and distal artery, young women). Renal artery duplex has a sensitivity of 85 to 95% and specificity of 90 to 95% for stenosis of 60% or more in experienced hands, but is highly operator dependent, with technically unsatisfactory examinations in 10 to 20% of patients.

Indications. Severe, resistant or accelerated hypertension. Hypertension of early onset (under 30 years), especially in women (fibromuscular dysplasia). Sudden rise in blood pressure in a previously stable patient.

Unexplained renal impairment or a fall in estimated glomerular filtration rate of over 30% after starting an angiotensin converting enzyme inhibitor or receptor blocker. Asymmetric kidney size (over 1.5 cm difference). Flash pulmonary oedema or unexplained heart failure with preserved ejection fraction. Abdominal bruit. Follow up after renal artery angioplasty or stenting (restenosis).

Assessment of the transplant renal artery (stenosis, thrombosis). Renal artery aneurysm, dissection, arteriovenous fistula after biopsy. Takayasu arteritis, other vasculitis. Not as a screening test in unselected hypertension.

Contraindications and precautions. None. Fasting reduces bowel gas, and simethicone may help. Large meals, carbonated drinks and gum chewing are avoided before the scan. The scan is more difficult in obesity, bowel gas, a deep aorta, ascites and after recent abdominal surgery. Antihypertensive drugs (especially blockers of the renin angiotensin system) do not affect the velocity measurements, but the patient's blood pressure is recorded.

Preparation and technique. The aorta is scanned in the transverse and longitudinal planes, with PSV measured at the level of the renal arteries (the reference value for the ratio).

Renal artery origins are found with colour Doppler from the anterior approach (the "banana peel" or "seagull" sign for the left renal vein crossing in front of the aorta, caudal to the superior mesenteric artery). The artery is followed through the proximal, mid and distal segments to the hilum, with Doppler angle of 60 degrees or less.

Accessory renal arteries are sought (in 20 to 30% of patients). Intrarenal Doppler is taken from the upper pole, mid-zone and lower pole segmental arteries in both kidneys, measuring the RI (peak systolic velocity minus end diastolic velocity, divided by the peak systolic velocity) and the early systolic peak, acceleration time and acceleration index.

Kidney length is measured, and the contralateral kidney is compared.

Normal appearances. The main renal artery shows a low resistance waveform with a rapid systolic upstroke, a broad systolic peak and continuous forward diastolic flow, with a clear spectral window and PSV under 180 to 200 cm each second. The RAR is under 3.5.

The intrarenal arteries show:

• A sharp early systolic peak (the early systolic notch or compliance peak).
• Acceleration time under 0.07 seconds.
• Acceleration index over 3 m each second squared.
• RI of 0.5 to 0.7 (under 0.7; up to 0.8 in the young and the old).

The RI differs between the two kidneys by under 0.05. Kidney length is 9 to 12 cm, with a difference between the two kidneys under 1.5 cm.

Renal vein flow is continuous and phasic.

Interpretation.

• Stenosis of 60% or more: PSV over 180 to 200 cm each second in the renal artery, RAR over 3.5, post-stenotic turbulence with spectral broadening, and a visible narrowing on colour Doppler (aliasing at the stenosis). End diastolic velocity over 150 cm each second suggests a very high grade stenosis (over 80%).
• Intrarenal waveform changes (tardus parvus pattern): acceleration time over 0.07 seconds, acceleration index under 3 m each second squared, loss of the early systolic peak, and a damped, rounded waveform, indicating a haemodynamically significant proximal stenosis; useful when the main artery cannot be seen.
• Occlusion: no flow in the main renal artery, a small kidney (under 8 to 9 cm) with reduced cortex, and very low velocity, damped intrarenal flow supplied by collaterals; the artery stump may be seen at the origin.
• Resistive index: a high RI (over 0.8) suggests parenchymal disease (diabetic or hypertensive nephropathy, interstitial nephritis, chronic kidney disease) and predicts that revascularisation is less likely to improve renal function or blood pressure. A low RI on the stenotic side is seen with a significant stenosis. A difference of over 0.05 between kidneys supports unilateral stenosis.
• Fibromuscular dysplasia: elevated velocities in the mid to distal artery with a "string of beads" appearance on angiography and tortuous, kinked vessels on ultrasound, often bilateral, and associated carotid involvement.
• Aneurysm: focal dilatation with swirling "yin yang" flow on colour Doppler, sometimes with mural thrombus and calcification.
• Dissection: intimal flap and false lumen.
• Arteriovenous fistula after biopsy: focal high velocity, low resistance flow with a draining vein showing arterialised flow and perivascular colour artefact (tissue vibration).
• Renal vein thrombosis: enlarged oedematous kidney, echogenic thrombus in the vein with absent flow, and a high resistance arterial waveform with reversed diastolic flow.
• Transplant renal artery stenosis: focal PSV over 200 to 250 cm each second at the anastomosis with turbulence, tardus parvus waveforms downstream and a rising creatinine or hypertension.
• Atrophic kidney: small size, thin and echogenic cortex.
• Aortic findings: atherosclerotic plaque, aneurysm, or dissection extending to the renal ostia.

Critical values and findings needing immediate action.

• Bilateral high grade stenosis or stenosis in a solitary kidney with acute kidney injury or flash pulmonary oedema: urgent nephrology and vascular review for revascularisation, and stopping of angiotensin converting enzyme inhibitors and receptor blockers.
• Acute renal artery occlusion (sudden flank pain, haematuria, raised lactate dehydrogenase, anuria, absent flow): urgent anticoagulation and vascular intervention within hours; renal viability is lost within 3 to 6 hours of complete ischaemia, although partial collateral flow may extend this.
• Renal artery dissection or rupture of an aneurysm (flank pain, shock): emergency CT angiography and vascular surgery or embolisation.
• Arteriovenous fistula or pseudoaneurysm after biopsy with bleeding: embolisation.
• Renal vein thrombosis with renal impairment: anticoagulation and specialist care.
• Transplant artery or vein thrombosis, or absent graft flow: emergency surgical review for graft salvage.
• Malignant (accelerated) hypertension with renal artery stenosis: blood pressure control in hospital and urgent evaluation.

Limitations and artefacts.

• Technical failure in 10 to 20% of patients (obesity, bowel gas, deep aorta, uncooperative patient, previous surgery, calcified plaques, respiratory motion). The entire length of the artery is seen in only 50 to 80% of cases; the origin and distal segments are the hardest to visualise.
• Accessory and multiple renal arteries (20 to 30%) can be missed, and a stenosis of a small accessory artery is difficult to detect.
• Velocity criteria depend on the equipment and angle correction; an angle over 60 degrees overestimates velocity.
• The PSV is raised in tortuous arteries and in hyperdynamic states and lowered in severe aortic stenosis, low cardiac output or an elevated aortic PSV. The ratio then underestimates stenosis; a ratio uses the aortic PSV and is unreliable if the aortic PSV is under 40 cm each second.
• Intrarenal waveform changes may be absent in mild stenosis, in stenosis with well developed collaterals, or with an elevated resistive index due to parenchymal disease, which masks the delayed upstroke.
• Intrarenal tardus parvus changes can occur with aortic stenosis, and aortic valve disease or proximal aortic lesions should be excluded.
• Stenting alters the velocity thresholds (a PSV over 280 cm each second suggests in stent stenosis of 60% or more), and local validation is needed.
• Duplex gives anatomical stenosis, not the functional or physiological importance of the lesion; many renal artery stenoses are incidental and do not cause hypertension, so correlation with the clinical picture is needed.
• The sensitivity of ultrasound for fibromuscular dysplasia is lower than for atherosclerotic disease because the lesions are distal.
• Operator dependent and time consuming; accreditation, training, and local audit against angiography improve reliability.

Special groups.

• Young women with hypertension: consider fibromuscular dysplasia; imaging of other vascular beds (carotid, cerebral) is often needed, and CT or MR angiography is preferred as the first line study when duplex is not suitable.
• Older adults with atherosclerosis: coexisting coronary and peripheral disease; renal artery disease is common (up to 40% in those with peripheral arterial disease), and many stenoses are managed medically.
• Children: renovascular hypertension from fibromuscular dysplasia, neurofibromatosis and Takayasu arteritis; the ultrasound is limited by technique and angiography may be needed.
• Pregnancy: ultrasound is safe; renal artery disease is rare.
• Patients with chronic kidney disease: avoids contrast, so duplex is the first line.
• Renal transplant recipients: routine post-operative scans and scans for hypertension or rising creatinine.
• Regions: Takayasu arteritis is an important cause of renovascular hypertension in young patients, along with fibromuscular dysplasia and neurofibromatosis. Late presentation with severe hypertension and renal failure is common. CT angiography or conventional angiography may be unavailable, so the duplex is often the only vascular test.

Follow up tests. CT angiography or MR angiography (non-contrast methods in advanced renal impairment) to confirm the stenosis, catheter angiography with pressure gradient measurement and angioplasty or stenting when the stenosis is significant and symptomatic, captopril isotope renography,

plasma renin activity and aldosterone (primary aldosteronism and secondary hyperaldosteronism), serum creatinine, estimated glomerular filtration rate and electrolytes, urinalysis and albumin to creatinine ratio, kidney ultrasound with size, ambulatory blood pressure monitoring, assessment of other vascular beds, inflammatory markers for arteritis, and referral to nephrology,

hypertension clinic or vascular surgery.