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Renal Artery Doppler

Renal artery duplex ultrasound is a non-invasive screening test for renal artery stenosis and renovascular hypertension. It combines direct spectral interrogation of the main renal arteries with indirect intrarenal (downstream) waveform analysis. Elevated peak systolic velocity and renal-aortic ratio indicate a haemodynamically significant stenosis, while a tardus-parvus intrarenal waveform provides supportive downstream evidence.

Indications

  • Suspected renovascular hypertension, especially resistant or early/late-onset hypertension
  • Unexplained or progressive renal impairment
  • Suspected fibromuscular dysplasia in younger hypertensive patients
  • Flash pulmonary oedema or worsening renal function after ACE inhibitor/ARB
  • Surveillance after renal artery stenting or angioplasty
  • Assessment of the transplant renal artery

Contraindications & Cautions

  • No absolute contraindications; ultrasound is safe and non-ionising
  • Bowel gas, obesity and patient motion frequently limit visualisation of the main renal artery origins
  • Technically demanding and operator-dependent; a negative or incomplete study may need CT/MR angiography
  • Accessory renal arteries are commonly missed and can cause false-negative direct criteria

Patient Preparation

  • Fast for 6-8 hours to reduce bowel gas obscuring the renal arteries and aorta
  • Schedule in the morning where possible
  • Supine and decubitus (flank) positions for anterior and posterolateral windows
  • Allow adequate time; the study is technically demanding
  • Record blood pressure medications and renal function

Technique & Parameters

  • Curvilinear 2-5 MHz probe for deep abdominal penetration
  • Sample the aorta at the level of the renal arteries for peak systolic velocity to calculate the renal-aortic ratio
  • Interrogate each main renal artery from origin to hilum with angle-corrected spectral Doppler at an angle <=60 degrees
  • Record segmental/interlobar intrarenal waveforms in upper, mid and lower poles
  • Measure intrarenal acceleration time, acceleration index and resistive index for tardus-parvus assessment
  • Measure bipolar kidney length and note asymmetry

Systematic Review

  • Aortic peak systolic velocity at the renal artery level
  • Main renal artery PSV at origin, mid and distal segments bilaterally
  • Renal-aortic ratio (renal artery PSV divided by aortic PSV)
  • Intrarenal waveforms for tardus-parvus (prolonged acceleration time, low acceleration index)
  • Resistive index in each pole and side-to-side comparison
  • Kidney length and cortical thickness for chronic atrophy

Key Findings & Significance

  • Main renal artery peak systolic velocity above ~180-200 cm/s suggests significant (>=60%) stenosis
  • Renal-aortic ratio greater than 3.5 supports a haemodynamically significant proximal stenosis
  • Post-stenotic turbulence and elevated velocity with distal spectral broadening
  • Tardus-parvus intrarenal waveform: acceleration time >0.07 s (prolonged upstroke) and reduced acceleration index
  • Resistive index side-to-side difference and low downstream RI may accompany proximal stenosis
  • Small kidney (<9 cm) with reduced flow suggests chronic ischaemic nephropathy

Differential Considerations

  • Elevated renal artery PSV: true stenosis vs tortuosity/angle error vs high-flow state
  • Tardus-parvus: proximal renal artery stenosis vs aortic stenosis (systemic upstroke change)
  • Renovascular disease: atherosclerotic (ostial/proximal, older) vs fibromuscular dysplasia (mid/distal, younger, string-of-beads)
  • Reduced intrarenal flow: stenosis vs medical renal disease vs poor cardiac output
  • Non-visualised artery: technical failure vs occlusion vs accessory artery

Pearls & Pitfalls

  • Keep the Doppler angle at or below 60 degrees; angle error is the commonest cause of spurious velocity elevation
  • Use both direct (PSV, RAR) and indirect (tardus-parvus, acceleration time) criteria for reliability
  • Beware accessory renal arteries as a cause of false-negative direct criteria
  • Fibromuscular dysplasia typically affects the mid-to-distal artery, unlike ostial atherosclerotic disease
  • Correlate with kidney size; a small kidney with slow flow suggests established ischaemic nephropathy

Structured Report

  • State fasting status, probe, positions and technical adequacy
  • Report aortic and renal artery PSV, renal-aortic ratio and side of any abnormality
  • Describe intrarenal waveforms with acceleration time, acceleration index and resistive index
  • Give bipolar kidney lengths and note asymmetry
  • Give an impression on the presence and likely aetiology of stenosis and recommend CTA/MRA or angiography when indicated

References

  • AIUM Practice Parameter for the Performance of Native Renal Artery Duplex Sonography
  • Radermacher J et al. Use of Doppler Ultrasonography to Predict Outcome of Renal Artery Stenosis. NEJM
  • Grant EG et al. Duplex Sonography of the Renal Arteries. Radiographics
  • ACC/AHA Guidelines on the Management of Peripheral and Renal Artery Disease

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.