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Notes #8:Renal Doppler |
Renal pathology or dysfunction can result in increased renal arterial impedance
Hypertension occurs in an estimated 58 million Americans
90% idiopathic, 10% results from renal disease or RA stenosis
Renal Artery stenosis only cause of hypertension in adults which can be treated by surgical means or by percutaneous transluminal angioplasty (PTA)
Angiography is the gold standard for diagnosing renal vascular disease
Duplex doppler non-invasive, less expensive alternative - but also less reliable - difficult ultrasound study
Direct Signs of RAS:
Velocities in the renal arteries are often overestimated due to suboptimal angles of incidence (>70 degrees). PSV reported to be diagnostic of RAS range from 100 to 180 cm/s. Cutoffs of 150 and below have a high number of false positives.
Ratio of PSV at maximum stenosis to PSV proximal to the stenosis of greater than 2 are diagnostic of stenosis in the carotid and femoral systems. Using such ratios partially normalizes velocities for cardiac factors and decreases patient to patient variation. A ratio of 2 will work in the renal artery if the stenosis is far enough distally to make a proximal PSV possible. Fibromuscular dysplasia patients often have distal stenosis. Patients with atherosclerosis have more proximal stenosis. Comparison is therefore made to the aorta and a ratio of 3.5 is utilized.
Indirect Signs of RAS:
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In large patient proximal renal arteries may be imaged with patient in lateral decubitus position imaging to the aorta. This view will display only most proximal portions as the renal arteries leave the aorta.
Obtain spectral waveforms at 60 degree (or less) angle at proximal, mid, and distal segments of each renal artery.
Patient should be in suspended inspiration when waveform is taken. Use small sample volume gate and high sweep speeds and place the gate midstream and parallel to vessel walls.
Record PSV's from renal artery waveforms
IVC can be used as a window to view RRA
In large patient or when the midline is obscured by gas, right renal artery can be viewed with the patient in left lateral decubitus position imaging through the liver to IVC
D. Measure kidney length - RAS causes decrease in length over time
E. Measure cortical thickness - less than 1 cm. equals thinning and possible renal vascular disease
1. Renal Artery to Aortic Ratio (RAR)
normal ratio less than 3.5
Utilize PSV (renal artery) from proximal, mid, and distal2. Resistive Index (RI)
3. Acceleration Index (AI), Acceleration Time (AT)
3. End Diastolic to Systolic velocity ratio (EDR)
4. Tardus-Parvus Waveform
Primary cause atherosclerosis
Other causes: fibromuscular dysplasia, syphilitic arteritis, vasculitis, neoplasm, developmental entrapment by crus of diaphragm.
Most of those resulting from atherosclerosis occur proximally within a few mm. of aorta --- not possible to assess renal artery velocity proximal to stenosis
A minority of patients (those with fibromuscular dysplasia) will have more distal stenosis
Severe unilateral RAS produces size asymmetry with the involved kidney smaller. Most cases however show no visible signs and Doppler or angiography is required for diagnosis
False positive and false negative examinations result from multiple renal arteries, branching arteries, and distal fibromuscular dysplasia
More frequent in hypertensive patients, and in patients with aortic aneurysms
Complication of renal biopsy
May be mistaken for cysts
Complication of renal biopsy
Ruptured aneurysm
Congenital malformation
Found within renal cell carcinoma
Turbulent, low resistance, high velocity flow close to AFV
Burst of color in adjacent kidney tissue due to vibrations form briskly flowing blood in the fistula
Pulsatile flow seen within the main renal vein if AFV is large
May resolve spontaneously or increase in size requiring intervention
May coexist with AVF after rupture of a pseudoaneurysm into adjacent venous structure.
May rupture producing a subcapsular hematoma which may lead to renal dysfunction secondary to compression or severe hemorrhage
Appear as simple cystic structure or small paravascular fluid collection
Doppler confirms turbulent flow within
May resolve spontaneously or increase in size requiring intervention
Acute or chronic event associated with pain, hematuria
Initially causes kidney to enlarge with decreased echogenicity.
Thrombus may be seen in the renal vein.
Occasionally areas of increased echogenicity due to hemorrhage within the kidney can be seen.
Sonography can detect dilatation of the vein proximal.
Chronic RVT results in shrunken, echogenic kidney
Renal arterial signal may exhibit reversed diastolic flow.
High resistance arterial flow with no flow in vein indicative of RVT even if thrombus is not seen
Often partial occlusion allows some venous flow to be documented