Prior to renal ultrasound exam, fasting for 8 hours is recommended to reduce bowel gas. During the exam, having the patient hold breath will push the kidneys inferiorly in the abdomen. This will also help with motion artifact.
In abdominal aorta, the blood flow at the level of celiac and SMA is low resistance. Below the level of renal arteries, the aorta assumes high resistance flow pattern. To calculate renal-to-aortic velocity ratio (RAR), aortic velocity should be taken at the level of SMA origin.
Renal Artery Velocity
- Normal main renal artery PSV < 180 cm/s
- Normal PSV for interlobar arteries: 30-40 cm/s
- Arcuate arteries: 20-30 cm/s
- Renal/Aortic ratio (RAR) < 3.5. If perirenal aorta PSV outside of 40- 100 cm/s (normal range), RAR is not reliable.
- Renal PSV > 180 cm/s and RAR > 3.5, it is > 60% stenosis
- PSV > 180, RAR < 3.5, < 60% stenosis
- PSV < 180, RAR < 3.5, normal
- For transplant kidney, renal-iliac ratio (RIR) of 2.5 -3.0 or greater is consistent with stenosis of allograft renal artery
- For renal stent or bypass PSV of 280 and RAR of 4.5 are used for thresholds
Renal Hilum Velocities
Renal hilum velocities can be easily obtained by scanning through the kidney at the patient’s flanks, and can yield clues to the renal artery. It can be used as adjunct but not suitable for primary screening for renovascular disease. Below are some normal values:
- For acceleration time (AT), more than 100 msec is abnormal
- For acceleration index (AI), or slope of the acceleration, less than 300 cm/s^2 is abnormal
Parenchymal Assessment
End diastolic ratio:
EDR = Parenchymal EDV/ Parenchymal PSV
Normal cortical EDR vales are 0.30 – 0.47
Resistive index:
RI = 1- EDR
High RI, or low EDR, signify worse kidney disease. RI > 0.8 identifies patients unlikely to respond to renal revascularization.
Size
- Normal 10-12 cm long
- 4.5 – 6 cm wide
- Cortical thickness 1 – 1.5 cm
- 1 cm decrease in length is considered renal atrophy
Renal Artery Stenosis Medical Management
For patients with atherosclerotic renal artery stenosis and hypertension, the patient should be on at least antiplatelet monotherapy and statin therapy. Blood pressure goals are the same as other hypertensive patients whether or not renal artery stenosis is present. The age-adjusted goals listed by ESVS are listed as follows:
- Age < 65: BP target <130/80 mmHg
- Age 65 – 79: BP target <140/80 mmHg
- Age 80 and up: SBP target 140-150 mmHg
ACE-I and ARB are first-line antihypertensives. Calcium channel blockers and thiazides are recommended as secondary additions.
Surgical Indications and Management for Renal Artery Stenosis
Indications
Renal artery stenosis and renovascular hypertension is considered medically-resistant and renal artery revascularization may be indicated if:
- The patient is on 3 anti-hypertensive medications including a diuretic and still has uncontrolled hypertension
- The patient is on 4 anti-hypertensive medications and does have controlled BP
Aside from BP and medication goals, select patients with following symptoms should also be considered for renal artery revascularization:
- Recurrent flash pulmonary edema
- Rapidly progressive renal failure
- Acute heart failure
- Solitary kidney or transplanted kidney
Interventions
- For atherosclerotic renal artery stenosis (90% of the cases), angioplasty with primary stenting is the treatment of choice
- For non-atherosclerotic renal artery stenosis, namely fibromuscular dysplasia, renal artery angioplasty without stenting is the treatment of choice and may be curative
- Open surgical revascularization is reserved for patients without endovascular options. Options include but not limited to: aorto-renal bypass, hepato-renal bypass, and spleno-renal bypass, endarterectomy, reimplantation, ex-vivo renal artery reconstruction and auto-transplantation