There are two methods of using duplex sonographic examination to evaluate the presence of renal artery stenosis (RAS).

A)  Direct Evaluation of main renal arteries

B)  Indirect evaluation of more distal portions of the renal arterial tree.

  (A) Direct Signs of RAS for > 60% RAS are

1. Renal to aortic velocity ratio (RAR) > 3.5
2. Peak systolic velocity (PSV) > 180 cm/second.

PSV reported to be diagnostic of RAS ranges from 100-180 cm/s. This is an extremely wide range but it is stated that PSV> 180 cm/s is more accurate in clinical practice.

Pic. 1

16/M – young hypertensive, both renal arteries show high velocity (>3.5 m/s), RAR was 3.6

However, direct examination is often plagued by relatively high technical failure or incomplete examination rate (bowel gas, obese patients, respiratory movements), long examination time, and difficulties in measuring accurate velocities due to sub optimal angle of incidence.

(B) Indirect Doppler findings of RAS are detectable in the renal arterial tree remotely distal to the site of actual stenosis. This includes distal main renal artery, the segmental and inter-lobar branches of renal artery. The indirect spectral Doppler parameters that can be helpful in evaluation of RAS include:

1. Acceleration Time (AT), Acceleration (ACC) & Acceleration Index (AI)
2. Early systolic Peak
3. Resistivity Index  (RI) Difference between ipsilateral and contralateral kidneys

 Acceleration Time (AT), Acceleration Index (AI) & Acceleration (ACC)

It has been found that the time taken from start of systole to its peak (AT) is prolonged and that the slope of early systolic rise (AI) decreases distal to the RAS. According to some, the best criteria for RAS > 50% are AT > .1 second and AI < 3.78 KHz/MHz/S²

Acceleration: Attempts were made to define velocity criteria for acceleration rather than frequency criteria of AI. It was found that acc < 3 m/s2 is suggestive for RAS > 60%.

AT and ACC must be measured from start of systole to the first systolic peak.

Pic 2.

Early Systolic Peak
In patients with RAS there is absence of early systolic peak (ESP) in distal renal arterial tree. This loss of ESP is detectable by pattern recognition alone and does not require measurements (AT & ACC) and therefore is quick and has an advantage over AT & ACC. This pattern is also called tardus-parvus pattern.
(Tardus-parvus pattern)

Pic 3.

A large RI difference between the kidneys supplied by stenotic renal artery and contralateral side is predictive of RAS.
RID: ipsilateral RI- contralateral RI.

There is decrease in pulsatality in ipsilateral kidney and therefore, reduction in measurements in pulsatality such as RI, PI & S/D.

Summary

It is recommended to use indirect method for initial evaluation but direct method should be used whenever the indirect method is equivocal or abnormal.

Advantages of indirect evaluation are:
Technical simplicity, less time-consuming & useful in assessing the effectiveness of revascularization procedures.

Limitations of indirect methods are: insensitivity for <50% diameter, inability to distinguish severe stenosis from complete occlusion and insensitivity for stenosis in an accessory renal artery.

Dr. Alpana Joshi, BYL Nair Hospital, Mumbai.