Editorial Comment on: The Influence of Super-Mini Percutaneous Nephrolithotomy on Renal Pelvic Pressure In Vivo by Alsmadi et al.

We have read with great interest the article by Alsmadi et al. who conducted a prospective cohort study to measure renal pelvic pressure (RPP) and its clinical impact on postoperative fever in patients treated with super mini percutaneous nephrolithotomy (SMP) for renal stones <2.5 cm in diameter. ¹ For this study, they used a 14F irrigation–suction access sheath to treat the kidney stone while continuous monitoring of RPP was performed with the use of a retrogradely inserted 5F catheter attached to an invasive blood pressure channel of PM9000 with a baroreceptor. Their results showed that the 14F irrigation–suction sheath was reasonably safe as it was able to maintain an RPP <30 mm Hg for almost the entire duration of each SMP in the study. In addition, they identified accumulative time (≥70 seconds) of high RPP (≥30 mm Hg) and mean RPP ≥20 mm Hg as the sole predictors of postoperative fever.

The necessity to control RPP originates from historic studies, which demonstrated that an increase in pressure above a threshold of 30–35 mm Hg could lead to pyelovenous backflow and substantial bacterial translocation into kidney parenchyma. ² However, recent studies on the effect of RPP on postoperative complications are controversial. ^{3 –5} It should be noted that the occurrence of RPP ≥30 mm Hg for a limited amount of time (50 seconds or less) did not correlate with postoperative fever. ⁴ It is plausible that operative time used as a surrogate for the accumulative time of high RPP, is the most important predictor of complications following percutaneous nephrolithotomy. ⁵ Thus confirming the importance of using a time-efficient stone procedure to decrease postoperative complication rates, namely sepsis. Consequently, the lower tract size used in mini percutaneous nephrolithotomy or SMP should not necessarily be considered a contraindication, but rather time of the procedure and efficacy of the percutaneous tract in clearing stone fragments using a suction device should be regarded as a primordial factor in preventing postoperative complications.

It is clear from this article that the main explanation for periodic increases in RPP is the decreased size of the tract and the minimal space between nephroscope and sheath. This decreased space can more easily be obstructed by stones that translates into an increased RPP. One possible solution is to withdraw the instrument frequently to remove these small obstructing stones. However, it is also plausible that SMP should not be considered an optimal choice for struvite kidney stones.

An important future study should compare these results with those of normal tract size percutaneous nephrolithotomy and ideally with flexible ureterscopy with and without a ureteral access sheath and with pressure irrigation.