SWL: Slower is better for upper ureteral stones

Researchers at the University of Toronto, the University of Western Ontario, and the University of Alberta reported that shock wave lithotripsy delivered at 60 shocks per minute was significantly more effective at destroying upper ureteral stones than was the same treatment delivered at the more familiar rate of 120 shocks per minute.

"Fewer shocks were needed to achieve stone fragmentation at the slower rate," lead author Kenneth Pace, MD, assistant professor of urology at St. Michael's Hospital, University of Toronto, told colleagues at the meeting. "Sixty shocks per minutes works better than 120 in ureters, just as it does on renal calculi. The cost of using the slower rate comes in increased treatment time."

Patients underwent shock wave lithotripsy at either 60 shocks per minute (74 patients) or 120 shocks per minute (83 patients). Researchers used a kidney, ureter, and bladder x-ray (KUB) to assess stone size before treatment and stone-free status after treatment.

Treatment for patients in both groups began at 17 kV and was increased to a maximum of 23 kV by 1 kV every 100 shocks. Treatment was halted either when the treating urologist determined that the stone had been completely fragmented or when a total of 3,000 shocks had been given. Fluoroscopy was used at least every 200 shocks to verify shock targeting and to assess the degree of stone fragmentation. Stone-free status was confirmed with either abdominal and pelvic ultrasound or with non-contrast spiral computed tomography.

Dr. Pace noted that the ureteral stone study follows earlier work with senior author R. John Honey, MD, of St. Michael's Hospital, that showed that 60 shocks per minute is clearly more effective in fragmenting renal stones than is the 120 shocks-per-minute rate more commonly used.

Results for ureteral stones were equally clear, Dr. Pace said. The 60-shocks-per-minute rate produced faster first fragmentation (1,113 shocks vs. 1,492 shocks, p=.002), required fewer shocks for total fragmentation (2,667 shocks vs. 2,938 shocks, p<.001), and better immediate complete fragmentation (46.6% of patients vs. 18.1%, p=.004).

No statistical difference in complication rates between the two groups was noted. Dr. Pace suggested that the slower shock rate may actually produce fewer adverse outcomes because patients need fewer total shocks to achieve complete stone fragmentation. An increasing number of shocks is associated with increasing renal parenchymal damage to the lower pole, such as hematoma and possibly new-onset hypertension, so fewer shocks should produce fewer renal complications.

The stone-free rate 2 weeks after treatment was 58.1% for the 60-shocks-per-minute group versus 39.8% for the 120-shocks-per-minute group (p=.022). The stone-free rate 3 months after treatment was 67.6% for the group who received 60 shocks per minute versus 50.6% for those administered 120 shocks per minute (p=.031).

The only negative result was in treatment time, Dr. Pace said. Mean treatment time for 60 shocks per minute was 44.1 minutes versus 25.4 minutes for 120 shocks per minute (p<.001). That equates to an increase in treatment time of about 78%. Increased treatment time is a reasonable tradeoff because higher success rates dramatically reduce the need for additional shock wave treatment or more invasive secondary procedures, Dr. Pace explained.

Why the slower shock rate is more effective remains unclear, Dr. Pace said, but he suggested that it could be related to decreased acoustic impedance mismatch, improved cavitation bubble production on the stone surface, or improved bubble dynamics due to water gas content surrounding the stone.