New Model Predicts Renal Pelvis Pressures During Ureteroscopic Lithotripsy
Key findings
- Urologists and mechanical engineers at Massachusetts General Hospital created a mathematical model that computes renal pelvis pressures during ureteroscopic lithotripsy, which if elevated can cause backflow and the potential for development of urosepsis
- The two strongest regulators of renal pelvis pressure during ureteroscopy were the size of the gap between the scope and the ureteral access sheath and the frequency and duration of scope withdrawal
- The theoretical results of the model were consistent with the experimental results of previously published studies
- The researchers plan a web-based or mobile application to allow surgeons to input the parameters of their proposed ureteroscopic lithotripsy technique for an individual patient, then see how varying those parameters would affect renal pelvis pressure
Elevated renal pelvis pressures during ureteroscopic lithotripsy can cause urine to flow into the bloodstream via pyelovenous backflow. If the urine contains bacteria, this backflow can result in a life-threatening systemic bacterial infection.
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The risk of backflow is believed to increase with the amount of time the renal pelvis pressure remains elevated. Clinical studies have shown that ureteral access sheaths and low-flow-rate irrigation reduce the pressure, but it is difficult or impossible for a clinical study to vary all operative parameters.
Brian H. Eisner, MD, co-director of the Kidney Stone Program at Massachusetts General Hospital, and colleagues are developing a model for urologists to use to determine how their equipment choices and techniques for ureteroscopy will affect renal pelvis pressure in individual patients. In PLOS One, they describe their initial work.
Model Development
The research team modeled ureteroscopic lithotripsy as flow through a series of distinctly connected pipes. They show the mathematics of computing renal pelvis pressure as a function of time, based on parameters that can be varied during ureteroscopy: irrigation pressure and the dimensions of the scope and sheath.
By inputting representative values of equipment dimensions, irrigation pressure and physiologic parameters (such as ureter diameter) into their mathematical model, the researchers derived a series of pressure curves. The model was able to predict quantitatively how renal pelvis pressure rises with time and the maximums it reaches.
Key Factors
Two key factors influenced both maximum pressure and rise time:
- The gap between the scope and the sheath. Outflow resistance was inversely proportional to the gap size, so a slight increase in the gap decreased the maximum pressure and the associated time. For example, increasing the gap size by just one Fr, by moving from no sheath to a 10/12 Fr sheath, reduced the maximum pressure by half.
- The gap between the scope and the ureteropelvic junction (UPJ). Outflow resistance depended on both UPJ and sheath gap sizes. For small sheath gap sizes, the influence of the UPJ was negligible and the sheath geometry essentially set the maximum pressure and the associated rise time. For sheath size larger than 12/14 Fr, the UPJ controlled the maximum pressure and the time.
Comparison with Published Data
To test the validity of the model, the researchers compared their theoretical results with two published studies of kidney stone treatment: a pig model of stones in the renal pelvis and a human study of stones in the ureter.
When the researchers replaced their representative values of equipment dimensions, irrigation pressure and physiologic parameters with the values given in the studies, the results were consistent with their theoretical predictions about time-averaged renal pelvis pressure.
Evaluating the Frequency of Scope Withdrawal
The researchers intend to use the model to identify operative parameters that could be modified for various ureteroscopic lithotripsy techniques in order to prevent urosepsis. As one example, they evaluated variation in the frequency of scope withdrawal for basket extraction of stone fragments.
The model predicted a tradeoff between renal pelvis pressure and operative time. Ureteroscope withdrawal decreased pressure at the expense of longer total operative time, and the magnitude of this effect was inversely related to sheath diameter.
Guidance for Surgeons
When a patient undergoing ureteroscopic lithotripsy is at higher risk of infectious complications, it may be worthwhile to take extra operative time to keep the pressure at the lowest possible levels throughout the procedure. That group includes patients who have struvite stones, stones colonized with bacteria or a history of urinary tract infection.
The research team is working toward an online or digital tool that will allow surgeons to input the parameters of their proposed procedure for an individual patient (e.g., use of sheath, sheath size, ureteroscope diameter, irrigation rate and/or basketing technique), then see how varying those parameters would affect renal pelvis pressures.
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