The objectives of partial nephrectomy (PN) for kidney cancer are to optimize cancer control by obtaining negative margins while preserving as much renal function as possible. Most conventional PNs are facilitated by temporary clamping of the renal vasculature along with hypothermia (ice slush applied to the surface of the kidney) if the ischemic interval is likely to extent beyond 20-25 minutes. This allows the surgery to be performed in a bloodless field and minimizes the potential for ischemic damage.

Until recently, the main focus in this field has been on minimizing the warm ischemic time (WIT), because this appeared to be the most important modifiable factor during PN. Indeed, most studies demonstrated a strong correlation between ischemic interval and ultimate renal function. In one such study, each additional minute of warm ischemia correlated with a 5%-6% increase in the risk of acute renal failure or new onset chronic kidney disease. Such data argue strongly in favor of any surgical approach that could minimize or eliminate warm ischemia.

However, most of these studies did not incorporate parenchymal volume loss into the analysis, and allowed ischemia time to serve to some extent as a surrogate predictor of ultimate renal function.  Our recent analysis of large subgroups of patients undergoing PN in a solitary kidney has demonstrated a strong correlation between volume loss and ischemia time, which is intuitive.  A more difficult PN requiring greater parenchymal volume loss will take longer, whereas a simple PN will be associated with a very brief ischemic interval. When both volume loss and WIT were incorporated into the analyses, volume loss stood out as the predominant factor determining ultimate renal function after PN, whereas WIT only achieved statistical significance on multivariate analysis when it was extended beyond 25 minutes. Most previous studies in this literature were misleading in that they did not incorporate the volume loss factor (figure), and thereby allowed WIT to subsume all of the predictive capacity.  

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In summary, recent studies suggest a more nuanced appreciation of the factors that determine renal function after PN, and this has direct implications with respect to surgical technique. Given the predominance of volume loss, precision during tumor excision and reconstruction of the kidney to preserve as much vascularized parenchyma, appears to be of primary importance.  As long as the ischemic interval is short or a hypothermic approach is applied, most nephrons will recover. A short ischemic interval, by providing a bloodless field, may facilitate a precise PN and thus allow for optimal recovery of renal function. 

Figure Legend: Initial studies using subjective estimates of volume loss have now been supported by quantitative analysis of volume loss, in this case by mathematical modeling of the kidney before and after surgery. In this case 76% of the normal, functional kidney was preserved, and this correlated with a 75% preservation of estimated glomerular filtration rate. The warm ischemia time was 14 minutes, and in this setting of limited warm ischemia (or hypothermia), our data demonstrates that most kidneys will ultimately recover to a level primarily determined by volume loss.  Extended warm ischemia (greater than 20-25 minutes), however, may lead to incomplete recovery from the ischemic injury. 

The authors are with the Center for Urologic Oncology at Cleveland Clinic’s Glickman Urological and Kidney Institute.