Clamping kidney blood vessels prior to surgery gives doctors a brief 30-minute window before ischemia damages the organ. Cooling the kidney, however, slows tissue deterioration due to lack of oxygen and extends surgery times. But how much cooling? Too much could freeze tissue, not enough shorten surgery period, and every kidney is different. To address the need, Argonne National Laboratory, Argonne, Ill., and the University of Chicago Medical School developed several coolants made of micro-particulate ice slurries.

“Slurries are made of finely chopped ice or by nucleation on a cold flexible membrane,” says Argonne's Energy Technology Division senior mechanical engineer Ken Kasza. “We chemically and thermally smooth these to less-than-100-micron ice particles so they can be pumped through small delivery tubes and injector tips.” The coolant extends safe clamp-off time (the surgical time) to more than 90 minutes.

To validate the slurry's efficiency, the team simulated kidney cooling using finite-element analysis. Kasza and colleague F. C. Chang wrote a commonly used bio-heat transfer equation into the FEA software. The equation models tissue metabolism and thermal perfusion behavior.

The researchers used cooling simulations to model kidney-temperature data from two cooling tests involving laparoscopic and transperitoneal surgical access to a kidney. Simulations involved covering the kidney with an ice slurry immediately after blood vessel clamp-off with a mesh-boundary temperature of 0°C, and metabolic and perfusion rates in the equation set to zero.

Results were compared to physical data from similar surgeries. Predictions agreed well with physical measurements during cool-down with clamped blood vessels. Results also agreed reasonably well during warm-up, when kidneys are unclamped, perfusion resumes, and the metabolic rate rises.

The next step is to create higher-resolution kidney models using CT or MRI data. These will show major blood vessels, voids, and tumors in patient kidneys. “The goal is to create a patient-specific kidney simulation that surgeons can review while planning cooling strategies for that patient's surgery,” says Kasza.