Near-infrared tissue oximetry and digital image analysis: Quantification of renal ischaemia in real time during partial nephrectomy

OBJECTIVE To determine the feasibility of using near-infrared tissue oximetry (TO) and digital image analysis for assessing renal function and to quantify local renal ischaemia in a porcine model. MATERIALS AND METHODS Tissue oximetry was performed and red/blue (R/B) colour ratios were determined on renal units of Yorkshire swine. Interval measurements were taken before clamping the renal hilum, during warm ischaemia, and after unclamping using a ViOptix T.Ox ^TM Tissue Oximeter and Matlab® digital image analysis. Matlab software analysed images from the laparoscopic camera and determined an R/B ratio to track renal ischaemia. The tissue oximeter used direct infrared light and was placed adjacent to the kidney parenchymal surface. The data were divided into preclamp, clamp and post-clamp, and compared between methodologies. RESULTS The R/B ratio showed a higher rate of change compared with TO during clamp time in both the 15-min experiment (R/B = 96.0 vs. TO = 52.1 unit/reference) and the 30-min experiment (R/B = 97.6 vs. TO = 45.9). The R/B ratio showed a higher rate of change compared with TO at 1 min after clamping in the 15-min experiment (R/B = 80.1 vs. TO = 12.4). Both detection devices showed similar changes in pre- and post-clamp measurements in the 15- min experiment (R/B = 1.6 vs. TO = 3.8) and the 30-min experiment (R/B = 4.7 vs. TO =-4.5). In the 30-min experiment the R/B ratio showed a significant difference between preclamp, clamp, and post-clamp states (P= 0.026). CONCLUSIONS Both TO and digital image analysis were able to calculate an ischaemic drop in tissue oxygen saturation during periods of acute renal ischaemia. The findings suggest that the R/B ratio observed during histogram analysis shows a greater sensitivity compared with TO in quantifying renal ischaemia.