The present study suggests that radioactive labeled microspheres (MS) can identify corticomedullary shunts after ischaemia and reperfusion of the human like polypapillary porcine kidney.
We studied renal blood flow by use of uptake of MS after unilateral renal ischaemia for 2 and 4 h followed by 4 h of observation after reperfusion. After intra-arterial injection of MS the absolute renal uptake of MS was significantly reduced in the kidneys subjected to 4 h total ischaemia. The uptake of MS after 2 h of renal ischaemia did not differ significantly from the uptake in the contralateral non-ischaemic kidney. After 4 h ischaemia, there was a relatively high uptake of MS in the central area of the kidney including the renal medulla both on the scintigrams and calculated from the absolute values. These observations may be explained by the special anatomy of blood supply to the kidneys. Blood supply to the nephron is maintained by two vascular systems organised in a serial manner: the capillaries in the glomeruli and the tubular capillaries in the renal medulla . Thus, intra-arterial injected MS reach the renal cortex with the arterial blood, and in the glomeruli they are trapped due to their diameter of 15 µm and thereby separated from the blood stream. Thus, measuring total renal blood flow may be done from the number of MS in the whole kidney, whereas regional distribution of MS is unreliable in calculation of the relative distribution of renal perfusion. However, this normal trapping of MS in the glomeruli explained by renal anatomy makes it possible to quantify functional active shunts from the number of MS present in the renal medulla. In contrast to the early anatomical studie by Trueta and others  we made a physiological study with MS as a tracer. We measured uptake of radiolabelled MS on planar images by use of a 2-dimensional technique. We did not have access to SPECT/CT images in these pig studies. Therefore, due to anatomical overlap of medulla and cortex the uptake in renal medulla will be overestimated, but the uptake in the central 75% includes the total medulla. Our results are consistent with the opening of corticomedullary shunts in the ischaemic kidney. However, we cannot exclude that vasodilation in cortical juxtamedullary afferent arterioles after long-term ischaemia could explain our findings, although we believe that our 15 µm MS would not be able to pass the capillaries in glomeruli. Our visualisation technique seems to indicate presence of MS in medulla and not in the juxtamedullary cortex.
Our findings of corticomedullary shunts in severely ischaemic kidneys are in agreement with a resent published study by Schutter and coworkers . By use of MRI they assessed the renal flow over time during normothermic machine perfusion (NMP) in porcine kidneys and human kidneys discarded for transplantation. Interestingly, they demonstrated by use or MRI, that the regional renal flow is entirely different from the total renal flow, and they found for all kidneys that the central region and medulla of the kidney was predominantely perfused initially, while the cortex reached a dominant perfusion state after 1–2 h, normalizing the flow distribution in the kidney. In addition, when they added a period of NMP hypoperfusion, demonstration a intrarenal shift of perfusion from the cortical area to the medullar regions. In their study, normal flow distribution was achieved in all kidneys, which was not the case in our study where the corticomedullary shunt remained in 38% and 5% of the kidneys subjected to 4 and 2 h ischemia followed by reperfusion. These are most likely the most damaged kidneys but it would have been interesting to see if these kidney could have improved had they been subjected to NMP instead of in situ reperfusion. From other experimental studies we know, that even severely damaged porcine kidneys with 75 min warm ischemia can be successfully transplanted in a survival model . Prolonged warm ischemia remains a challenge in donation after circulatory death and a better understanding of the cotticomedullary shunts may be of benefit.