Gadolinium-enhanced MRI visualizing backflow at increasing intra-renal pressure in a porcine model

Abstract

Introduction Intrarenal backflow (IRB) is known to occur at increased intrarenal pressure (IRP). Irrigation during ureteroscopy increases IRP. Complications such as sepsis is more frequent after prolonged high-pressure ureteroscopy. We evaluated a new method to document and visualize intrarenal backflow as a function of IRP and time in a pig model. Methods Studies were performed on five female pigs. A ureteral catheter was placed in the renal pelvis and connected to a Gadolinium/ saline solution 3 ml/L for irrigation. An occlusion balloon-catheter was left inflated at the uretero-pelvic junction and connected to a pressure monitor. Irrigation was successively regulated to maintain steady IRP levels at 10, 20, 30, 40 and 50 mmHg. MRI of the kidneys was performed at 5-minute intervals. PCR and immunoassay analyses were executed on the harvested kidneys to detect potential changes in inflammatory markers. Results MRI showed backflow of Gadolinium into the kidney cortex in all cases. The mean time to first visual damage was 15 minutes and the mean registered pressure at first visual damage was 21 mmHg. On the final MRI the mean percentage of IRB affected kidney was 66% after irrigation with a mean maximum pressure of 43 mmHg for a mean duration of 70 minutes. Immunoassay analyses showed increased MCP-1 mRNA expression in the treated kidneys compared to contralateral control kidneys. Conclusions Gadolinium enhanced MRI provided detailed information about IRB that has not previously been documented. IRB occurs at even very low pressures, and these findings are in conflict with the general consensus that keeping IRP below 30–35 mmHg eliminates the risk of post-operative infection and sepsis. Moreover, the level of IRB was documented to be a function of both IRP and time. The results of this study emphasize the importance of keeping IRP and OR time low during ureteroscopy.

Understanding intrarenal backflow: Intrarenal pressure during ureteroscopy and beyond

“…[ 11 ], showing that damage due to experimental vesicoureteral reflux in some of the less severely affected areas of the kidney, appeared to be limited to the center of the medullary pyramid with normal renal parenchyma at the borders. Additionally, it appears that IRB happens first and at lower pressures in the upper pole [ 13 ], which is consistent with compound papillae being more frequently observed in the upper calyces in both human and pig kidneys [ 11 ].…”

“…Based on animal studies and studies during retrograde pyelography in humans, pyelotubular backflow occurs at 20–30 mmHg (27–41 cmH 2 O), pyelovenous backflow at 30–50 mmHg (41–68 cmH 2 O), and forniceal rupture at 70–200 mmHg (95–272 cmH 2 O) [ 1 , 2 ]. Newer studies using dynamic gadolinium-enhanced MRI have suggested, however, that IRB may occur at considerably lower pressures, starting at mean pressures as low as 15 mmHg (20 cmH 2 O) [ 13 ]. Thus, it seems logic that IRB happens as a continuous function of pressure, initially appearing in the center of the medullary pyramid, where the collecting ducts are less protected against reflux, compared to the peripheral ducts that end on the oblique part of the papillary surface.…”

“…The limitation of the study is that the experiment was done in ex-vivo kidneys, and, thus, data do not consider the dynamics of the system. Recently, for the first time, IRB was documented dynamically in-vivo by MRI, using gadolinium-saline solution for irrigation in a pig model [ 13 ]. Under controlled irrigation, the IRP was increased while repeating MRI scans until a maximum pressure of 50 mmHg was reached.…”

“…The mean time to the first visualization of IRB to the cortex was 15 min, and the mean IRP at first visual changes was 21 mmHg, which is considerably lower than previously assumed from in-vitro studies [ 14 , 15 ]. The data suggested that IRB is a function of both pressure and time, and it was suggested to use a measure for accumulated ureteroscopy-induced pressure impact on the kidney (IRB multiplied by operation time) to be used for comparison of postoperative clinical outcome in ureterorenoscopic procedures [ 13 ]. Subsequent evaluation of renal cortex tissue showed significant up-regulation of the proinflammatory cytokine, monocyte chemoattractant protein-1, suggesting that early IRB may induce inflammation [ 13 ].…”

“…The data suggested that IRB is a function of both pressure and time, and it was suggested to use a measure for accumulated ureteroscopy-induced pressure impact on the kidney (IRB multiplied by operation time) to be used for comparison of postoperative clinical outcome in ureterorenoscopic procedures [ 13 ]. Subsequent evaluation of renal cortex tissue showed significant up-regulation of the proinflammatory cytokine, monocyte chemoattractant protein-1, suggesting that early IRB may induce inflammation [ 13 ].

Figure 1 MRI showing intrarenal backflow into the renal cortex during irrigation with gadolinium-saline solution, visualized as pyramid shaped changes first appearing at the upper and lower poles (dark blue).

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