EXPERIMENTAL AND CLINICAL TRANSPLANTATION

About 5% to 6% of kidney transplants are lost within 30 days of implant.¹ Vascular problems constitute 30% to 35% of this graft loss.² New technologies like the implantable Doppler probe may reduce the preventable causes of graft loss. The implantable Doppler probe is a blood-flow monitoring device that provides real-time information and direct assessment of anastomotic patency. Traditional clinical assessment with routine color duplex ultrasonography remains the gold standard postoperative technique to monitor kidney transplant recipients. The implantable Doppler probe has the potential for use as a beneficial adjunct to monitor postoperative vascular function.³ However, the implantable Doppler probe has some limitations; for example, it provides indirect evidence of blood flow in the transplant renal vein. To our knowledge, this is the first description of a false-negative case (ie, normal Doppler signals despite no blood flow in the vessel) displayed by the implantable Doppler probe in a kidney transplant recipient. This case report is aimed to alert clinical practitioners to the limitations of implantable Doppler probes in kidney transplant surgery. Informed consent was obtained from the patient before publication of this case report.

A 41-year-old male kidney transplant recipient whose primary cause of the established renal disease was oligomeganephronia hypoplasia presented to our unit with a failed third graft attributed to chronic antibody-mediated rejection. Epilepsy and hyper-tension were his only comorbidities. He was offered a reasonably matched (011 mismatch) kidney from a 52-year-old male deceased donor (circulatory death). The transplant patient had a complex surgery due to retroperitoneal fibrosis scar tissue from previous transplant procedures. The kidney was from the right-hand side, with single vessels, and was implanted intra-abdominally with the renal artery anastomosed to the external iliac artery and the renal vein to the external iliac vein. The cold ischemia time was 16 hours. An implantable Doppler probe was attached to the renal artery, and satisfactory Doppler signals were recorded during the closure of the wound. On the first postoperative day, the patient developed tachycardia and hypotension. He remained anuric. Our investigations showed elevated inflam-matory markers and lactate. An urgent color duplex ultrasonography scan was requested that revealed reversed diastolic flow in the artery with complete renal vein thrombosis. The patient was reexplored immediately in the surgical theater, and a grossly discolored and congested allograft with renal vein thrombosis was revealed (Figure 1). A continuous trace of Doppler signals persisted during the surgical exploration, which is indicative of renal artery patency. The operative findings were concordant with the report of the color duplex ultrasonography scan. Unfortunately, the patient underwent a transplant nephrectomy and reverted to hemodialysis.

During the past 7 years at our center, the implantable Doppler probe has been used to postoperatively monitor more than 250 kidney transplant patients. Because of its capability to provide continuous and real-time data, we advocate that this device may have a role as a beneficial adjunct in clinical practice.⁴ However, this discovery of a false-negative case displayed by the implantable Doppler probe and its potential implications in a kidney transplant patient is important information for renal transplant clinicians. Despite the renal vein thrombosis in the allograft, the persistent Doppler signals were due to the back-and-forth blood flow in the renal artery. This is caused by the blood pressure shifts during the cardiac cycle against the high intrarenal resistance of the thrombosed graft.⁵ This characteristic flow pattern in the renal artery is also distinguished by color duplex ultrasonography scan as reverse diastolic flow and is known to be a reliable indicator of renal vein thrombosis with a sensitivity of 97%.⁶ The false-negative cases provide erroneous reassurance of patency and thereby lead to a critical delay in the diagnosis and further intervention to salvage an allograft. This limitation of the implantable Doppler probe has also been highlighted in previously published studies. Swartz and colleagues studied 63 patients undergoing free-tissue transfers who were clinically monitored postope-ratively with an implantable Doppler probe for up to 4 weeks. They reported 3 false-negative cases that displayed a normal trace of Doppler signals despite venous thrombosis of the supplying pedicle.⁷ In another clinical study involving 133 patients undergoing free-tissue transfers who were monitored with implantable Doppler probes, Swartz and colleagues demonstrated that the sensitivity of the implantable Doppler probe to detect microvascular thrombosis was optimized when placed on the vein versus the artery.⁷ In an experimental animal model, Swartz and colleagues further showed that, when the probe was placed on the artery, the implantable Doppler probe did not allow for rapid detection of venous thrombosis and a continuous trace of Doppler signals persisted for several hours despite venous thrombosis.⁸ Although these outcomes are concordant with the findings of our case report, we acknowledge the limitation that an implantable Doppler probe should not be placed on the renal vein, which has a thin wall and is frail. Therefore, when the probe is removed at the end of the monitoring by a gentle but firm pull, the traction applied to disengage the probe might result in the avulsion of the renal vein.⁹ Kind and Oliva conducted a study of 300 patients who underwent microvascular flap procedures for breast reconstruction and were monitored with implantable Doppler probes. Three false-negative cases were reported in their study. The Doppler signals in these patients remained unchanged despite the observation of internal mammary vein thrombosis.¹⁰ Their study reflects the importance of caution for interpretation of the signals from the implantable Doppler probe, when applied on the vessels of buried tissues. In a kidney transplant, the loss of Doppler signals may indicate loss of blood flow in the renal artery; however, the presence of these Doppler signals may not necessarily rule out venous complications.

An implantable Doppler probe may be a beneficial adjunct to postoperatively monitor kidney transplant patients. However, the Doppler signals should be interpreted alongside the traditional clinical assessment techniques to ensure safe and effective postoperative monitoring. Further controlled studies are recommended to evaluate the effectiveness of implantable Doppler probes in clinical settings.

Volume: 21 Issue: 1 January 2023

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