Objectives: In the past decade, the implantable Doppler probe has been studied widely as a blood flow-monitoring device in reconstructive and transplant surgical specialities. Its utility as an effective postoperative monitoring technique is still debatable, with no clear guidelines in clinical practice. Here, we mapped the current evidence on the usefulness of the implantable Doppler probe as a blood flow-monitoring device. The objective was to present an up-to-date assessment of the benefits and limitations of using implantable Doppler probes in clinical and experimental clinical settings.
Materials and Methods: We conducted a literature search using the Cochrane Library and Healthcare Databases Advanced Search and using implantable Doppler probe, transplant, graft, and flap as key words. The search yielded 184 studies, with 73 studies included after exclusions. We evaluated, synthesized, and summarized the evidence from the studies in tabular form.
Results: There is clinical equipoise regarding the effectiveness of implantable Doppler probe as a flow sensing technique. The main reason is the lack of information and gaps in the evidence regarding the benefits and limitations of using implantable Doppler probes in clinical practice.
Conclusions: The implantable Doppler probe has the potential to be used as an adjunct postoperative blood flow-monitoring device. However, keeping in view of technical limitations, its signals should be interpreted alongside traditional clinical assessment techniques to determine the patency of microvascular anastomosis. Although evidence in this review will inform clinical practice in transplant and reconstructive surgical specialties, a prospective randomized controlled study with a larger patient cohort is required to evaluate the effectiveness of this probe in clinical settings.

Key words : Free flap, Graft, Transplantation, Vascular flow monitoring

Introduction

In the constant pursuit to improve patient outcomes and reduce graft loss, new flow-sensing technologies are advocated to detect vascular complications promptly and to allow an opportunity for surgical salvage.¹ One such novel technology is an implantable Doppler (ID) probe, a postoperative blood flow-monitoring device.² When placed around a pulsatile main graft vessel, it can provide continuous and easily recordable audible Doppler signals. These signals may be used to monitor blood flow in the vessel directly, indicating patency of the vascular anastomosis and graft perfusion.²

Implantable Doppler probes have been widely studied in reconstructive and transplant surgical specialities with varying outcomes.³ The experience of using an ID probe in monitoring the vascularization of free flaps has suggested its possible role in the early identification of complications in the microvascular anastomosis.⁴ A prompt diagnosis offers an op-portunity for a surgical intervention to rescue the free flaps. Following the same principle, the ID probe may have a role in early diagnosis of vascular comp-lications critical to salvaging transplanted grafts.⁵

Vascular anastomotic occlusion has a catastrophic effect on graft outcomes.⁶ There is a persevering need for a monitoring device for vascular anastomotic surveillance and graft perfusion postoperatively.⁷ However, there is no consensus or any guidelines regarding the usefulness of the ID probe in the clinical practice.⁸

This review aimed to map the current evidence on the usefulness of the ID probe as a blood flow-monitoring device. The objective was to present an up-to-date assessment of the benefits and limitations of using an ID probe in clinical and experimental clinical settings. The evidence in this review will inform clinical practice in transplant and reconstructive surgical specialities.

Function of the Implantable Doppler Probe

The ID probe (Cook-Swartz Doppler Probe, Cook Medical, Bloomington, 2023) comprises a 1-mm2 piezoelectric crystal embedded in a 20-MHz transducer.² The duo is attached to a silicone cuff placed around the vessel and secured with a suture.³ A wire connects the transducer to an external monitoring device3 (Figure 1). The piezoelectric crystal embedded in the transducer converts the kinetic energy of the pulsatile blood flow in the vessel into electric energy.⁴ The connecting wire transmits the electric energy from the transducer to the monitoring device, where it is converted into audible Doppler signals. Interruption of blood flow in the vessels results in the cessation of audible signals.⁴ This is considered an early warning sign for graft hypoperfusion, warranting urgent definitive diagnostic investigations.⁷

The audible Doppler signals can provide continuous monitoring of blood flow in vessels, indicating patency of vascular anastomosis and hence graft perfusion.^2,3 Thus, the ID probe may have a possible role in the early identification of vascular complications critical to reducing graft loss.⁸ The ID probe’s crystal, transducer, and connecting wire are easily removed by a gentle traction that disconnects the wire from the silicone cuff left around the vessel.

Materials and Methods

A literature search was conducted in the Healthcare Databases Advanced Search (HDAS). The HDAS is a medical search catalogue created in collaboration with the National Institute for Health and Care Excellence, National Health Services (NHS), and Health Education England. It is a widely used and recommended database catalogue for conducting research in the NHS.⁹ It includes the databases AMED, CINAHL, EMCARE, MEDLINE, BNI, EMBASE, HMIC, PubMed, and PsycINFO. The literature search inclu-ded the Cochrane Library. No filters were applied. The databases were searched from inception to April 2022.

Key words used with the addition of the Boolean operator (AND) were graft, flap, transplantation, Vascular flow monitoring. Only full-text original research studies in the English language that reported the application of the ID probe in clinical or experimental settings were included in the review. Abstracts, conference proceedings, editorials, letters to the editor, and study protocols were considered.

We identified 171 studies by the database search (Figure 2). After we searched for references related to these studies, we identified a further 13 studies for a total of 184 studies. After removal of 73 duplicates, 111 studies remained. Full-text articles could not be retrieved in 11 studies, and a further 27 studies were ineligible as they did not involve application of the ID probe. Thus, we included 73 studies in the review after their satisfactory full-text assessment.

To date and to our knowledge, no randomized controlled trial investigating the usefulness of the ID probe in clinical practice has been published in the medical literature. Consequently, the studies found in the literature search were preliminary and observational and nonexperimental. The data obtained from the studies included in the review were evaluated, synthesized, and summarized in tabular form (Table 1).

Role of the implantable Doppler probe in experimental animal models
Animal experimental models are used to investigate interventions because they closely replicate human experiences due to similarities in response to injury, anatomy, and physiology.¹⁰ The experimental studies investigating the probe in animal models show inconsistent results. Although the authors describe an overall positive experience, they report the benefits of the monitoring device as overrated Table 1).

Swartz and colleagues studied the monitoring device’s signals on the femoral artery of a dog and a rabbit. The authors noted a 14% error at maximum flow and an 18% error at minimum flow, when comparing the signals with a simultaneously placed electromagnetic flowmeter. In the second part of the study, the device was applied to 63 patients undergoing free tissue transfers. In 3 patients, persistent tracing was noted despite venous thrombosis, and 2 patients experienced the loss of arterial tracing due to malfunction and had records of false positive and false negative results, respectively. There were no complications related to the probe. The authors concluded that the ID probe might hold promise as a vascular monitoring device, although it had limitations.¹¹

In a porcine experimental model study conducted in Denmark, the ID probe was assessed by the stepwise reduction of the renal venous blood flow in 20 pigs; the results were compared with a microdialysis catheter in the pelvis that simultaneously measured glutamate and lactate levels. The results revealed that the device could not detect flow changes until there was total venous occlusion. Moreover, the probe could only detect occlusion of the vessels to which it was attached. Based on these findings, Amdisen and associates proposed that the ID probe is unsuitable for early detection of renal vascular complications.¹²

An experimental study conducted by Kind and associates on the femoral vessels of 6 rabbits investigated the sensitivity of the ID probe in detecting vascular occlusion when placed on an artery compared with a vein. The results showed a persistent trace of audio signals in the artery for 220 ± 40 minutes after the vein was occluded, with a trace of 6 ± 2.5 minutes in the vein when the artery was occluded. The results demonstrated a greater degree of sensitivity for the vein, and Kind and associates reiterated that the probe can only reliably detect occlusion of the vessel on which it is affixed.¹³

Role of the implantable Doppler probe in jejunal transplant surgery
Free jejunal flaps are used to reconstruct circumferential defects in the upper gastrointestinal tracts. Jones and associates investigated the ID probe, first in the isolated jejunal segment of a dog and then in 16 patients undergoing free jejunal transfer procedures. Despite false positive cases, the probe allowed early detection of hypoperfusion due to pedicle kinking or increased wound tension during the wound closure¹⁴. In their study, Cuthbert and associates highlighted the importance of a continuous vascular monitoring device that could be instrumental in reducing graft loss due to vascular complications in free jejunal flap procedures.¹⁵ These early feasibility studies were not followed up by further research in this field.

Role of the implantable Doppler probe in plastic and reconstructive surgery
Most studies investigating the ID probe have been conducted in the speciality of plastic and recon-structive surgery. Differing results are shown due to the lack of standardization and limitations in the research design. These studies have been sum-marized based on their surgical outcomes (Table 1).

The first group of studies showed positive results and suggest significant improvements in the free flap outcome with ID probe monitoring.^4,5 Improved salvage rates were recorded that were attributed to the efficacy of the ID probe monitoring.^6,8,16 The second group of studies highlighted the limitations of the ID probe, implying that the monitoring device had no additional benefits.^11,13,17 These studies have higher false positive rates in the ID probe group.18,19 The third group of studies showed equivocal results that neither advocate nor dissuade the use of the ID probe.^20-22

It is acknowledged that, although the ID probe is associated with high false positive rates, it is an early warning system for detecting vascular complications.^23,24 The authors concluded that the ID probe could be used only as a screening test, and all the positive results should be confirmed with color duplex ultrasonography.¹⁹

Role of the implantable Doppler probe in cardiothoracic surgery
The literature reveals evidence supporting the use of ID probe for assessing graft blood flow in cardiothoracic surgery. The signals provided by the ID probe helped to effectively monitor the blood flow in the internal thoracic artery and gastroepiploic artery following the coronary artery bypass grafting procedure.^25-27 Despite the reported favorable outcomes, monitoring with the ID probe did not find wide acceptance in cardiothoracic surgery practice.

Role of the implantable Doppler probe in liver transplant surgery
Vascular thrombosis leading to graft loss is a dreadful complication in liver transplant as it is associated with high patient mortality. Pediatric liver transplant is linked to a high incidence of vascular thrombosis due to the small size and fragility of the vessels. Graft salvage is dependent on the early detection of vascular thrombosis. There are studies that have investigated the possible role of ID probe monitoring in adult and pediatric liver transplantation.

Cronin and associates described the outcomes of 4 consecutive pediatric liver transplant procedures using ID probe monitoring. Dual monitoring was achieved by attaching the ID probe to the hepatic artery and portal vein. The authors reported that the ID probe provided safe, reliable, and real-time monitoring of the graft vascular patency for up to 7 days postoperatively.¹ In a similar observational study, Kaneko and colleagues studied the outcomes of living donor liver transplant procedures in 10 adults with dual monitoring provided by the ID probe for the first 48 hours postoperatively. The clinicians and medical staff included in the study reflected positively on the ease of signal monitoring and reliability of the ID probe.²⁸

Favorable outcomes were also reported by Kulkarni and colleagues in their observational study that included 40 adult transplant procedures with ID probe monitoring attached to the hepatic artery only. With the reported hepatic artery thrombosis rate of 10.53%, ID probe monitoring displayed a sensitivity and negative predictive value of 100%.²⁹ Byam and colleagues compared graft loss in adult liver transplant procedures with and without the ID probe in the previous 12 years at their transplant center. Two graft losses were reported in the group without ID probe monitoring. Although 2 cases of hepatic artery thrombosis were recorded in the group with ID probe monitoring, the grafts were salvaged. The authors implied that ID probe monitoring was instrumental in the early diagnosis, which allowed a timely surgical correction.³⁰ The above preliminary observational studies displayed favorable surgical outcomes with the use of the ID probe in liver transplant surgery.

Role of the implantable Doppler probe in breast reconstructive surgery
Buried free flaps after skin and nipple-areola complex-sparing mastectomy is the gold standard in breast reconstruction. The studies evaluating the ID probe in monitoring of buried free flaps have conflicting results. These studies have been sum-marized based on their surgical outcomes (Table 1). The first group of studies revealed positive results and reported reliable and safe monitoring of the flap viability by ID probe.^3,31,32 The second group consisted of studies with equivocal results that showed no statistically significant difference in the flap salvage rates.^33,34 The third group comprised studies with negative results. These implied that ID probe monitoring can offer misleading reassurance and delay further interventions critical to salvaging the compromised graft.^35,36 These findings are in line with earlier studies.^8,11

Role of the implantable Doppler probe in neurosurgery
The ID probe has not been studied directly in neurosurgical patients. However, a similar device was used to measure the cerebrospinal fluid flow in the ventriculoperitoneal shunt used in hydrocephalus patients. Numoto and colleagues correctly established cerebrospinal flow in 11 clinical cases.³⁷ Similarly, Miller and Armonda have described the possible role of transcranial Doppler ultrasonography in monitoring cerebral blood flow in critically ill patients of neurocritical care unit.³⁸ More developed methods of monitoring have been implemented in neurocritical settings, and further research on the ID probe was not pursued.

Role of the implantable Doppler probe in uterus transplant surgery
Uterine transplantation, if successful, may provide thousands of women worldwide an opportunity to have children. To date, more than 30 uterus transplant procedures have been performed worldwide. Kengelbach-Weigand and colleagues studied the vascular monitoring of the transplanted uterus in a sheep. The ID probe was applied to the uterine vessels monitoring the patency of the vascular anastomosis. The authors reported that the ID probe proved to be valuable for the surgeons and researchers in assessing the viability of the uterus.³⁹ Further research is anticipated in this field.

Role of the implantable Doppler probe in kidney transplant surgery
Vascular complications account for one-third of the kidney grafts lost in the first month after transplant.⁴⁰ Graft loss may be reduced by the timely detection of vascular complications, allowing an opportunity for a prompt intervention.² The role of the ID probe in the postoperative vascular monitoring of the kidney graft has been debated in the medical literature.²

Crane and Hakim conducted an observational study in which ID probe monitoring was used in 15 consecutive living kidney donor transplant procedures. Although no vascular complications were reported in these cases, the authors concluded that the ID probe is a beneficial vascular monitoring device in the postoperative care of kidney transplant patients.² Hakim and colleagues reported a case of successful identification of vascular complications by ID probe monitoring in living donor kidney transplant procedure. The timely diagnosis allowed a prompt surgical correction that saved the compromised graft.⁷ Although a case report lies at the bottom of the hierarchy of evidence, this is the only description in the medical literature where a compromised graft was successfully salvaged by the timely identifi-cation of vascular complications by ID monitoring.⁷

Malik and associates conducted a retrospective cohort study in which they compared the surgical outcomes of 324 kidney transplant procedures with and without the ID probe monitoring device over 5 years at their unit. They reported a lower graft loss (1.5% vs 3.1%; relative risk = 0.48) in the group with ID probe monitoring compared with the group without. Although this is the largest reported series of kidney transplant patients with ID probe monitoring, the authors described technical limitations and inherent selection bias in the study.^41,42 Further cont-rolled studies are recommended to fully understand the use of ID probe in routine clinical practice.

Results

The studies encompassed a full spectrum of experimental and clinical settings in which the utility of ID probe was studied in different surgical disciplines (Table 1). This review comprises a comprehensive descriptive synthesis of a significant knowledge base obtained from the published information. The benefits and limitations of using an ID probe as a blood flow-monitoring device are summarized below.

Benefits of the implantable Doppler probe
Several of the studies mentioned in our article recommend the ID probe as a helpful adjunct to traditional clinical monitoring in the early posto-perative care of free flaps and grafts.^1,5 The authors advocated that the ID probe was well received by surgeons, nursing staff, and patients.¹⁶ The device has a high sensitivity and specificity rate in detecting loss of graft perfusion when used in collaboration with other assessment techniques.²⁸ The ID probe uses Doppler technology to deliver real-time and continuous information of the vessel patency to which it is attached. This feature allows rapid detection and timely management of postoperative vascular complications.⁸ This means that the ID probe can contribute to increased salvage of flaps and grafts in patients experiencing vascular thrombosis.^6,7

Minimal additional surgical time is required in the application of this device.²⁹ It is convenient to use and easy to monitor by clinicians and nursing staff.³⁰ It has the potential to be used as an early screening tool to detect vascular impairment in microvascular anastomosis.^4,25

Qualitatively, the patients found the sound of the blood flow into their graft reassuring.⁴¹ To allow the patients to sleep, there is a provision to turn down the sound output, which can then be turned up for monitoring as required.

Limitations of the implantable Doppler probe
The medical literature emphasized a few short-comings of the monitoring device. After the duration of the required monitoring, the wire attached to the probe is disengaged by slight traction. The wire, along with the transducer, is pulled out of the patient; however, the silicon cuff is left behind attached to the artery supplying the graft. The clinical safety of the retained probes and complications like tension on the anastomosis and possible vessel constriction have not been studied adequately with long-term follow-up. Kreutz-Rodrigues and colleagues in their retrospective descriptive study assessed 18 free flap patients with retained ID probes for about 3 years. They reported no adverse clinical outcomes related to the probe,16 but further studies looking at the long-term consequences of retained items from the monitoring device are recommended.

As demonstrated in animal models, the continuous audible signal of the monitoring device stops only after the absolute cessation of blood flow in the vessel. The authors implied that, because the ID probe cannot reveal a reduction of blood flow to the graft, the diagnosis may be delayed until complete thrombosis of the vessels.^11,13

The ID probe can only reliably detect vascular occlusion in the vessel to which it is attached.35 An earlier study conducted on experimental and clinical models demonstrated that the ID probe has a greater sensitivity in detecting microvascular thrombosis when placed on a vein compared with the artery.³⁶ However, in some specialities (ie, transplant surgery), due to the thin wall of the veins, the probe is usually placed on the artery.^41,42 Consequently, the probe can only offer an indirect assessment of the venous return. It is unlikely to detect venous thrombosis until the arterial flow is eventually impaired.¹²

The ID probe is sensitive to traction and is easily dislodged. Any accidental entanglement of the probe’s connecting wire during patient movements may result in detachment. This inadvertent traction leads to false positive results (ie, cessation of signals despite normal blood flow in the vessels). A few studies have reported false positive rates as high as 88%, resulting in increased negative surgical reexplorations.⁴³ Color duplex ultrasonography is a rapid and noninvasive test that is the gold standard investigation to assess the graft perfusion postoperatively.¹⁹ Color duplex ultrasonography should confirm all positive cases detected by the ID probe before planning a surgical reexploration.⁴⁴

Although rare, false negative cases (normal Doppler signals despite no blood flow in the vessels) associated with the monitoring device have been reported in the literature. Malik and colleagues reported a case of a complex kidney transplant recipient in which the ID probe was attached around the renal artery for postoperative monitoring. Despite displaying normal Doppler signals, the patient had deteriorating vital signs, elevated lactate, and metabolic acidosis. An urgent color Duplex ultrasonography diagnosed renal vein thrombosis. The patient was reexplored immediately, revealing a grossly discolored and congested allograft with renal vein thrombosis. Interestingly, a continuous trace of Doppler signals persisted during the surgical exploration indicative of the renal artery patency.⁴² A study that included 300 patients undergoing microvascular flaps for breast reconstruction reported 3 false negative cases. The authors advised caution while interpreting ID probe signals.³⁵ Another study also reported prolonged trace of Doppler signals despite venous thrombosis when the probe was attached to the artery for monitoring.³⁶ Persistent signals can falsely reassure the clinicians and cause a critical delay in the diagnosis. The role of traditional clinical assessment and color Duplex ultrasonography in the postoperative monitoring of patients cannot be overemphasized.^11,13

During the detachment of the probe, there is a risk of avulsion of the vessel to which it is attached. Because the probe is not under direct vision during the disengagement, any resultant bleeding can be catastrophic for patients, particularly if they have transplanted organs (ie, liver, kidney). Anctil and associates reported a case of free flap surgery in which the ID probe was attached to the feeding artery for postoperative monitoring. After removal of the probe on postoperative day 7, a leakage in the arterial anastomosis was identified that led to complete occlusion of vessels within minutes.⁴⁵ Likewise, Hayler and associates, in their prospective observational study assessing the ID probe in 100 consecutive free flap operations, also reported a case of pedicle laceration following probe extraction.⁴⁶

Discussion

Multiple studies were found in the medical literature investigating the role of the ID probe in both experimental and clinical settings.^11,25 Although studies have been conducted in different specialities, a consensus on the usefulness of the probe as a postoperative monitoring device has not been established.^4,11 The main reason for this appears to be methodological limitations in the studies that have resulted in inconsistent results.^18,23

Most of the studies are preliminary and comprise retrospective observational analysis describing local experience.^1,4,5 The nonexperimental study design is not considered high on the hierarchy of evidence.⁴⁷ The authors did not discuss any risk stratification during the recruitment of sample populations.^5,16 Recruitment of low-risk participants is commonly seen in preliminary studies aimed at obtaining initial information about an intervention because it facilitates the simple and smooth conduction of study.⁴⁸ High-risk participants with comorbidities that are more susceptible to vascular complications require additional diligence and safeguarding measures.⁴⁹ The drawback of this approach is that the study sample is not fully representative of the target population, including high-risk cases. It is paramount for the accuracy and reliability of the research results that study samples should be representative of the target population.⁵⁰ Furthermore, studies with low-risk patients are unlikely to have sufficient events identified for a benefit (or otherwise) from the intervention to be statistically significant.

None of the studies included measures to reduce confounding factors like randomization or allocation concealment.^2,5,37 In studies without methods to prevent selection bias, unclear recruitment criteria may lead to inaccurate results.⁵¹ Other limitations observed in the methodology include the absence of power calculation and long-term follow-up.^8,11,21 Complications of a procedure are unpredictable and require a rational sample size and sustained follow-up to investigate.^46,52

The authors may intentionally limit the internal validity of preliminary feasibility studies, as a study with simplified methodology allows uncomplicated collection of data in a short time. Because most of the studies were conducted on patients in the standard health care systems, the external validity favors further evaluation in conventional medical setups.^33,53

The studies included in the narrative review have described the application of the Cook-Swartz Doppler probe in different surgical specialities. This is the standard implantable blood flow-monitoring device used in clinical practice.^2,4 Moving forward, developing a wireless monitoring system attached to the ID probe via Wi-Fi or a blue tooth system would be convenient and reduce complications related to the application of the device.

Conclusions

The ID probe has the potential to be used as an adjunct postoperative blood flow-monitoring device. However, in view of the technical limitations, its signals should be interpreted alongside the traditional clinical assessment techniques to determine the patency of microvascular anastomosis. The evidence in this review will inform the clinical practice in transplant and reconstructive surgical specialities. A prospective randomized controlled study with a larger patient cohort is required to evaluate further the effectiveness of the ID probe in clinical settings.