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Cevallos et al. Plast Aesthet Res 2023;10:30 https://dx.doi.org/10.20517/2347-9264.2023.01 Page 9 of 17

Invasive techniques
Invasive techniques for free flap monitoring are utilized for buried flaps when surface monitoring is not
feasible. An ideal implantable device should possess the following characteristics: (1) enable continuous
real-time monitoring of anastomotic patency; (2) reliably distinguish between arterial and venous
occlusions; and (3) be applicable for monitoring both cutaneous and buried flaps. Various devices have been
developed for invasive flap monitoring and ongoing research focuses on device refinement [42-47] . Current
electronic flap monitoring techniques offer qualitative and quantitative information on flap perfusion,
including flow patterns and quality [48-50] . However, there are limitations associated with the use of invasive
monitoring techniques, such as the requirement for expert interpretation of results, inadequate equipment
[48]
designs and potential malfunction, and high costs .
Implantable Doppler & venous coupler
Swartz et al. were the first to publish on the utilization of an implantable probe for continuous monitoring
[92]
of microvascular anastomoses . Their study employed a 20-MHz ultrasonic Doppler probe encased in a
silicone sleeve positioned distal to an anastomosis, demonstrating the probe’s effectiveness in monitoring
[92]
patency and occlusion up to 4 weeks post-operatively . This system, now referred to as the Cook-Swartz
implantable Doppler (Cook Medical®, Ireland), has since become a valuable tool in clinical post-operative
monitoring, particularly for buried flaps, such as those in nipple-sparing mastectomy procedures
[Figure 1B]. Apart from the Cook-Swartz implantable Doppler used in buried flaps, an external
transcutaneous Doppler signal can be marked on the mastectomy skin intra-operatively, following
temporary clamping of the flap vascular pedicle, to minimize potential signals confusion between the
mastectomy skin and the flap. In 1994, Swartz et al. discovered that Cook monitoring of venous
anastomoses exhibited greater sensitivity to both venous and arterial thrombosis compared to solely
detecting arterial anastomoses [92,93] . Since its introduction, there have been numerous advancements aimed
at enhancing the monitoring of capabilities of Cook Doppler devices, which heavily rely on their
positioning. These improvements encompass non-attachment around a venous pedicle, microclip fixation,
suture fixation, and elongation of the silicone cuff. The success of these developments has varied, and their
implementation may lead to prolonged operating times and necessitate technical expertise to ensure proper
intraoperative placement [94,95] . Additionally, complications associated with Cook Dopplers include retained
wires, pedicle laceration during extraction, and signal interruption due to clot formation around the
[96]
probes .

The Flow Coupler is an additional invasive post-operative monitoring device designed to detect the patency
of microsurgical anastomoses [Figure 1B]. Developed by Synovis Life Technologies in 2010, this device
consists of a removable 20-MHz Doppler that provides real-time data on vessel patency. Preliminary data
suggests that the Flow Coupler is both effective and reliable. However, studies comparing the results of the
Cook-Swartz implantable Doppler and the Synovis Flow Coupler have not identified any significant
differences in false positive or false negative results, or the need for salvage procedures . The Flow Coupler
[97]
may have a higher false positive rate and a higher incidence of vascular thrombotic events compared to
[98]
non-flow coupler devices . A pooled meta-analysis revealed that the use of a Cook-Swartz Doppler in
combination with clinical monitoring is associated with significantly higher rates of flap survival but also
[96]
increased Incidence of false positive results . When compared to microdialysis, the implantable Cook-
Swartz Doppler demonstrated earlier detection of flap compromise by 60 min and reduced time to re-
exploration compared to external Doppler monitoring [99,100] .

Transcutaneous oxygen tension monitoring
Tissue oxygen tension monitoring is a commonly used invasive method to assess flap perfusion and viability
[101]
[Figure 1C] . This technique involves the use of probes that compare the partial pressure gradient