Implantable Doppler Probe
The Cook-Swartz Doppler Flow Monitoring System (CDSP) is a technique introduced in 1988 by Swartz et al. [74] This method involves securing a 1.0-mm Doppler probe to a cuff of expanded polytetrafluoroethylene (e.g., GORE-TEX), which is then placed around the flap vessel and sutured in place. [75]
The Doppler probe utilizes a 20-MHz ultrasonic frequency and features a 1-mm2 piezoelectric crystal embedded within a soft silicone sleeve. It connects to a battery-operated or line-powered portable monitor. The probe is secured around the vessel with a small 8x5 mm2 thin silicone sheet cuff, using sutures or clips. Its proximal end exits as a thin wire through the wound and connects to an intermediate extension cable, sutured to the patient using specially designed retention tabs. This cable plugs into a transportable monitor at the patient’s bedside, powered by either battery or mains.[76]
This design ensures precise detection of blood flow velocity and facilitates secure attachment to the blood vessel distal to the anastomosis site, ensuring compatibility with surrounding tissue.[74] During operation, the probe enables simultaneous measurement of velocity and blood flow, establishing a direct correlation between flow and velocity with a relatively low error margin, even under varying flow conditions. [74]
Some authors postulate the addition of implantable systems such as the Cook-Schwartz Doppler probe to the clinical assessment of free tissue transfers. [77] Several cohort studies have demonstrated their usefulness in terms of sensitivity, specificity and false-negative and positive results. This technology can quickly detect compromise, especially in flaps that are not visible and therefore cannot be assessed clinically. [78] However, in a large cohort of 398 breast reconstruction patients, other authors recognized no advantage in the use of Cook-Schwartz Doppler probes; on the contrary, false negative findings of flap compromise were significantly increased. [8]
Comparative Analysis
In the domain of flap surgery, reconstructive surgeons employ a spectrum of both invasive and non-invasive techniques. We provided a comparative analysis of these different techniques where we delve into the nuanced strengths and weaknesses of each approach, with a keen focus on factors such as accuracy, cost-effectiveness, ease of use, and clinical applicability.
Non-invasive techniques offer diverse options. Acoustic Doppler Sonography (HHD) and colour-coded duplex sonography (DS) provide cost-effective solutions, particularly valuable for identifying perforators. HHD excels in breast surgery but may yield false positives, while DS, offering detailed data, can be time-consuming and technically challenging due to its requirement for extensive anatomical knowledge. Near Infrared Spectroscopy (NIRS) effectively measures oxygen level changes and distinguishes between venous and arterial compromise in free flaps, yet lacks dedicated flap assessment systems. Thermal Imaging (TI), a smartphone-based technique, enables early detection of vascular insults but is limited to surface temperature changes, with variable specificity. Photoplethysmography (rPPG) assists in differentiating between venous and arterial thrombosis but may require prior visualization of vascular anatomy.
Invasive techniques introduce different considerations. Contrast-enhanced ultrasound (CEUS) provides accuracy in locating small-lumen perforating vessels but carries risks associated with intravenous contrast agents. Computed Tomography Angiography (CTA) offers high accuracy and shorter examination times but involves ionizing radiation, limiting its use in specific patient populations. NIRF with ICG, an invasive method, enables real-time visualization of vessel structure but entails potential risks associated with intravenous injection. The implantable Doppler probe rapidly detects compromise, especially in non-visible flaps, though controversial findings regarding sensitivity and false-negative results raise concerns about its advantages.
In the overall evaluation, each technique, be it non-invasive or invasive, presents a unique set of advantages and limitations. Non-invasive methods excel in accessibility and safety but may sacrifice specificity. Invasive techniques offer high accuracy but introduce additional risks. The selection between these approaches necessitates a tailored decision-making process, taking into account the specific clinical scenario, patient characteristics, procedural requirements, and the surgeon’s expertise. A concise overview of the strengths and weaknesses of each flap-assessment technique, categorized by non-invasive and invasive approaches, is provided in table 1.