Methods

Thirty-six patients undergoing microvascular FTT for head and neck cancer were recruited into this prospective open non-randomised study between June 2005 and June 2007. Each patient underwent standard clinical monitoring (SCM) and CEU of the free flap several times following surgery. The techniques were assessed for their ability to predict the primary outcome measure – a successful or failed/failing free tissue flap. Successful was defined as a healthy tissue flap on clinical inspection 14 days postoperatively that did not require any prior intervention. A failed or failing flap was one that demonstrated evidence of ischaemia or necrosis at re-exploration surgery and would not have survived unless operated upon. The time taken by SCM and CEU to detect free flap failure was also recorded.

As this was a feasibility study designed to assess the accuracy of CEU in monitoring for healthy or ischaemic flap perfusion and its relation to outcome, the results of the CEU were not made known to the chief surgeon and not allowed to influence the decision to return to theatre, which was based on SCM alone. Ethical approval was granted by the NHS Research Ethics Committee and patient consent obtained.

For SCM, the non-buried free flaps were assessed for flap colour, temperature, capillary refill time, and bleeding on pinprick (if indicated). Both buried and non-buried flaps underwent hand-held CWD of the arterial vascular pedicle. SCM took place hourly for the first 2 days and then 4-hourly for 11 days. The flap was recorded as either healthy or not-healthy.

All patients underwent CEU 12, 24 and 48 h postoperatively. Contrast (Sonovue ^® , Bracco Int SPA, Milan) was given as a continuous intravenous infusion in a 5-ml solution. Images were obtained with a Sonos 5500 ultrasound platform (Philips Medical Systems, UK) with a 5-MHz transducer. Tests were executed by one of two clinicians involved in the surgery together and a cardiologist operating the ultrasound platform; although the complete imaging protocol could easily be achieved by a single operator. Flap identification, orientation and margin delineation were made using two-dimensional (2D) imaging with a surgical instrument placed on one surface of the flap (which is clearly seen on ultrasound). Buried flaps were located using anatomical landmarks appropriate to the location of the flap. The flap was reviewed in three planes to obtain the optimum scanning plane. Following this, CEU took place in real time using power modulation mode with a mechanical index of 0.1. Microbubbles were continuously infused at a rate of 1.5 ml/min and, once a steady state was reached (60 s), the ultrasound gain, image depth, focus and sector angle settings were adjusted optimally. Following flash imaging (microbubble destruction), 15 cardiac cycles were digitally recorded in DICOM format to demonstrate tissue replenishment with microbubbles.

Three sequences were recorded at each stage. Using this technique the flap was assessed qualitatively and quantitatively. Qualitative analysis required the observer to decide visually whether microbubbles were flowing into the whole flap following microbubble destruction and decide whether the flap was perfused (yes or no). Quantitative analysis was performed offline in which free flap blood velocity and volume was calculated using QLAB Advanced Quantification software (Philips Medical Systems, Nederlands BV). A region of interest was selected in the centre of flap but away from the arterial pedicle. The software generated a replenishment curve following bubble destruction according to the replenishment curve-fit-model: y ( t ) = α (1 − exp(− βt )) + C (where α is peak intensity, β is refilling rate, t is time and C is baseline intensity). The frame immediately following bubble destruction was selected to represent background signal intensity and this was subtracted to zero the software. All readings were carried out with results blinded to the surgeon in charge.

For statistical analysis, CEU and SCM were compared in their abilities to predict the primary outcome measure as defined above. Sensitivities, specificities, positive and negative predictive values were calculated for each technique. A paired t -test was used to compare the times taken by each method to detect flap failure. A P value of <0.05 was taken as statistically significant. Logistic regression was used to model the relationship between flap outcome and the α value. Odds ratios were obtained for the chances of flap success per dB change in the α value.

Results

Thirty-six patients undergoing FTT for maxillofacial cancer were recruited between June 2005 and May 2007. The mean (range) age of the patients was 57 (29–82) years; 22 (61%) were male. Eight (22%) flaps were buried and 28 (78%) were non-buried. Table 1 shows that the majority of procedures performed were glossectomies ( N = 14, 39%) and the main FTT method used was the radial forearm fasciocutaneous flap ( N = 21, 58%). Six (17%) flaps failed (one buried) and 30 (83%) were successful (five buried). Overall success rate (including non-recruited patients and following salvage) was 94%. There were no adverse incidents associated with the administration of contrast agent. 2D imaging, used to define the flap boundaries, was quick and simple to perform but localisation with a surgical instrument was not possible with buried flaps. Identification of the pedicle was straightforward but occasionally maintaining an ideal image for Doppler readings was difficult due to the small size of the vessels and movement (voluntary or related to normal respiratory movements or swallowing). The flap boundaries were clearly visible due to the different characteristics of the flap tissue adjacent to native tissue (e.g. radial fascia adjacent to native tongue muscle). Differences in intra-flap tissue vascularity could also be seen, for example skin, muscle and fascia could be differentiated on the basis of their reflectivity and contrast intensities. Bony flaps cast acoustic shadows in the far field but the tissue in this area was visible when imaged in a different plane.