Online rapid sampling microdialysis (rsMD) using enzyme-based electroanalysis for dynamic detection of ischaemia during free flap reconstructive surgery

Abstract

We describe an enzyme-based electroanalysis system for real-time analysis of a clinical microdialysis sampling stream during surgery. Free flap tissue transfer is used widely in reconstructive surgery after resection of tumours or in other situations such as following major trauma. However, there is a risk of flap failure, due to thrombosis in the flap pedicle, leading to tissue ischaemia. Conventional clinical assessment is particularly difficult in such 'buried' flaps where access to the tissue is limited. Rapid sampling microdialysis (rsMD) is an enzyme-based electrochemical detection method, which is particularly suited to monitoring metabolism. This online flow injection system analyses a dialysate flow stream from an implanted microdialysis probe every 30 s for levels of glucose and lactate. Here, we report its first use in the monitoring of free flap reconstructive surgery, from flap detachment to re-vascularisation and overnight in the intensive care unit. The on-set of ischaemia by both arterial clamping and failure of venous drainage was seen as an increase in lactate and decrease in glucose levels. Glucose levels returned to normal within 10 min of successful arterial anastomosis, whilst lactate took longer to clear. The use of the lactate/glucose ratio provides a clear predictor of ischaemia on-set and subsequent recovery, as it is insensitive to changes in blood flow such as those caused by topical vasodilators, like papaverine. The use of storage tubing to preserve the time course of dialysate, when technical difficulties arise, until offline analysis can occur, is also shown. The potential use of rsMD in free flap surgery and tissue monitoring is highly promising.

Figure

Free flap surgery timeline: The flap is raised and MD probe inserted. Glucose and lactate levels were monitored at 1 minute intervals throughout flap removal and the reconstruction of the tongue. Grey lines indicate key events as communicated by the surgeons in real time.

Fig. 1

The microdialysis probe is inserted into the tissue flap subcutaneously and sutured into place, shown in the photograph in (a) before the flap has been detached from the blood vessels. The dialysate is introduced to the rsMD injection valve via low-volume FEP connection tubing (blue dotted line). The reaction is mediated by ferrocene monocarboxylate, which is perfused at 200 μL/min using an HPLC pump. The enzyme is loaded onto membranes placed before a glassy carbon electrode which is held at 0 V (b). The current is recorded as peaks, where amplitude is related to concentration as shown in the calibration curve in (c). Raw data of a successful arterial anastomosis (indicated by the dotted grey line) are shown in (d). A time line indicating the calibration protocol throughout monitoring is shown in (e)

Fig. 2

Raw data from dialysate collected during surgery into a collection tubing. The probe was removed from the tissue during collection. The dialysate was stored for 10 days at 4 °C and analysed using rsMD. The data shown have had no noise removal algorithms applied. The top graph shows glucose data and the bottom graph shows lactate data

Fig. 3

Analysis of data collected during free flap surgery. The top graph (blue) shows the lactate/glucose ratio. The bottom graph shows glucose (red) and lactate (green) levels throughout the reconstructive surgery. Time zero is the time from stable baseline (10 min after probe insertion). The dotted lines indicate key events logged as communicated by the surgeons to the monitoring team in real time

Fig. 4

Analysis of data collected during ICU monitoring. The top graph shows the lactate/glucose ratio. The bottom graph shows the glucose (red) and lactate (green) levels

Fig. 5

Two extracts from one surgery showing (a) a failed anastomosis and (b) a successful anastomosis in terms of lactate (green), glucose (red) and the lactate/glucose ratio (blue). Time zero is the time of connection, conveyed by the surgeons and recorded in real time

Fig. 6

Histograms highlighting key moments during rsMD monitoring: (i) Baseline. (ii) Flap detached. (iii) Flap reattached. (iv) Levels at the end of monitoring in ICU. A Mann–Whitney–Wilcoxon test was used to assess the changes from baseline within each case