Monitoring of microvascular free flaps following oropharyngeal ...

Eur Arch Otorhinolaryngol DOI 10.1007/s00405-015-3780-9

HEAD AND NECK

Monitoring of microvascular free flaps following oropharyngeal reconstruction using infrared thermography: first clinical experiences Maren Just1 • Claire Chalopin2 • Michael Unger2 • Dirk Halama3 Thomas Neumuth2 • Andreas Dietz1 • Milosˇ Fischer1

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Received: 17 May 2015 / Accepted: 8 September 2015 Ó Springer-Verlag Berlin Heidelberg 2015

Abstract The aim of this study is to investigate static and dynamic infrared (IR) thermography for intra- and postoperative free-flap monitoring following oropharyngeal reconstruction. Sixteen patients with oropharyngeal reconstruction by free radial forearm flap were included in this prospective, clinical study (05/2013–08/2014). Prior (‘‘intraop_pre’’) and following (‘‘intraop_post’’) completion of the microvascular anastomoses, IR thermography was performed for intraoperative flap monitoring. Further IR images were acquired one day (‘‘postop_1’’) and 10 days (‘‘postop_10’’) after surgery for postoperative flap monitoring. Of the 16, 15 transferred free radial forearm flaps did not show any perfusion failure. A significant decreasing mean temperature difference (DT: temperature difference between the flap surface and the surrounding tissue in Kelvin) was measured at all investigation points in comparison with the temperature difference at ‘‘intraop_pre’’ (mean values on all patients: DTintraop_pre = -2.64 K; DTintraop_post = -1.22 K, p \ 0.0015; DTpostop_1 = -0.54 K, p \ 0.0001; DTpostop_10 = -0.58 K, p \ 0.0001). Intraoperative dynamic IR thermography showed typical pattern of nonpathological rewarming due to re-established flap perfusion

& Milosˇ Fischer [email protected] 1

Clinic of Otolaryngology, Head and Neck Surgery and Department of Head Medicine and Oral Health, University of Leipzig, Liebigstr. 10-14, 04103 Leipzig, Germany

2

Innovation Center Computer Assisted Surgery, ICCAS, University of Leipzig, Leipzig, Germany

3

Clinic of Maxillo-Facial-Surgery and Department of Head Medicine and Oral Health, University of Leipzig, Leipzig, Germany

after completion of the microvascular anastomoses. Static and dynamic IR thermography is a promising, objective method for intraoperative and postoperative monitoring of free-flap reconstructions in head and neck surgery and to detect perfusion failure, before macroscopic changes in the tissue surface are obvious. A lack of significant decrease of the temperature difference compared to surrounding tissue following completion of microvascular anastomoses and an atypical rewarming following a thermal challenge are suggestive of flap perfusion failure. Keywords Free tissue transfer Microsurgery Free-flap monitoring Perfusion failure Thermal imaging Dynamic infrared thermography

Introduction Free microvascular tissue transfer, like the free radial forearm flap, has become a reliable and well-established technique in reconstructive surgery with success rates higher than 95 % [1–3]. Postoperative monitoring of free flaps relies on skin colour, capillary refill, turgor and palpation of the arterial pulse wave to provide visual and manual evidence of flap perfusion. Recognizing the visual cues of flap failure requires considerable clinical experience [4]. To date, objective flap monitoring for oropharyngeal reconstructions is limited. Recently, systemic analysis showed that techniques such as implantable Doppler, indocyanine green (ICG) angiography, tissue oximetry as well as dynamic infrared thermography (DIRT) have the potential to provide objective data for intraoperative perfusion assessment [5]. Infrared (IR) thermography measures the electromagnetic radiations emitted by a warm body which are proportional to its

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temperature. Thermal cameras include sensors that convert IR radiation into images where the pixel values are proportional to the temperature values. This technique is an interesting alternative imaging method because it is easy to use, non-invasive and contactless. Therefore, it is convenient for an intraoperative use. Medical applications of thermography are wide [6] and are most often used to investigate a temperature change revealing a possible pathology. These applications include among others the monitoring of skin temperature [7, 8], the detection of breast cancer [9, 10], skin cancer [11] and inflammation [12]. In the operating room, thermography was investigated for neurosurgical applications [13, 14] and plastic breast reconstruction [4]. The purpose of this study is to evaluate the static and dynamic infrared thermography as a non-invasive monitoring technique and its potential to provide objective intraand postoperative data for perfusion assessment of free tissue transfer following oropharyngeal reconstruction. It was demonstrated that a suitable dermal perfusion in the transferred tissue results in higher thermal radiation of the tissue surface [4].

Materials and methods Subjects Altogether, 16 patients (2 female, 14 male) suffering from resectable oropharyngeal carcinoma were included in this clinical and prospective study in the period from May 2013 to August 2014. The mean age was 58.4 years (range 51–69 years). According to the pathological examination (TNM classification, 7th edition), the tumour states were distributed as following: 1x T1, 2x T2, 11x T3 and 2x T4a. All patients underwent oropharyngeal reconstruction regarding soft palate and the tonsillar region. Inclusion criteria were the age of the patient between 18 and 70 years, estimated complete resection of the tumour (R0resection, free margin [5 mm) as well as an adequate preoperative general state of health (ASA 1–3). Exclusion criteria were the missing informed consent and a flap reconstruction, which could not be observed by transoral investigation (e.g. complex hypopharyngeal reconstructions). Patients with serious arteriosclerotic disease were excluded from free-flap reconstruction. Eight of the 16 patients suffered from arterial hypertension and one patient suffered from mild peripheral arterial occlusive disease and s/p coronary artery bypass graft. All patients received the same dose of 0.6 low molecular weight heparins per day for postoperative thromboembolism prophylaxis. The study protocol was approved by the institutional review board (AZ.: 099-13-22042013).

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Technical details of the thermal camera The thermal camera PI 450 (Optris GmbH, Berlin, Germany) used to measure the flap temperature, is a commercial model principally available for technical investigations (e.g. architectural thermography, heat loss detection on buildings, security techniques or identification of heat sources in civil emergency). It measures the radiations in the infrared (IR) field (7.5–13 lm) and it is CE approved. This small (46 mm 9 56 mm 9 90 mm) and even lightweight (320 g) IR camera has a thermal resolution of 40 mK and an optic resolution of 382 9 288 pixel. This compact model is suitable for an intraoperative use and the high thermal resolution enables the detection of minimal temperature differences. The calibration of the IR cameras is automatically performed by the company, Optris. The process consists in measuring 17 well-reproducible temperatures, like the melting point of pure metals, as it is proposed in the International Temperature Scale of 1990 (ITS-90). These temperature points are provided with very high accuracy by a radiating system emitter and are carefully controlled using a laser infrared thermometer insuring high precision in the calibration [15]. The IR camera is plugged to a regular laptop by a USB-connection. The software ‘‘Optris PI Connect’’ (Optris GmbH, Berlin, Germany) offers the acquisition of the thermal images in real time, their storage and the analysis of the recorded thermal images. ‘‘Regions of interest’’ (ROI) can be selected and their characteristics such as minimal, maximal and average temperature can be displayed and illustrated as time-dependent graphs. Surgical technique All patients signed informed consent for participation in the study. To evaluate the suitable forearm for harvesting the radial forearm flap the Allen-Test (recovered perfusion B5–7 s), was performed. All arterial anastomoses were sutured end-to-end. Of the 16, 15 venous anastomoses were sutured end-to-side connecting with the internal jugular vein. In one patient, the venous anastomosis was performed as an end-to-end anastomosis using a microvascular coupler system (GEM Microvascular Anastomotic COUPLER SystemTM, Synovis Micro Companies Alliance, Inc., Birmingham, AL, USA). After all, the sufficient reperfusion of the pedicle was controlled by palpation and by external Doppler. A protocol was used to document the flap volume (by displacement), total operation time, ischemic time (from cutting the proximal end of the radial artery until release of clamps after completion of the arterial anastomosis), noflow-time (from cutting the proximal end of the radial artery until completion of the venous anastomosis), and


time needed to suture the arterial and venous anastomosis. Additionally, the flap pedicle was irrigated with heparin and the sufficient arterial anastomosis was monitored by external Doppler (Dopplex Mini D900, 8 MHz probe, Huntleigh Diagnostics Ltd., Cardiff UK). Flap monitoring by infrared thermography The first IR thermography acquisition was performed just before the start of the arterial anastomosis (‘‘intraop_pre’’). To provide constant investigating conditions the mouth of the patient was closed for 10 min prior to the first IR thermography acquisition. The IR camera was positioned at a fixed distance of 35–40 cm from the flap on a tripod to ensure an acquisition without motion (Fig. 1). The focus of the camera was manually tuned to obtain sharp images and the acquisition was performed during a couple of seconds. The temperature difference between the flap surface and regular surrounding tissue was measured synchronized by thermal imaging, and therefore independently from external factors. The surrounding tissue and the free-flap reconstruction were in a same plane perpendicular to the camera axis. Therefore, there was no different measuring angle between the thermal camera and the tissue surface. In order to dynamically study the perfusion effect, the dermal flap was shortly cooled by pressure with a metal plate (2 9 2 cm) at room temperature.

The rewarming of the dermal flap surface was then recorded. Each thermal image acquisition took about 5 min. After the successful completion of the anastomoses and the completion of the neck closure, the second static IR thermography acquisition was performed (‘‘intraop_post’’) with the same protocol at least 30 min after completion of the anastomoses. Further, IR thermography acquisitions were performed on the first (‘‘postop_1’’) and tenth (‘‘postop_10’’) postoperative day. Patients were asked to stay motionless during the postoperative acquisition. Consequence of patient motions is the shift of the examination plane out of the focus of the camera. To avoid confounding factors (e.g. cooling of the flap by respiratory flow), all postoperative IR thermography acquisitions were performed using a cuffed tracheal cannula and with closed mouths during 10 min prior to thermal imaging, too. The dynamic examination was performed intraoperatively ‘‘intraop_pre’’ and ‘‘intraop_post’’ for the last six patients included in the study. Additionally, a daily clinical standard follow-up was performed by physical examination which included visual (colour, capillary refill) and manual (tissue turgor) evaluation as well as endoscopic photo documentation and external Doppler (first and tenth postoperative day) (Fig. 2). The effect of antihypertensive medication was not evaluated. Analysis of the IR data Static thermography The flap’s surface temperature and the values of the temperatures measured by the IR camera depend on multiple factors. In particular: flap size and localization as well as body temperature, fever, respiratory flow and vascular circulation depending on the applied anaesthesia, angle between the tissue surface and the camera lens, room temperature and air humidity [16, 17]. To eliminate these confounding factors the temperature difference between flap surface and regular surrounding tissue was measured instead of just measuring the absolute temperature of the flap’s tissue surface [18]. In correlation to the endoscopic picture, a rectangle or elliptic measuring field was selected in the flap region (Area 1) as well as a second measuring field representing the surrounding, regular tissue (Area 2). The mean temperatures in the selected measuring fields were measured. Dynamic thermography

Fig. 1 Setting of the thermal imaging shown with a patient model

The rewarming of the flap after cold challenge performed before and after completion of the anastomoses was qualitatively evaluated. The mean temperatures in the area1 (flap

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Fig. 2 Comparison of endoscopic photographs and IR thermal images of an oropharyngeal reconstruction with a free radial forearm flap

region) were plotted as time–temperature curves for the last six patients included in the study. The shapes of both curves were visually compared. Moreover, the mean temperatures were measured at two time points, right after tissue cooling and 5 s later and their difference calculated. Statistical analysis Statistical analysis was done with IBM SPSS Statistics 20 (SPSS Inc.) and R version 2.14.1 (R Foundation for Statistical Computing). A descriptive analysis of the data was performed. For continuous variables mean and standard deviation was calculated. A paired t test was used to calculate significance of group differences. Categorial data were compared with the Fisher’s exact test. A p value of less than 0.05 was considered significant.

Results Information of each patient, operation and thermographic data are shown in Table 1. The mean flap volume was 16.5 ml (range 12.0–26.0 ml), mean entire operation time 08:13 h (range 06:30–10:17 h). The median ischemic time of the free flaps was 02:55 h (range 02:11–06:28 h). The median time performing the anastomoses was 57 min (range 37–220 min). The IR thermography showed a mean temperature difference DTintraop_pre between the free-flap surface and the surrounding tissue after oropharyngeal reconstruction just before completion of the microvascular anastomoses of -2.64 K (range -0.76 to -5.49 K). A significant decrease of the mean temperature difference at the investigation

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points ‘‘intraop_post’’, ‘‘postop_1’’ and ‘‘postop_10’’ was obtained (DTintraop_pre = -2.64 K vs. DTintraop_post = -1.22 K, p \ 0.0015; DTintraop_pre = -2.64 K vs. DTpostop_1 = -0.54 K, p \ 0.0001; DTintraop_pre = -2.64 K vs. DTpostop_10 = -0.58 K, p \ 0.0001) (Fig. 3). The mean temperature difference decreased from ‘‘intraop_pre’’ to ‘‘postop_1’’ and remains constant at ‘‘postop_10’’. If there was a decrease of the mean temperature difference of more than 0.5 K on the first postoperative day this was a prognostic factor for the absence of perfusion failure. However, the correlation was not statistically significant (DTintraop_post = -1.22 K vs. DTpostop_1 = -0.54 K, p = 0.0619). After performing the thermal challenge additional dynamic IR thermography showed a typical shaped graph (logarithm function) of the rewarming due to re-established perfusion following successful completion of the microvascular anastomoses in six patients at investigation point ‘‘intraop_post’’ (Fig. 4). Mean temperature differences before and after thermal challenge were not significant (1.825 vs. 2.833 K, p = 0.2013). Though, this was examined for the last six patients only, a robust, statistically significant result is not expected. Eventually, 15 of the 16 transferred free radial forearm flaps did not show major perfusion failure neither with common examinations nor with IR thermography. External Doppler signals were detectable in all cases if physical examination was suspicious of perfusion failure. Patient 8 showed a temperature difference DTpostop_1 of -2.34 K at ‘‘postop_1’’ (first postoperative day, approximately 20 h after surgery). In comparison to the previous measured thermographic data DTintraop_pre = -2.37 K and DTintraop_post = -2.41 K the mean measured value was

Eur Arch Otorhinolaryngol Table 1 Individual Patient Information including surgical and thermographic data Patient

FV

OT

TIT

TA

TAA

TVA

DS

OR

DTt0

DTt1

DTt2

DTt3

DT (t0, t1)

DT (t2, t3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Mean Median

17 16 22 18 17 15 13 15 12 26 22 13 13 14 19 12 16.5 15.5

617 546 488 405 448 568 539 568 411 390 558 420 478 490 545 426 493.6 489

388 177 237 131 172 285 185 153 163 140 188 218 144 159 141 182 191.4 174.5

220 72 57 69 53 51 60 51 61 51 67 56 57 61 53 37 67.3 57

220 27 22 23 22 16 24 19 15 20 15 16 21 21 13 16 31.9 20.5

40 37 21 31 22 22 26 16 20 17 30 22 24 25 26 7a 24.1 23

? ? ? ? ? ? ? ± ? ? ? ? ? ? ? ?

? ? ? ? ? ? ? ± ? ? ? ? ? ? ? ?

-2.5 -1.45 -4.27 -1.46 -1.6 -0.76 -1.03 -2.37 -1.91 -2.09 -3.52 -3.42 -4.42 -2.47 -5.49 -3.48 -2.64 -2.42

-2.3 -1.27 -1.99 -1.07 -0.24 -1.26 -0.28 -2.41 -0.07 -2.14 -0.18 -0.49 -1.63 -1.34 -2.47 -0.30 -1.22 -1.27

0.07 -0.55 -0.5 0.15 -0.14 -0.13 -0.67 -2.34 -0.22 -0.27 -0.62 -0.74 -0.69 -0.95 -0.49 -0.58 -0.54 -0.53

-0.05 -0.6 -0.77 -0.85 -0.41 -1.16 0.32 XXX -1.44 -0.81 -0.73 -0.68 -0.4 -0.44 -0.18 -0.52 -0.58 -0.60

? ? ? ? ? ? ? ? ? ?

? ? ? ? ? ? ? ? ?

FV, flap volume (ml); OT, operation time (minutes); TIT, total ischemic time (minutes); TA, time performing anastomoses (minutes); TAA, time performing arterial anastomosis (minutes); TVA, time performing venous anastomosis (minutes); DS, Doppler Sonography (?, signal; -, no signal); OR, operation result (?, successful; -, unsuccessful); DTt0, intraoperative performed infrared thermography before microvascular anastomosis (relative temperature difference between flap and surrounding vascularized tissue in Kelvin); DTt1, intraoperative performed infrared thermography after microvascular anastomosis (relative temperature difference between flap and surrounding vascularized tissue in Kelvin); DTt2, postoperative performed infrared thermography at 1st postoperative day (relative temperature difference between flap and surrounding vascularized tissue in Kelvin); DTt3, postoperative performed infrared thermography at 10th postoperative day (relative temperature difference between flap and surrounding vascularized tissue in Kelvin); DT, dynamic infrared thermography performed with applied cold challenge (?, yes; -, no) at time point t0 or t1 (intraoperative) or at time point t2 or t3 (postoperative) a

Coupler-system used for venous anastomosis

examination at postop_1 showed a pale and devitalized flap without signs of capillary refill, reduced turgor and without external Doppler signal (Fig. 5). Salvage surgery was immediately performed. A thromboembolism proximal of the arterial microvascular anastomosis was proved and has been successfully cleared. After salvage flap surgery, a pulsation of the flap pedicle and a positive external Doppler signal were detectable. Unfortunately, the flap did not recover from the lack of perfusion and needed to be removed eventually.

Discussion Summary of own results

Fig. 3 Mean temperature differences at investigation points

almost constant, showing just a minimal adjustment of 0.03 K. Intraoperatively the flap was clinically assessed without perfusion failure. Furthermore, the clinical

Although advances in microvascular techniques and conditioning of vascular risk factors have contributed to the improvement in the recent decades, a pillar of permanent free-flap vitality is the frequent flap monitoring. To our knowledge, this study is the first one to investigate IR thermography for intraoperative and

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Fig. 4 Time-temperature-diagram before and following thermal challenge, a before completion of microvascular anastomoses (intraop_pre): rewarming DT 0.65 K after 5 s, b after completion of

microvascular anastomoses (postop_1): rewarming. DT 1.35 K after 5 s (typical log-based line)

postoperative monitoring of free tissue transfer in head and neck reconstructions. The results with static thermography showed significant decrease of the temperature difference measured between the flap surface and the surrounding tissue after completion of the anastomoses, and on the first and tenth postoperative day. Dynamic IR thermography clearly revealed a typical rewarming of the tissue surface

depending on the dermal perfusion following a thermal challenge based on the observation of the time–temperature curves. Positive correlation of thermal imaging after completion of the anastomoses compared to thermal imaging on the first postoperative day could be shown. Therefore, IR thermography offers a real-time imaging solution for the control of microvasculature in surgery [19].

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method. This method was already tested with success for the detection of perforators in DIEP flaps with a cold thermal challenge [22]. After successful microvascular anastomoses and re-established perfusion there was improved dynamics of rewarming following a thermal challenge. Although the fasciocutaneous radial forearm flap is not considered a typical perforator flap it was possible to demonstrate that the rewarming follows a typical log-based line after successful completion of the microvascular anastomoses compared to a non-perfused flap during primary ischemia as shown by Chubb et al. [23]. Adequate arterial inflow and venous drainage result in fast and overall rewarming in the sense of a typical dynamics proved by IR thermography [4, 27]. Although the number of dynamic IR thermography acquisitions is limited in our study. The same typical rewarming dynamics was observed in previously published data from the field of plastic surgery [4, 23]. Postoperative flap monitoring by thermal imaging

Fig. 5 Patient 8 with flap perfusion failure at postop_1; a endoscopic picture of the pale flap intraorally, b thermal image showing temperature difference between flap surface and regular surrounding tissue

Intraoperative flap monitoring by thermal imaging As published by Rosenbaum et al. the time window of increased thrombogenicity and consecutive lysis following microvascular anastomoses is about 30 min [20]. Wolff et al. showed development of 14 of 16 thromboses in the pedicle during the first 45 min following microvascular anastomoses in 350 head and neck microsurgeries [21]. Following the completion of the microvascular anastomoses and the closure of the neck the second IR image was acquired. Thus, the duration up to the second IR image acquisition included that vulnerable time window. 13 of the 16 flaps showed significant decrease of temperature difference after successful completion of the microvascular anastomoses (‘‘intraop_post’’). Nevertheless, reperfusion problems can occur in a flap which is partially rewarmed. Diagnosing perfusion failure just from static IR images may be difficult. The judicious choice of the ROI for the measures is crucial. It is time consuming and complex to perform online in the operating room [4]. On the other hand dynamic thermography is a more reliable

Furthermore, the IR thermography was used to monitor the free-flap reconstructions in the postoperative course. On the first postoperative day and after ten days a third and fourth IR thermography acquisition were performed. The rational for these time points was the general knowledge that vascular compromise resulting in flap perfusion failure usually occurs during the first 24 h. Neovascularisation is established by peripheral ingrowth after eight days. Thus, flap perfusion is secured even if vascular compromise occurs in the flap pedicle. Additionally, assuming regular wound healing, patients start oral feeding on the tenth postoperative day. The results showed a significant decrease of the temperature difference on the first and tenth postoperative day in 15 of 16 patients in comparison with DTintraop_pre. Moreover, the mean temperature difference decreases after completion of the anastomoses and remains constant. This shows an average improvement of the flap perfusion with the time. One patient did not show a decrease in the relative temperature of the flap surface compared to the first IR image (‘‘intraop_pre’’). Despite thermographic examination showed a lack of significant temperature decrease during the second acquisition in the OR, no reliable clinical signs of perfusion failure were observed until the end of the surgery. However, postoperatively the flap showed clinical signs of perfusion failure. Though, further acquisitions on flaps with perfusion deficit have to be performed. This study showed moreover that these are rare cases. So far, it can only be concluded that a small temperature difference should lead to additional examinations to rule out perfusion failure.

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Flap perfusion failure and thermal imaging Failure of flap perfusion can occur while the patient and surgeon are still in the operating room [1, 24–26]. The risk of arterial thrombus formation is highest at 15 min after clamp release and usually occurs during the first 45 min [20, 21]. Due to this fact, many studies deal with intraoperative flap monitoring to evaluate patency of microvascular anastomoses in real time [27]. Routine has shown that flap failure due to vascular compromise occurs between 12 and 24 h after surgery [5]. Vascular compromise or perfusion failure results in the need for revision surgery or in flap loss by exceeding the ischemic tolerance of the free flap in the postoperative course, eventually. However, permanent damage to the flap may already be present before clinical signs are apparent [18]. To date, intraoperative and postoperative monitoring of sufficient perfusion of free-flap reconstructions rely on the assessment of clinical signs by physical examination of the free flap: skin colour, tissue tension, capillary refill time and arterial pulse wave by manual palpation [5]. Though, this technique needs considerable experience and the description of the findings is subjective. Therefore, objective monitoring methods are desirable. The aim of an objective monitoring method is to detect perfusion failure of the free flap before clinical signs or permanent damage occurs. So far, no studies are available using IR thermography for free-flap monitoring in head and neck reconstructions. Thus, the usability of IR thermography to monitor perfusion in freeflap surgery was proven for breast reconstructions with the DIEP flap as well as for perforator selection and planning of flap design [4, 18, 22, 27]. In comparison to other objective monitoring methods IR thermography has several advantages. It is non-invasive and contactless. The expenses for the camera are moderate and there are no expenses for consumables and no need for applying a contrast medium. In a small meta-analysis of four publications (65 flaps) dealing with IR thermography the sensitivity was about 33 % and the specificity was 100 %. To predict overall complications the sensitivity was less compared to implantable Doppler and tissue oxygen measurements, but it was 100 % sensitive for anastomotic problems [27]. This is supported by the results of this study. In 15 of 15 cases without signs of perfusion failure thermal imaging showed a decrease in the temperature difference compared to regular surrounding tissue by time (specificity 100 %). In one flap there was a lack of decrease of the temperature difference in a flap with delayed clinical signs of postoperative perfusion failure. Due to this single case no general conclusions can be drawn regarding sensitivity. Static and dynamic IR thermography showed reliably the rewarming of the flap surface. Additionally, the appearance of ‘‘hot spots’’ was proven to correlate with

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perforator vessel localisation compared with Doppler sonography [4, 22]. Taking into account the results of this study a lack of significant decrease of the temperature difference between flap surface and surrounding tissue following microvascular anastomoses, after 24 h and after 10 days may be suggestive of flap perfusion failure. Additionally, a temperature difference between the flap surface and the surrounding tissue higher 1 K on the first postoperative day may be suggestive of flap perfusion failure, too. If regular perfusion in the free flap is re-established, the rewarming following a thermal challenge should show a typical pattern. Limitations of the study The IR thermography camera was a commercially available model not adapted for medical purposes. An adapted model combined with a visual camera will assist the acquisition by having quickly a good orientation of the scene. An IR camera with auto focus will facilitate the acquisition in sterile environment. Moreover, endoscopic camera to examine buried flaps would be desirable. The monitoring of flap reconstructions in the oropharynx is subject to specific conditions. Internal factors such as body temperature, respiratory flow and local inflammatory reactions, as well as external factors like room temperature, air humidity or angle of the camera axis with the patient, may affect measurement results. To avoid this, constant investigation conditions were provided and no absolute temperatures were measured in this study.

Conclusion In summary, the static and dynamic IR thermography is a promising objective method for intraoperative and postoperative monitoring of free-flap reconstructions in head and neck surgery and to detect perfusion failure before macroscopic changes in the tissue surface are obvious. However, further research is necessary to improve this method for additional applications such as preoperative flap design in head and neck reconstructions or prognosis of partial flap necrosis. Compliance with ethical standards Conflict of interest of interest.

The authors declare that they have no conflict

Ethical approval The study protocol was approved by the IRB (AZ.: 099-13-22042013). All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and

Eur Arch Otorhinolaryngol with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. Informed consent An informed consent was obtained from each individual participant and is included in the study.

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