A previous study showed that the success rate of microvascular free-tissue transfer is >90% [12]. Despite the good outcomes of head and neck surgery, the outcomes of older individuals and those with obesity and atherosclerosis are challenging to manage. If a malperfusion of a free flap takes place, a prompt reintervention improves flap salvage [13].
The technical noninvasive monitoring methods include NIR [14], laser Doppler flowmetry/tissue spectrophotometry [15], and HSI [11] (Table 2). NIR has been used clinically since 1995 [16], and it has several advantages. In a retrospective trial of 900 microsurgical breast reconstructions, NIR proved to significantly decrease the flap loss rate and improved the flap salvage rate [17]. In a second retrospective study of 1050 patients who underwent microsurgical tissue reconstruction, the rate of flap salvage increased to 96.6%, and the number of complete flap losses decreased. Even the rate of reintervention decreased over time [18]. In a systematic review, the overall flap success rate was 99.5%, and the flap salvage rate was 91.1% [14]. Otherwise, compared with laser flowmetry, NIR cannot predict wound complications, including fat necrosis [19], or provide information on blood flow [20].
Table 2
Different types of noninvasive monitoring devices
Method
|
HSI [6, 27]
|
NIR [14, 28]
|
Laser Doppler flowmetry/tissue spectrophotometry [15]
|
Device/company
|
Tivita Tissue®/Diaspective Vision, Germany
|
Tissue Oximeter®/T.Ox, the USA
|
O2C®LEA Medizintechnik, Germany
|
Range
|
500–1000 nm
|
690–850 nm
|
830 nm/500–800 nm
|
Modality
|
Noninvasive
|
Noninvasive
|
Noninvasive
|
Assessment frequency
|
Discontinuous
|
Every 2–4 s
|
Continuous
|
Monitoring modality
|
Imaging
(30 × 30 cm]
|
No imaging (probe)
|
No imaging (probe)
|
Laser Doppler flowmetry, similar to the O2C device, can determine blood flow (flow), velocity, hemoglobin oxygenation, and relative hemoglobin concentrations [15]. Furthermore, the O2C device can provide data regarding intracapillary hemoglobin oxygenation in correlation with intracapillary hemoglobin saturation and the local blood flow rate [15]. Vessel occlusion was detected in five of five patients who underwent free-flap breast reconstruction. Thus, O2C may improve flap survival rates at an early stage [15]. The disadvantage of the O2C device is its cost and cautious placement of sensor as shearing motions and excessive pressure can lead false measurements [15].
HSI and NIR are both optical devices measuring the tissue perfusion in different electromagnetic ranges. In contrast to O2C or NIR, HSI can facilitate a contact-free, noninvasive imaging analysis. In a recent systematic review, the most frequently reported limitations of clinical monitoring were need for expert interpretation (25% of related papers), unsuitability of buried flaps (21%), and a lack of quantitative/objective values (19%) [21]. To provide sufficient quantitative data and reduce the need for expert interpretation, different diagnostical studies were published to establish quantitative thresholds about HSI in microsurgical tissue reconstruction in recent years [8, 9, 11, 22, 23]. All studies have a prospective character, a low number of presented cases, and a re-exploration rate based on clinical assessments in common. Furthermore, every study postulated that malperfusion goes hand in hand with reduced oxygen saturation in form of a decreased StO2 and NIR. Due to the low number of cases and the different kinds of flap surgeries, different thresholds were postulated (Table 3). However, a distinction between arterial occlusion and venous congestion was not made in every trial. So, the role of other parameters like the THI or TWI remains unclear.
Table 3
Various studies related to HSI and its diagnostic value.
author
|
Thiem et al. [9]
|
Thiem et al. [29]
|
Kohler et al. [8]
|
Schulz et al. [11]
|
year
|
2021
|
2020
|
2021
|
2021
|
design
|
prospective
|
prospective
|
prospective
|
prospective
|
cases
|
63
|
30
|
22
|
19
|
distinction between arterial occlusion and venous congestion
|
none
|
yes
|
none
|
yes
|
thresholds
|
StO2 ≤ 32%
StO2∆reference > −38%
NRI ≤ 32.9
NRI∆reference ≥ −13.4%
|
StO2 < 40%
NIR < 25%
THI < 40%
|
NIR ≤ 40
|
THI ≥ 53%
NIR ≤ 25%
TWI ≤ 43%
StO2 ≤ 22%
|
For the first time, our study has tried to move the role of the THI into the clinical focus and derive a therapeutic outcome from it. In the current study, HSI could predict poor tissue perfusion in all seven patients with total flap necrosis. Moreover, HSI confirmed adequate tissue perfusion in 108 of 124 measurements (specificity: nearly 97%). Only three measurements were documented as false positives and two as false negatives (sensitivity: approximately 84%). In the case of venous congestion, a decrease in StO2 and NIR was observed in subgroups A, B, and C. However, it was only in the case of the NIR that this was statistically significant in contrast to the previous publications in Table 3. Otherwise, the THI was already statistically significant from subgroup A onwards to differentiate between vital and non-vital flaps as a dichotomous target variable. The diagnostic study presented here points out that in the case of venous congestion of the free flaps, the THI plays a role just as essential as the StO2 or NIR. A high THI ≥53% seems to indicate a venous stasis. A simultaneous decrease in NIR and StO2 suggests venous congestion or arterial occlusion [11].
One reason for the above-mentioned false positive results (cases 9 and 33) in our patient collective may be postoperative reactive hyperemia of both flaps. In both cases, the intraoperative anastomosis of a second vein was not taken into account. As a result, the venous flow may have increased compared with free flaps with two anastomosed veins, and a higher proportion of hemoglobin may have accumulated. An explanation for the two false positives assessments (case 10 and 20) may be that both assessments were taken between 48 and 72 h. Previously, there had been a prolonged malperfusion of both flaps that could explain the final drop in the THI as an expression of necrosis of the superficial skin tissue that had already begun.
The major role of HSI could be the early pre-clinical identification of flap compromises. Thiem et al. in a previous publication stated to detect perfusion compromises significantly earlier than clinical monitoring based on a decreased StO2 or NIR [9]. The same would have to be checked in regards to the THI. In case 41, HSI could report a revision indication within the first 24 h (t1). However, the clinical decision did take place on the following day. In this case, based on the HSI data, an earlier operational revision could have been initiated. However, due to the retrospective design of this study, this cannot be directly implemented and further prospective studies are necessary.
To create clarity and to be able to better control the massive amount of data, artificial intelligence could be a suitable alternative for predicting malperfusion. A large number of studies have been published in which diagnostic statements were made based on the data generated by the HSI. For example, different types of mouth tissue could be classified by their HS-signature using a deep learning approach [24]. Also, an algorithm for brain cancer classification [25] or for the differentiation between skin complications of diabetes mellitus and healthy volunteers using HSI and machine learning algorithms was demonstrated [26]. The further investigation of the flap perfusion by artificial intelligence seems to be possible and necessary. The lack of quantitative values mentioned at the beginning and the necessary interpretation for noninvasive flap monitoring devices could thus be remedied. Nonetheless, possible disadvantages of artificial intelligence are the large amounts of data that are required and the corresponding expense for programming.
The current study has several limitations. First, it was retrospective in nature. Moreover, because of the relatively small sample size, information regarding the areas of partial flap necrosis was not examined and there were missing data about flaps with arterial occlusion. Second, the measurements did not affect the usual clinical care; therefore, the flap salvage rate was not reproducible. The diagnostic value of postoperative assessment is, at least, defined by the flap salvage rate [17]. Fourth, the occurrence of late-onset complications, including wound edge necrosis, was not assessed [19].