| | Continuous postoperative monitoring of cutaneous free flaps using near infrared spectroscopy☆Received 9 October 2006; accepted 14 April 2007. published online 29 May 2007. Summary Reliable detection of circulatory compromise threatening free-flap viability is essential for prompt surgical intervention and flap salvage. Numerous techniques have been developed to address the issue of postoperative flap monitoring but none have achieved universal acceptance. Near infrared spectroscopy (NIRS) is a noninvasive technique that allows continuous monitoring of tissue oxygenation and perfusion. It is increasingly recognised to be a reliable method for flap viability assessment. This study was designed to investigate the ability of NIRS to detect and identify microvascular thrombosis endangering flap survival. To our knowledge, this is the first clinical evaluation of NIRS used for continuous monitoring of free flaps. MethodsFifty flaps used for autologous breast reconstruction in 48 patients were included in this prospective clinical study. NIRS was employed for 72-h continuous postoperative monitoring. The data were compared to findings of clinical assessments. ResultsTen flaps (20%) developed 13 anastomosis thromboses (two arterial and 11 venous). NIRS detected all cases of flow failure prior to clinical observation with no false positives or negatives. Based on consistent patterns of NIRS parameter changes, it was possible to differentiate between changes caused by arterial and venous thrombosis with accuracy before surgical re-exploration. The salvage rate was 70%. Overall flap viability was 94%. ConclusionsContinuous NIRS monitoring can reliably detect and identify early stages of arterial and venous thrombosis, and is a credible method for noninvasive postoperative flap surveillance. Based on these findings, we advocate its use for monitoring of flaps with a cutaneous component. Due to advanced microsurgical techniques and the development of more reliable flaps, free-tissue transfer has become a routine procedure in reconstructive surgery. Since its introduction in the late 1960s, flap viability has improved significantly and the current literature reports success rates of greater than 95%.1, 2, 3, 4 Nevertheless, even in the most experienced hands, postoperative circulatory failure, warranting immediate re-exploration and restoration of adequate anastomosis patency, still occurs in 6%–25% of cases.1, 5 Salvage rates exceeding 50% have been reported.3, 5, 6 The time interval between onset and surgical repair of a microvascular problem largely influences the success of a salvage procedure.7 For this reason, an effort to detect postoperative flap ischaemia and permit timely re-exploration is necessary to minimise the incidence of flap failure and its detrimental consequences.8 The ideal monitoring technique is non-invasive, continuous, reliable, accurate, instantaneous, easy to use and inexpensive.9, 10 More than 60% of microsurgeons routinely utilise one of many adjuvant monitoring devices in addition to clinical flap assessment.11, 12 To date, no single monitoring technique described in the extensive literature addressing the issue of postoperative surveillance9, 10, 13, 14, 15, 16, 17, 18, 19, 20 has fulfilled all the proposed criteria and has been generally accepted for clinical use. Despite its inherent problems, clinical observation remains the ubiquitous standard of good practice.6, 11, 12 Thus, the search for an optimal monitoring system continues. Near infrared spectroscopy (NIRS) is a noninvasive instrumental technique, which allows continuous bedside monitoring of flap tissue oxygenation and perfusion. NIRS measures relative changes in concentration of oxygenated (HbO2) and deoxygenated (Hb) haemoglobin. Their sum provides an estimation of total tissue haemoglobin concentration (HbT). The latter reflects changes in tissue blood volume and thus indirectly the status of tissue perfusion. Tissue oxygen saturation (StO2) is the percentage of saturated haemoglobin (StO2 =100×HbO2/HbT). It is correlated with tissue oxygenation and determined by the balance between oxygen delivery and consumption. Two studies published by Irwin et al.21 and Thorniley et al.22 introduced NIRS as a potential monitor of flap viability. Their findings demonstrated NIRS ability to detect and discriminate between changes caused by experimental induction of arterial, venous or total (combined) occlusion in rabbit hind limbs and prefabricated porcine myocutaneous flaps, respectively. In the article we describe the results of a prospective study designed to examine NIRS use for continuous free-flap monitoring in a clinical setting. A series of 50 microsurgical autologous breast tissue reconstructions was used as a relatively uniform model. The NIRS system was employed for 72-h postoperative flap monitoring. The results were compared to findings of standard clinical observations. We aimed to investigate NIRS reliability in detecting vascular compromise threatening flap viability and its ability to distinguish between changes caused by arterial and venous occlusion. The overall aim was to ascertain whether NIRS could provide a trustworthy means of postoperative flap surveillance in routine clinical work. Patients and methods  The study was conducted in strict adherence to Slovene law with the approval of the National Medical Ethics Committee. A consent form was signed by all patients. Fifty consecutive free flaps used in 46 patients undergoing unilateral and in two patients undergoing bilateral autologous breast reconstruction were included in the study. All 48 patients were operated upon between August 2004 and September 2005 at the Ljubljana Clinical Department of Plastic Surgery and Burns. Their age ranged from 31 to 64 years, with a mean age of 47 years. The average body mass index was 26, extending from 22 to 35. All patients were treated for mammary carcinoma (nine patients by simple and 39 by modified radical mastectomy). Unilateral secondary breast reconstruction was performed in 21 patients treated by mastectomy 1–9 years (mean 3.7 years) prior to reconstruction. Twenty-nine flaps were used for primary reconstruction in 27 patients. Two patients with BRCA mutation had bilateral reconstruction following prophylactic mastectomy of contralateral breast. Seven patients were smokers; one patient had controlled diabetes mellitus; no patient had a known vascular disease. Flap selection In patients with suitable abdominal skin laxity and lack of relevant scars in the region, reconstruction was accomplished using either a deep inferior epigastric perforator (DIEP) or superficial inferior epigastric artery (SIEA) flap.23 Superior gluteal artery perforator (s-GAP) flap was used in patients with inappropriate abdominal donor area. Internal mammary artery and comitans vein were used as recipient vessels for end-to-end microvascular anastomosis. The same surgical team, led by the senior author (Z.M.A.), performed all reconstructions. NIRS principles and instrument NIRS employs the principles of optical spectrometry to measure haemoglobin content and oxygenation in local tissues. The system delivers selected and calibrated wavelengths of near infrared light to illuminate biological tissues which are relatively transparent for this part of the electromagnetic spectrum (700–1100nm). Selective light absorption by oxygen-dependent tissue chromophores, primarily haemoglobin, results in the reduction of light intensity. Attenuated optical signal exiting the tissue is analysed using spectrophotometric principles that relate light absorption to the tissue concentration of the chromophore.24 Characteristic absorption spectra of oxygenated and deoxygenated haemoglobin permit the system to calculate concentration of both haemoglobin forms.25 NIRS measures haemoglobin content and oxygenation in all small-diameter vascular compartments of tissue (arterioles, venules and capillaries), as vessels of larger calibre absorb the light completely. The tissue spectrometer (InSpectra™ Model 325; Hutchinson Technology Inc., Hutchinson, MN, USA) used in our study continuously measures relative changes in tissue concentrations of oxygenated, deoxygenated and total haemoglobin (HbO2, Hb and HbT) expressed in arbitrary units and tissue oxygen saturation (StO2) presented as a percentage of saturated haemoglobin. The apparatus emits light at four selected wavelengths (680, 720, 760 and 800nm) and analyses the light reflected from tissue. In order to provide a spectral measurement independent of total haemoglobin concentration and optical path-length changes, tissue absorbance is transformed into a scaled second derivative value. A wide gap second derivative algorithm is used to quantify changes in haemoglobin oxygenation, as described in detail elsewhere.26 The probe is attached to the skin via a disposable plastic shield (Fig. 1). The inferior side of the shield is covered with an adhesive surface to attach the probe to the flap skin island and prevent undesired probe movement. The shield is designed to prevent ambient light intrusion. The depth of tissue measured by spectrometer is directly correlated to the distance in between the tips of probes emitting and receiving optical fibre. Mean depth of measurement is roughly half this length. A probe with 25-mm spacing was used. The tissue spectrometer is connected to a regular laptop computer. Its screen displays real-time graphic and numeric StO2, HbO2, Hb and HbT values. Measurements are taken every 3s and stored on the hard disc for later analysis. Study protocol of flap monitoring NIRS was employed for 72-h continuous postoperative monitoring. This duration of intensive flap monitoring is common in the majority of reconstructive surgical units.11 It has been reported4, 27 that only 10% of thromboses develop after postoperative day three and no flaps experiencing delayed flow failure were salvaged successfully. At the end of the surgical procedure a probe is attached to the flap skin island and the StO2 value is read. An adjustable sound alarm built into the tissue spectrometer is set at an arbitrary 50% of the initial StO2 level. The protective shield of the probe remains fixed to flap skin throughout the observation period to prevent dislocation of the monitoring site. For bilateral breast reconstructions we employed an additional NIRS monitor. Simultaneous clinical monitoring is performed at hourly intervals by experienced nursing staff recording colour, capillary refill and surface temperature of the flap. If StO2 decreases by 50% of the reference (initial) value, the alarm alerts the nursing team to notify the attending doctor and observe the flap. Upon arrival, the physician investigates NIRS parameters and re-examines the flap. When a diagnosis of circulatory compromise is confirmed by clinical findings, the patient is taken back to the operating room for flap revision. In the case of successful salvage operation, NIRS monitoring is resumed for an additional 72h. Statistical analysis The average daily StO2 values measured in the three flap-type groups were tested for differences using one-way ANOVA. Day-to-day changes in average StO2 values were assessed by paired-samples t-test. The difference in average initial StO2 levels measured between the groups of failing and unproblematic flaps was evaluated using independent-samples t-test. The same test was employed to analyse differences in means of NIRS parameter changes caused by venous and arterial thromboses. The results were considered statistically significant when P<0.05. Results In the series of 50 breast reconstructions we used 37 DIEP (24 primary; 13 secondary), 5 SIEA (all primary) and 8 s-GAP (3 primary; 5 secondary) free flaps. Forty (80%) experienced an uneventful postoperative period. There were 13 cases of microvascular thrombosis observed in 10 free flaps; two flaps suffered more than one complication. The exact data on the timing, management and outcome of vascular complications are compiled in Table 1. All flow failures occurred within 3 days of reconstruction. On average, the first incident occurred 16h after the operation, ranging from 1.5 to 64h postoperatively. NIRS identified all cases of circulatory compromise prior to clinical assessment. Subsequent clinical exploration supported the NIRS findings on every occasion. Thirteen re-explorative procedures were indicated, finally confirming the diagnosis in all cases. We lost a total of three flaps (6%). Two of the flaps that ultimately failed were revised more than once with immediate but transitory success. NIRS parameters reversed to pre-occlusion levels and the clinical appearance of the flaps recuperated, as seen in the other seven cases of successful salvage procedures. However, venous thrombosis reoccurred and salvage was finally given up due to massive venous anastomosis clots stretching deep into the flap. The salvage rate, including transiently successful procedures on the flaps that failed eventually, was 77% (10 out of 13 re-explorations). The overall salvage rate was 70% (seven out of 10 failing flaps), leading to an overall free-tissue transfer success rate of 94%. NIRS monitoring findings Typically, NIRS parameters remained stable throughout the monitoring period with the exception of circulatory failure incidents. The average daily StO2 levels for various flap types are shown in Table 2. No significant trends of either StO2 increase or decrease were noticed in the days following the operation. There were no significant differences based on flap type. Substantial inter-flap variability was present as daily StO2 averages measured in individual flaps ranged from 20% to 72%. | | |  | Flap type | Cases (n=50) | StO2 (%) | |
|---|
| Day 1 | Day 2 | Day 3 | |
|---|
| DIEP | 37 | 47±12 (23–66) | 48±14 (24–72) | 48±13 (24–71) | | | SIEA | 5 | 51±20 (28–65) | 55±21 (30–71) | 54±22 (29–70) | | | s-GAP | 8 | 46±15 (20–60) | 48±17 (20–59) | 47±14 (25–60) | | | | |
The occurrence of circulatory failure caused an abrupt deviation of all parameters from their baseline values. These patterns of parameter changes were characteristic for both types of thrombosis and were recorded repeatedly in all 13 cases of flow failure. The average time for changes to develop with the entire pattern shifting from the baseline pre-occlusion to new plateau values, indicating complete flap deoxygenation, was 44.5min. These events were not preceded by any noticeable trends in NIRS parameters, which would permit detection of thrombosis before the actual start of pattern development. All cases of flow failure were detected by NIRS monitoring first. Initial clinical assessments incited by NIRS alarm revealed only the most subtle clinical evidence of flap distress. Clinical signs gradually increased, and by the time NIRS patterns were fully manifested the clinical diagnosis became confirmative. Following a successful salvage operation parameters recovered to their pre-occlusion level, as mentioned before. In the group of failing flaps the average StO2 level measured at the beginning of observation period was 43% (SD±13). In the flap group without postoperative complications, it was marginally higher at 45% (SD±18). The difference was not statistically significant. Venous thromboses. Based on consistent patterns of changes NIRS identified all 11 cases of venous thrombosis. They were characterised by increase in Hb and HbT combined with decrease in HbO2 and StO2 (Table 3). Changes were gradual and took from 33 to 68 (mean 46) min to develop. By this time StO2 fell to zero and Hb, HbO2 and HbT stabilised at new plateau values (Fig. 2). Alterations in NIRS parameters were followed by clinical findings supporting diagnosis of venous congestion. Flap skin became increasingly darker with bluish hue and capillary refill brisker. Flap appeared progressively swollen. Surgical exploration confirmed the diagnosis of venous anastomosis thrombosis in all 11 cases. Arterial thromboses. NIRS identified both arterial thromboses due to their distinctive pattern of changes. The main feature was a decrease in HbO2, HbT and StO2 paralleled by Hb increase (Table 3). Concentration of Hb manifested an initial short-lived (3 and 5min) decline followed by an overall increase in Hb concentration. Parameters changed gradually and took 32 and 42min (mean 37) to entirely develop the pattern. In this time StO2 dropped to zero and Hb, HbO2 and HbT curves reached their new plateau values (Fig. 3). Clinical signs consistently followed the advance of NIRS changes. Flap skin became increasingly paler in colour and started to cool down. Refill slowed progressively. Salvage operation confirmed thrombosis at the site of arterial anastomosis in both cases. Comparison of arterial and venous thromboses. Both arterial and venous anastomosis occlusion cause progressive flap ischaemia. Oxygen depletion is manifested by a decline in HbO2 coupled with an increase in Hb concentration, as saturated haemoglobin off-loads the oxygen. The metabolism consumes the oxygen contained in the flap and tissue saturation gradually falls to zero, as recorded in both categories of thrombosis. The rise in Hb concentration is significantly less prominent in cases of arterial occlusions due to its initial decline caused by the loss of artery inflow and resulting reduction in flap blood content. The main and discriminative distinction is the change in total haemoglobin concentration, indicating quantity of blood in the flap. In the case of arterial occlusion, HbT drops, whereas in venous congestion, it rises significantly. Characteristic changes caused by arterial thromboses were faster to develop the entire pattern (complete ischaemia) by approximately 9min on average. NIRS reliability NIRS monitoring detected all events of vascular compromise threatening flap viability before exhibiting signs detectable by experienced personnel. Based on consistent patterns of NIRS parameter changes, we were able to distinguish between incidents caused by arterial and venous thrombosis with accuracy before the re-explorative procedure. Clinical judgement and findings at salvage operation invariably supported the diagnosis indicated by NIRS monitoring. In this series, NIRS monitoring was completely reliable with no false positives or negatives. Discussion The ability to assess tissue viability remains an important requirement in surgical work. This is particularly true in free-flap transfer where vital blood supply depends solely on microsurgical arterial and venous anastomosis. Early and reliable detection of circulatory failure is essential if a salvage procedure is to prove successful. Previous studies on flap viability evaluation reported evidence supporting the use of noninvasive optical spectroscopy for flap monitoring.21, 22, 28, 29, 30, 31, 32, 33 Despite the differences in spectroscopy devices and study designs, all earlier reports agree on NIRS reliability in flap viability assessment and justify this method as a promising clinical tool. The findings of our study support the data and premonitions contributed by earlier works. As far as we are aware, this is the only clinical study to employ NIRS for free-flap monitoring in a continuous fashion. Continuity of surveillance has many important advantages that will be argued further in this discussion. NIRS reliably detected changes in tissue oxygen saturation and blood volume caused by circulatory failure prior to clearly identifiable clinical signs. By the time that full-blown changes were manifested by NIRS monitoring, the clinical diagnosis had become evident. Characteristic patterns of parameter changes induced by postoperative thromboses were recorded in all failing flaps. This allowed exact identification of arterial and venous thromboses. Furthermore, Irwin21 and Thorniley22 with colleagues reported comparable patterns characterising arterial and venous occlusions induced in an experimental flap model. Our findings confirm the assumption that NIRS can be reliably employed to monitor changes in flap oxygenation and haemodynamics in the clinical setting. In our series, there was no case of simultaneous occlusion of arterial and venous anastomosis. If total occlusion should occur, the authors believe it would be detected and recognised by parameter changes similar to those described in previous experimental work21, 22: unchanged HbT, a fall in HbO2 and StO2, a rise in Hb. With the mean depth of measurement around 12.5mm, the values reported by NIRS monitoring reflect changes in the skin and subcutaneous fat tissues of the cutaneous flaps surveyed. In these tissues NIRS parameters vary to a notable extent between individual patients, observed as inter-flap variability (Table 2), and even between adjacent locations within the small area of flap skin island. Any dislocation of probe position influences the measurement considerably, as already reported.29, 31 In addition, parameters are expected to fluctuate to a certain degree during a lasting observation. This inherent variability seriously obstructs any reasonable standardisation of NIRS threshold values which would reliably indicate imminent threat to flap. For this reason, we believe it is important to observe the trends of NIRS parameter changes rather than absolute values. We opted for continuous measurement with the probe firmly fixed to the flap skin throughout the entire monitoring process. Continuous monitoring at a constant location on the flap yields temporal trends of NIRS parameters and shows how they change in relation to each other. Characteristic patterns provide the data essential for correct interpretation of the NIRS signal. Continuous monitoring obviates the need for absolute standards, offers full-time monitoring, and permits immediate detection and accurate identification of parameter changes indicating impending flap failure. NIRS can provide the surgeon with an early, reliable and objective indication of forthcoming flap failure, allowing timely surgical intervention. It draws immediate attention to the failing flap and adds certainty to clinical judgement. This might possibly reduce the time to detect the failure and time spent in flap viability evaluation; however, this question was beyond the scope of the present study. NIRS was shown to be a convenient, noninvasive method of monitoring, harmless for the patient and the flap. There were no false positives or negatives due to machine, probe or operator malfunction. NIRS is easy to use and adds an insignificant burden to the personnel. NIRS monitoring can be used with minimal training. The system is portable, unaffected by ambient light and therefore suitable for bedside use in any hospital unit. NIRS measurements are not influenced by skin pigmentation. Melanin pigment in the skin causes a steady background light attenuation and does not interfere with relative changes in haemoglobin concentrations measured by the system. The skin colour of our patients ranged from pale to deeply pigmented (constitutional and suntanned); however, there were no black patients. NIRS monitoring does not inflict any discomfort on the patient and they tend to tolerate the presence of the system without difficulty. Measurement is impervious to all but the most violent movements of the patient. The probe can be temporarily disconnected when necessary and repositioned to exactly the same spot as long as the shield is left attached to the skin. There is no need for recalibration or setting new baseline values after disconnecting and reinserting the probe. As suggested previously21, 29, 31, 32, 33 NIRS was found to feature tangible benefits over laser Doppler flowmetry and other monitoring techniques in current use. One of its main advantages when compared to other noninvasive modalities is the capacity of near infrared light to penetrate to a considerable depth of tissue and provide data on microcirculatory events occurring in a comparatively large volume of inspected tissues. Conversely, laser Doppler flowmetry is limited to observing relatively superficial (1–2mm) cutaneous circulatory phenomena susceptible to changes in local environment and microvascular heterogeneity.9 NIRS monitoring is not influenced by patient movement whereas laser Doppler is highly susceptible to artefacts due to probe dislocation. Many clinically used monitoring devices fail to detect venous congestion which, in our series, was reliably identified at early stages by NIRS. Furthermore, according to the experimental study on a pedicled flap model by Payette et al.32 optical spectroscopy was consistently more reliable in detecting problems with arterial inflow when compared to laser Doppler assessments. A rather important drawback among the undisputed qualities of NIRS monitoring is the system cost. The price of the spectrometry system used in this study is approximately €35,000. The disposable shield of the probe costs €18. This price might prove unjustifiable for departments with a low volume of flap transfers. We have recently started to use NIRS for monitoring of skin-grafted muscle flaps. Preliminary results are promising but the series needs additional cases before a final positive report can be given. A larger series of free-tissue transfers is needed to evaluate the possible effects on flap salvage and overall viability rates attributable to the sole use of NIRS continuous monitoring. In conclusion, our data show that NIRS allows objective and early detection of flow failure when used for continuous flap monitoring. It can accurately identify early signs of arterial and venous thrombosis. It offers the surgeon a dependable means of noninvasive postoperative free-flap surveillance, which is in many respects superior to other monitoring techniques in present use. We believe NIRS meets all the criteria of ideal monitoring apart from the high cost of the system. Based on these findings, we advocate its use for flaps with a cutaneous component. Further studies are necessary to evaluate the use of NIRS for free muscle and buried flap monitoring. Financial disclosure The authors have no financial interest or commercial association with any of the subject matter or products mentioned in this manuscript. References 1. 1Jones NF. Intraoperative and postoperative monitoring of microsurgical free tissue transfers. Clin Plast Surg. 1992;19:783–797. MEDLINE 2. 2Kroll SS, Schusterman MA, Reece GP, et al. Choice of flap and incidence of free flap success. Plast Reconstr Surg. 1996;98:459–463. MEDLINE |
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a Clinical Department of Plastic Surgery and Burns, University Medical Centre, Zaloška 7,1000 Ljubljana, Slovenia b Clinical Department of Anaesthesiology and Intensive Care, University Medical Centre, Zaloška 7, 1000 Ljubljana, Slovenia  Corresponding author. Tel.: +386 1 522 33 16; fax: +386 1 522 22 39.
☆ Presented at the 17th Annual Meeting of European Association of Plastic Surgeons, Istanbul, Turkey, May 2006. PII: S1748-6815(07)00247-1 doi:10.1016/j.bjps.2007.04.003 © 2007 Published by Elsevier Inc. | |
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