Accuracy of early burn depth assessment by laser Doppler imaging on different days post burn
Introduction
Clinical experience has shown that early excision and skin grafting of deep dermal and full thickness burns results in a faster healing time, fewer complications including infection, reduced length of hospital stay and lower treatment costs as well as a better functional and aesthetic outcome [1], [2], [3], [4], [5], [6], [7]. However, especially at the early stage, it is difficult to distinguish the more superficial dermal burns, which will heal well without surgical intervention, from the deep dermal burns where a prolonged healing time will result in hypertrophic scarring and contractures unless excision of necrotic tissue and skin grafting is performed. Therefore, it is essential to determine burn wound depth and predict healing time accurately as early as possible. The most common technique currently available for determining burn depth is clinical visual and tactile assessment of the burn wound by an experienced surgeon [8]. These assessments are reliable in clearly superficial and clearly full thickness burns. However, it has been demonstrated in several studies that a purely clinical, bedside evaluation of the burn depth in dermal burns is accurate only in about 50–75% of the cases [8], [9], [10], [11], [12], [13], therefore stressing the need for a more reliable indicator of burn depth especially in the category that is often referred to as burns of intermediate or indeterminate depth.
Laser Doppler is a very sensitive technique for monitoring differences in tissue perfusion. It operates on the principle that laser light undergoes a Doppler frequency shift when reflected by moving objects such as red blood cells. Therefore cutaneous perfusion can be determined and processed on a ‘flux’ scale. The use of LDI in burn depth assessment is based on the well established relationship between the depth of the burn and the subsequent changes in microvascular blood flow in the remaining dermis [10], [14], [15]. During the inflammatory phase of a burn wound, there is always an increased blood flow due to local vasodilation. However, if the dermal microvasculature is destroyed, as in deeper burns, this increased blood flow does not happen. The degree of blood flow is therefore a parameter for burn depth or subsequent healing time of a burn wound. The laser Doppler technique has evolved from an early apparatus requiring a surface contact probe and measuring perfusion in one single spot (laser Doppler flowmetry) [10], [14], [16] to the highly sophisticated laser Doppler imaging systems which produce a two dimensional, color coded image of skin blood movement. The recent development of an advanced and computerized LDI system provides a less time-consuming, non-invasive system that can register blood flow over larger burned areas and is convenient for patient and operator [17].
The earliest studies of burns using the laser Doppler technique were with contact probes and frequently used local skin heating to aid discrimination of wound types [10], [14], [15], [16], [18]. These studies were very important in demonstrating the worth of laser Doppler to improve the accuracy of burn assessment. The single point (and later four-point) measurements were only of very local areas that might not be representative of the whole; coupled with the need for patient contact, with associated risk of infection and patient discomfort, the technique was not always practical for routine clinical use. These problems were overcome by the introduction of laser Doppler imaging [19].
Since the first article of its use in burns, by Niazi et al. [11] in 1993, there have been several reports on the use of the laser Doppler imaging (LDI) to objectively determine the depth of a burn wound [12], [20], [21], [22]. There have been several studies that have assessed laser Doppler perfusion at different times post burn. The very early post burn period has been studied in experimental burn wounds to assess blood flow and burn wound conversion, between 30 min and 5 h post burn, during resuscitation with varying volumes of fluid [23]. Kloppenberg et al. [24] in 2001 described blood flows and their changes at days 1, 4 and 12 post burn, identifying patterns for wounds that healed before 7, 14 and 21 days post burn and full thickness burns that required surgery. They also provided a laser Doppler cut-off value (2 V) for predicting wounds requiring surgery, with 93.8% specificity and 100% sensitivity. Laser Doppler images were also obtained by Riordan et al. [25] in 2003 at 24, 48 and 72 h after burn; a progressive decline in flux and increase in depth during the 72 h period was observed. For images obtained at 48 h, a cut-off value (1.3 V) was found to define superficial dermal wounds; specificity 94%, sensitivity 95%. Atiles et al. [26], [27] in 1995 performed the earliest investigations of contact laser Doppler, using a 4-point probe, on successive post burn days (0, 1, 2, 3 days for paediatrics, 4 and 5 days PB also for adults). Their studies on adults and children showed similar results: wounds that did not heal within 21 days had LD flux values below 80 PU; for burn wounds that healed before 21 days, LD flux was greater than 80 PU but did not stabilize until day 2 PB. A significant limitation to these studies, acknowledged by the authors, was that wounds treated surgically were included in the group of wounds considered as not healed at day 21, without any confirmation of burn depth by histology. It was acknowledged that some of these wounds might have healed without surgery and that this would have affected the results. In a similar study by Mileski et al. [28] in 2003 the same protocol that was used on children was used for a further study on adults. The serial measurements were used to predict healing: i.e. LD flux consistently below 80 PU on 0, 1, 2 and 3 days after burn was used to predict non-healing at day 21; LD flux above 80 PU on any of the 4 days was used to predict healing within 21 days. This technique enabled outcomes to be predicted with a sensitivity of 80% and specificity of 88%. It is possible that the accuracies would have been higher without the above limitation.
The aim of this study was to investigate, prospectively, the changing accuracies of burn depth assessments by laser Doppler imaging during the important early after burn days and to compare these with the changing accuracies of clinical diagnosis.
Section snippets
Material and methods
The investigation was conducted at the Burns Centre, University Hospital of Gent, Belgium, a primary referral centre for burns.
After approval of the local medical ethical committee and after informed consent was obtained, we prospectively assessed 40 patients with intermediate depth burns on days 0, 1, 3, 5, 8, after burn when possible, both clinically and with LDI. Patient selection was as follows:
- 1.
Admitted and adequately resuscitated;
- 2.
Dermal burn wound;
- 3.
Not be obviously superficial or full
Results
In a period of 12 months, 40 patients with intermediate depth burn wounds on clinical assessment were included, 7 women and 33 men (male to female ratio was 4.7/1). Patient characteristics showed a mean age of 37 years (range: 4–83 years). The mean percentage of total body surface area was 15% (range: 3–50%).
Concerning the etiology of the burn wound, 52% were flame burns followed by scalds (43%) and other causes (5%). The anatomic location of the injuries varied: burn wounds on abdomen and
Discussion
The ultimate goal in the treatment of severely burned patients is first to save lives, but it is also important to obtain early wound closure with an optimal functional as well as aesthetic outcome [1], [2], [3], [4], [5], [6], [7].
Worldwide, the standard technique for determining burn depth consists almost exclusively of clinical assessment of the burn wound. Unfortunately, clinical bedside evaluation only is not very reliable and has been shown to be accurate in only 50–75% of the cases, even
Conclusion
This study of 40 burns showed that burn depth assessment with LDI was more reliable than clinical evaluation only. The accuracies of burn depth assessments with LDI at days 0, 1, 3, 5 and 8 were 54.8%, 79.5%, 95%, 97% and 100% compared with the accuracies of clinical assessment alone of 40.6%, 61.5%, 52.5%, 71.4% and 100%, respectively (Table 5). Except on day 8, where both techniques were completely accurate, LDI was the most accurate on days 3 and 5 with differences in accuracies that were
Conflict of interest statement
None of the authors has any financial interest whatsoever in any of the techniques or instruments used or mentioned in this article.
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