Solving the Burn Depth Puzzle

Knowing burn wound depth helps determine therapy and predicts healing time and potential for complications such as infection or excessive scarring. Experienced burn surgeons usually determine burn depth clinically with 60%–75% accuracy.¹ Burn depth assessment methods include histological biopsy evaluation, thermography, vital dye techniques, video angiography, video microscopy, and laser Doppler flowmetry (LDF). Only LDF, a noninvasive measure of microvascular perfusion, has sufficient evidence of accuracy in predicting burn wound outcomes to merit regulatory clearance for assessing burn depth² although it still needs to be standardized.²  

This Evidence Corner reviews two recent publications that confirm the validity of LDF as a measure of burn depth among adult Caucasian and pediatric Asian populations. Both studies reinforce the accuracy of LDF in assessing burn wound depth and support the conclusion that partial-thickness burns heal faster than full-thickness burns.

Reference: Merz KM, Pfau M, Blumenstock G, Tenenhaus M, Schaller HE, Rennekampff HO. Cutaneous microcirculatory assessment of the burn wound is associated with depth of injury and predicts healing time. Burns. 2009 Oct 23. [Epub ahead of print].    

Rationale: Burn surgeons need early, accurate knowledge of burn depth to decide whether or not excision and skin grafting is needed to minimize the risk for infection, scarring, and long hospital stays associated with deep partial-thickness or full-thickness burns as compared to superficial burns. The recognized standard clinical assessment accurately differentiates very superficial burns from full-thickness burns, but is only 50% accurate for intermediate depth burns. Since the first use of LDF on burns in 1984, growing evidence suggests that burns with elevated microvascular perfusion during the first 72 hours post burn will heal without grafting.

Objective: Use a portable LDF device to noninvasively analyze sequential patterns of microvascular perfusion in superficial and deep partial-thickness burns and identify LDF cut-off values resulting in a high positive predictive value for spontaneous healing by 3 weeks post-burn.

Methods: A prospective observational study used LDF to measure microvascular perfusion noninvasively on all 28 patients with 173 superficial or deep dermal or full-thickness burn wound sites who were admitted to the Tübingen, Germany burn unit from February 2003–April 2004. A portable LDF unit measured arbitrary units (AU) of blood flow amount and velocity, oxygen saturation, and relative hemoglobin at skin depths of 2 mm and 8 mm on each burn site, and on contralateral unburned sites within 24 hours and at 3 and 6 days post-burn. Treatment decisions were made based on clinical assessment by experienced burn physicians. Superficial burns were treated topically with collagenase ointment and Vaseline gauze. Full-thickness burns were all excised and grafted with split-thickness skin grafts. Intermediate-depth burns were initially treated as superficial until a clinical decision was made that they would not heal in 3 weeks. At that point, they were then excised and grafted as a full-thickness burn. LDF diagnostic and screening validity were calculated respectively as percent of healing outcomes correctly predicted and the percent of burns correctly predicted on admission to heal or not heal spontaneously within 21 days. The LDF cut-off point for predictive validity was determined to optimize overall accuracy using receiver operating curve (ROC) analysis.

Results: The most significant predictor of healing was LDF flow at the 2 mm skin depth within 24 hours post-burn. Burns of LDF AU 100 were more likely to heal within 21 days. Lower vascular flux within 24 hours of admission and at days 3 and 6 post-burn were associated with deeper clinical burn depth assessment (P < 0.001). All 72 burns clinically assessed as superficial partial-thickness (mean flow 145 AU) healed in 3 weeks; the 58 burns clinically assessed as full-thickness (mean LDF 15 AU) were excised as early as feasible and were grafted. Of the 43 burns (mean LDF 60 AU) clinically assessed as deep partial-thickness burns, 30 (70%) required subsequent excision and grafting at a mean of 8 days post-burn. The remaining 30% healed within 21 days. All burns that healed in 21 days had a mean LDF of 100 AU—higher than the 30 AU for burns requiring excision and grafting (P < 0.005). Contralateral intact skin flow was also reduced (P < 0.007) but only in patients with full-thickness burns. Using a cut-off LDF blood flow value of 100, AU optimized overall accuracy (83.2%), diagnostic sensitivity (80.6%), specificity (88.2%), and screening positive (93.1%) and negative (69.8%) predictive value for predicting healing within 21 days.      

Authors’ Conclusions: Using LDF can accurately diagnose burn depth within 24 hours post-burn, predict healing time and help avoid unnecessary operations.

Reference: Cho JK, Moon DJ, Kim SG, Lee HG, Chung SP, Yoon CJ. Relationship between healing time and mean perfusion units of laser Doppler imaging (LDI) in pediatric burns. Burns. 2009;35(6):818–823.

Rationale: LDF assesses burn depth. At the time of this study no research had used LDI to predict healing time.

Objective: Evaluate LDF imaging as a predictor of healing time and define ideal perfusion unit (PU) cut-off point to predict that a pediatric burn will heal within 14 days.

Methods: A prospective observational study used scanning LDF imaging on 103 consecutive patients < 15 years old with 181 partial-thickness burns in the Department of Emergency Medicine of Bestian Hospital, Seoul, Republic of Korea from May to November 2006. Mean perfusion units (PU) on each burn site were measured 2–3 days post-burn. Healing time was independently reported by two experienced physicians as the time to complete re-epithelization. Mean PU was compared between burns healed within 14 days (early) or after 14 days (late). A ROC curve analysis determined the ideal PU cut-off point to optimize LDF prediction of early healing.

Results: Early healers had a higher mean ± standard deviation PU of 380 ± 158 than late healers 186 ± 116 (P < 0.001). Using 250 PU as a cut-off value, early healing was predicted with 80.6% sensitivity and 76.9% specificity.

Authors’ Conclusions: LDF imaging is a good predictor of pediatric partial-thickness burn healing within 14 days and can be used to improve clinical therapy decision making.

Together, these two studies further validate LDF as a quick, reliable, noninvasive tool to assess burn depth and facilitate informed clinical decisions about the need for immediate burn excision and grafting. These studies also underscore the need for clear global operational definitions of effective LDF units and cut-offs for decision-making, as well as depth metrics and expected healing time for burns and wounds in general. Without these, important nuggets of truth may be obscured by babble. These studies raise interesting questions: What constitutes healing early enough to support the decision not to excise and graft? What is the better operational definition of “early healing,” 14 or 21 days?

Now is the time to unify the language and units of laser Doppler flux, while the field is young, so that results can be compared across studies. It’s time to clarify the common global language of wound depth and of LDF measures so all can use the results more consistently to improve patient care and outcomes. These researchers have shown LDF to be a robust tool in estimating burn depth and consistently predicting burn healing times. The findings are consistent across age and skin pigment groups. They have verified its diagnostic and screening validity.  

These results have clinical implications beyond burn depth diagnosis. LDF is useful for diagnosing other skin and wound conditions³ and for evaluating potential wound treatment applications. For example, LDF has been instrumental in discovering that hydrogen peroxide cream⁴ and retinoic acid pretreatment⁵ can ameliorate ischemic tissue damage. What else can LDF measures of vascular perfusion tell us? What if the wound depth findings could be applied to other chronic wounds such as pressure ulcers of “unknown depth” or “deep tissue injury”? Could it help improve reliability of pressure ulcer staging? Would LDF of chronic ulcers predict their timely or delayed healing? Could LDF cue staff when to call for a vascular consult? Once we all speak the same language, and the technology size and price improve, imagine how LDF could help the field of wound care clarify operational definitions of wound depth, recognize risk of delayed healing, and identify effective preventive or treatment modalities for wound care.

Laura Bolton, PhD, FAPWCA
Adjunct Associate Professor
Department of Surgery
UMDNJ

WOUNDS Editorial Advisory Board Member and Department Editor