Elsevier

Burns

Volume 43, Issue 8, December 2017, Pages 1624-1639
Burns

Review
A review of the evidence for threshold of burn injury

https://doi.org/10.1016/j.burns.2017.04.003Get rights and content

Highlights

  • Earliest perception of pain occurs just above 43 °C in adult human skin.

  • Burn injury occurs when the temperature at the dermoepidermal junction exceeds 44 °C.

  • From 44 to 70 °C rate of damage increases logarithmically with linear increase in temperature.

  • Time-temperature relationships in superficial burns are reliable but for deeper burns have limited clinical validation.

  • Time-temperature relationships have not been clinically validated in children.

Abstract

Introduction

Burn injury is common and depth is one measure of severity. Although the depth of burn injury is determined by many factors, the relationship between the temperature of the injurious agent and exposure duration, known as the time-temperature relationship, is widely accepted as one of the cornerstones of burn research. Moritz and Henriques first proposed this relationship in 1947 and their seminal work has been cited extensively. However, over the years, readers have misinterpreted their findings and incorporated misleading information about the time-temperature relationship into a wide range of industrial standards, burn prevention literature and medicolegal opinion.

Aim

The purpose of this paper is to present a critical review of the evidence that relates temperature and time to cell death and the depth of burn injury. These concepts are used by researchers, burn prevention strategists, burn care teams and child protection professionals involved in ascertaining how the mechanism of burning relates to the injury pattern and whether the injury is consistent with the history.

Review methods

This review explores the robustness of the currently available evidence. The paper summarises the research from burn damage experimental work as well as bioheat transfer models and discusses the merits and limitations of these approaches.

Review findings

There is broad agreement between in vitro and in vivo studies for superficial burns. There is clear evidence that the perception of pain in adult human skin occurs just above 43 °C. When the basal layer of the epidermis reaches 44 °C, burn injury occurs. For superficial dermal burns, the rate of tissue damage increases logarithmically with a linear increase in temperature. Beyond 70 °C, rate of damage is so rapid that interpretation can be difficult. Depth of injury is also influenced by skin thickness, blood flow and cooling after injury. There is less clinical evidence for a time-temperature relationship for deep or subdermal burns. Bioheat transfer models are useful in research and becoming increasingly sophisticated but currently have limited practical use. Time-temperature relationships have not been established for burns in children’s skin, although standards for domestic hot water suggest that the maximum temperature should be revised downward by 3–4 °C to provide adequate burn protection for children.

Conclusion

Time-temperature relationships established for pain and superficial dermal burns in adult human skin have an extensive experimental modeling basis and reasonable clinical validation. However, time-temperature relationships for subdermal burns, full thickness burns and burn injury in children have limited clinical validation, being extrapolated from other data, and should be used with caution, particularly if presented during expert evidence.

Introduction

Burns are common [1], [2], [3], [4], [5]. Severe burns impact every organ system. Severity is related to the percentage total body surface area (%TBSA) burned and the burn depth, but even relatively small burns can be life-threatening and life-changing.

The understanding of cutaneous thermal injury and its classification relies on a basic knowledge of skin anatomy. The skin is the largest organ and covers the entire external surface of the human body [6]. Even until quite recently, it was thought to be a passive barrier between the host and its hostile environment [7]. However, the skin is a complicated organ that provides not only an active barrier against trauma, ultraviolet radiation, microbes, dehydration and extremes of temperature but also continuous cutaneous immune surveillance and sensory awareness of the environment [7], [8], [9], [10].

The avascular epidermis is comprised of keratinocytes that progressively differentiate from the basal layer toward the outermost stratum corneum, gradually replacing their cytoplasm with keratin and embedding themselves in a complex lipid skeleton to form the principal protective barrier against the external environment. The epidermis ranges from 10 to 100 or more cells thick depending on the anatomical location. The dermis provides the epidermis with mechanical strength as well as metabolic and trophic sustenance through the dermoepidermal junction. The dermis rests on the subcutis, which is comprised largely of areolar and fatty connective tissue. All layers of the skin are populated by immune competent cells [8], [10], [11] and permeated by a number of skin appendages, such as hair follicles and glands [6], which act as a reservoir of stem cells for re-epithelialisation following injury.

The classification of burn injury is based on the depth of skin damage. Along with the extent of burn and the age of the patient, burn depth is a primary prognostic indicator of both mortality and morbidity based on the time taken for the burns to heal [12]. A number of terms relating to depth are used in the literature, which can be confusing. Thermal injury to the epidermis only is not usually considered to be a true burn but is still referred to as an epidermal burn (superficial or first-degree). There is minimal structural damage and no blistering. This is little more than simple erythema and, apart from the accompanying pain, typically resolves completely within a few days. Examples include sunburn or minor flash burns. A thermal injury that extends through the dermoepidermal junction into the dermis is referred to as a dermal burn (partial thickness or second-degree). This can be further classified as superficial or deep. Disruption of the dermoepidermal junction causes blistering and healing can only occur once the basal layer of keratinocytes has been restored. A superficial dermal burn is painful and produces a lot of exudate but, with appropriate treatment, should heal within fourteen days with little or no residual scarring. A thermal injury that extends through the entire dermis is referred to as a subdermal burn (full thickness or third/fourth-degree). The healing is slow and complicated by extensive scarring and surgical excision and reconstruction are often required.

Although the depth of burn injury is determined by many factors, the relationship between the temperature of the injurious agent and the exposure duration, known as the time-temperature relationship, is widely accepted as one of the cornerstones of burn research. Henriques and Moritz [13], [14], [15], [16], [17] first proposed this relationship in 1947 and their seminal work has been cited extensively. However, over the years, readers have misinterpreted their findings and incorporated misleading information about the time-temperature relationship into a wide range of industrial standards, burn prevention literature and opinion in medicolegal cases [18], [19].

Section snippets

Aim

The purpose of this paper is to present a critical review of the evidence that relates temperature and time to cell death and the depth of burn injury. These concepts are used by researchers, burn prevention strategists and burn care teams involved in ascertaining how the mechanism of burning relates to the injury pattern and, perhaps more importantly, whether the injury is consistent with the history. This review discusses the robustness of the currently available evidence.

The current evidence describing heat transfer in tissues

Under normal conditions, the skin is maintained at a comfortable temperature for metabolic function and a state of thermal equilibrium exists between the skin surface and the environment. The transport of thermal energy in living tissues is a complex process involving conduction, convection, radiation, metabolic heat generation and phase changes. When a heat source interacts with the skin, thermal energy is transferred by two principal mechanisms. Initially, a conductive heat exchange occurs

Experimental evidence

The published evidence shows that there is a broad agreement between the in vitro and in vivo studies for superficial burns. The seminal work of Henriques and Moritz [13], [14] has been corroborated by others, albeit with modifications to the thermophysical properties and acceptance that the experimental methodology differs between the studies. There is clear evidence that the perception of pain in adult human skin occurs just above 43 °C [61], [62], [64]. A pathological burn injury, defined as

Summary points

  • The earliest perception of pain occurs just above 43 °C in adult human skin but has not been validated in children [61], [62], [64].

  • Burn injury, defined as irreversible necrosis of the uppermost dermis, occurs when the temperature at the dermoepidermal junction exceeds 44 °C [14].

  • Between 44 °C-70 °C, the rate of tissue damage increases logarithmically with a linear increase in temperature [14], [58].

  • Time-temperature relationships established for pain and superficial dermal burns in adult human skin

Conflict of interest

Mr. N.A. Martin has no conflict of interest to declare.

Ms. S. Falder acts as an Expert Witness in court cases regarding children’s burn and scald injuries.

Acknowledgements

The authors are grateful to Ayesha Khaleel who conducted an initial review of the literature and to Mr AJ Stephenson for his comments on an early draft of this manuscript.

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