ReviewA review of the evidence for threshold of burn injury
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
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The earliest perception of pain occurs just above 43 °C in adult human skin but has not been validated in children [61], [62], [64].
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Burn injury, defined as irreversible necrosis of the uppermost dermis, occurs when the temperature at the dermoepidermal junction exceeds 44 °C [14].
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Between 44 °C-70 °C, the rate of tissue damage increases logarithmically with a linear increase in temperature [14], [58].
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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.
References (218)
- et al.
Resident and “inflammatory” dendritic cells in human skin
J Invest Dermatol
(2009) Prognostic indicators and burns
Burn
(2003)- et al.
Correcting a prevalent misunderstanding of burns
Burns
(2016) - et al.
The relationship between heating time and temperature: its relevance to clinical hyperthermia
Radiother Oncol
(1983) - et al.
The relative thermal stability of tissue macromolecules and cellular structure in burn injury
Burns
(2005) Heat shock proteins and molecular chaperones: implications for adaptive responses in the skin
J Invest Dermatol
(1995)Thermal properties of human skin related to nondestructive measurement of epidermal thickness
J Invest Dermatol
(1977)- et al.
Estimating the time and temperature relationship for causation of deep-partial thickness skin burns
Burns
(2015) - et al.
Rationalization of thermal injury quantification methods: application to skin burns
Burns
(2014) - et al.
Technique for quantifying low temperature burns
J Surg Res
(1964)
The theory and practice of safe handling temperatures
Appl Ergon
The nature of pain
J Chronic Dis
A simple algebraic model to predict burn depth and injury
Int Commun Heat Mass Transf
A thermal-ablation bioheat model including liquid-to-vapor phase change, pressure- and necrosis-dependent perfusion, and moisture-dependent properties
Int J Heat Mass Transf
Review on modeling heat transfer and thermoregulatory responses in human body
J Therm Biol
Mathematical models of bioheat transfer
Adv. Heat Transfer
Patterns of burns and scalds in children
Arch Dis Child
Burn injuries among children from a region-wide paediatric burns unit
Br J Nurs
Children with burn injuries—assessment of trauma, neglect, violence and abuse
J Inj Violence Res
Ten-year epidemiological study of pediatric burns in Canada
J Burn Care Res
Epidemiology and outcome analysis of 208 children with burns attending an emergency department
Pediatr Emerg Care
Anatomy, histology and immunohistochemistry of normal human skin
Eur J Dermatol
Immune surveillance in the skin: mechanisms and clinical consequences
Nat Rev Immunol
Mechanisms regulating skin immunity and inflammation
Nat Rev Immunol
Skin immune sentinels in health and disease
Nat Rev Immunol
The skin-resident immune network
Curr Dermatol Rep
Studies of thermal injury: I. The conduction of heat to and through skin and the temperatures attained therein. A theoretical and an experimental investigation
Am J Pathol
Studies of thermal injury: II. The relative importance of time and surface temperature in the causation of cutaneous burns
Am J Pathol
Studies of thermal injury: III. The pathology and pathogenesis of cutaneous burns. An experimental study
Am J Pathol
Studies of thermal injury IV; exploration of the casualty-producing attributes of conflagrations; local and systemic effects of general cutaneous exposure to excessive circumambient (air) and circumradiant heat of varying duration and intensity
Arch Pathol
Studies of thermal injury V: the predictability and the significance of thermally induced rate processes leading to irreversible epidermal injury
Arch Pathol
When are full thickness burns not so?: The importance of source reference material
Br Burn Assoc Annu Conf
A new computational thermal model of the whole human body: applications to patient warming blankets
Numer Heat Transf A: Appl
Models for thermal damage in tissues: processes and applications
Crit Rev Biomed Eng
How do cells respond to their thermal environment?
Int J Hyperth
Basic principles of thermal dosimetry and thermal thresholds for tissue damage from hyperthermia
Int J Hyperth
Arrhenius relationships from the molecule and cell to the clinic
Int J Hyperth
Effects of hyperthermia on survival and progression of Chinese hamster ovary cells
Cancer Res
Role of thermo TRPA1 and TRPV1 channels in heat, cold, and mechanical nociception of rats
Behav Pharmacol
A structural view of ligand-dependent activation in thermoTRP channels
Front Physiol
Molecular mechanism of TRP channels
Compr Physiol
A multi-scale view of skin thermal pain: from nociception to pain sensation
Philos Trans R Soc London A Math Phys Eng Sci
Modeling of nociceptor transduction in skin thermal pain sensation
J Biomech Eng
The principle of temperature-dependent gating in cold- and heat-sensitive TRP channels
Nature
Sensory systems: thermoTRP channels and beyond: mechanisms of temperature sensation
Nat Rev Neurosci
Thermal stability of proteins
Ann N Y Acad Sci
Extracellular heat shock proteins
Shock
Heat shock proteins: modifying factors in physiological stress responses and acquired thermotolerance
J Appl Physiol
Heat shock proteins and the skin
Clin Exp Dermatol
Molecular chaperone function of mammalian Hsp70 and Hsp40—a review
Int J Hyperth
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2022, Journal of Pharmaceutical and Biomedical AnalysisCitation Excerpt :Burn wounds are tissues lesion that can destroy part or all of the skin dermis. Their severity degree increases with the depth of the injury: they are classified into first degree (epidermal burn), second degree (dermal burn) and third/fourth degree (subdermal burn) [1]. The World Health Organization (WHO) considers burn wounds as a serious public health problem, with approximately 24 million people suffering from burns each year worldwide, the majority of them from weak and middle-income countries [2].