Evaluation of human brain hyperthermia using exergy balance equation
Introduction
The human body possesses an ability to maintain its temperature around 37 °C, mainly through functions of the hypothalamus (Kiyatkin, 2004). But sometimes the dynamic thermal equilibrium is disturbed and consequently, the body produces/absorbs more heat than it dissipates. This may result in overheating the core to around 40 °C, called “severe hyperthermia” (Beachy and Repasky, 2011; Bouchama et al., 2007; Kiyatkin, 2004). This condition may occur under intense environmental exposures, e.g. harsh industrial environments, prolonged sun-exposure during outdoor activities, or due to intensive physical exercise or sport. In continuous exposures, body temperature may also rise above 40 °C, called hyperpyrexia (Trautner et al., 2006), which may put the life at risk with threats including failure of vital organs, unconsciousness, coma or death (Duthie and Katz, 1998; Fauci, 2008). The studies reveal that the initial few hours are very crucial for hyperthermia patients (Duthie and Katz, 1998; Smith and Zhu, 2010) and antipyretics do not characteristically reduce the temperature (Bouchama et al., 2007; Fauci, 2008). Therefore, medicated treatment alone becomes less effective. In such situations, physical cooling therapy in conjunction with intravenous fluids provides an immediate and optimal way to cope with the situation (Fauci, 2008; Smith and Zhu, 2010).
In this context, and prior to designing any effective therapeutic cooling device, it is vital to find the human body's heat storage potentials and maximum heat-transfer ability under brain-hyperthermia. The literature reveals that most of the bio-heat transfer models are built from a viewpoint of physiological thermal comfort with human occupancy or cryosurgical applications. The model of brain-hyperthermia, on the other hand, should be pathophysiological because the physiological understanding of thermoregulation is required to be associated with a pathological state. Prior studies used to evaluate human body heat transfer under physiological conditions along with dependant parameters (i.e. blood flow rate, oxygen consumption, skin and core body temperature) are defined under normal functionality of body's self-thermoregulatory system. A gap remains there to evaluate severe hyperthermia which is a pathological condition with impaired body's thermoregulation. Therefore, the application of existing physiological models (developed for physiological heat transfer) for the hyperthermia evaluation requires certain parameters (metabolic rate, exergy consumption, environment, skin and core temperatures etc.) to be evaluated at pathological extremities, implies a pathophysiological state evaluation. Furthermore, the domain-specific thermal models, i.e. Penne's bioheat transfer model (Liu, 2007; Smith and Zhu, 2010) for targeted tissue therapy, energy balance equation (Rhoades and Bell, 2012) for conventional thermal comfort analysis, and radiofrequency exposure (D'Ambrosio and Dughiero, 2007; Hirata et al., 2009) for estimation of core body temperature and burning of tumour tissues, are insufficient to define an ailing body's heat transfer anomalies. Moreover, hemodynamic and respiratory mechanics are not handled in existing physiological modelling approaches which are equally important in finding brain-hyperthermia thresholds.
The exergy balance equation (EBE) (Tokunaga and Shukuya, 2011), unlike the energy balance equation, is the first model that defines a body's heat transfer in a constituent breakup, i.e. exergy-entropy form. This model can be helpful in estimating the heat stored in a body in relation to its evacuation capacity under intense environmental exposures. It highlights that energy storage effects may be neglected from a physiological heat transfer perspective but are vital for defining the pathological state(s), especially in situations like hyperthermia where heat builds up to a dangerous extent. The research presented herein, in this context, showcases a new approach to evaluate the pathological anomalies using the exergy balance equation (Shukuya, 2012). The thresholds of hyperthermia, metabolic heat generation, and impulsive heat evacuation have been evaluated with respect to typical South Asian environmental exposure in summers.
The research also makes use of exercise-induced-hyperthermia studies due to ethical constraints. Moreover, the dependent parameters of EBE, of which metabolic rate is of prime importance, have been analysed herein at induced-hyperthermia extremities. A case study is also presented to substantiate the findings along with their biological explanation. Furthermore, a 3-node thermal manikin has been designed and developed to experimentally verify the computed thresholds. The thermal manikin reproduced the peripheral blood perfusion and heat transfer characteristics in line with real human body thermodynamics. Both the steady-state and transient responses showed that the thermal manikin is in close approximation with the computed thresholds. The authors believe that the study can greatly help in designing an effective cooling therapy for brain-hyperthermia, e.g. the heat transfer coefficient and heat flux which is necessary to develop any heat exchanger for the patients. The thermal manikin, developed herein, may also prove to be vital in designing therapeutic equipment.
The next section briefly discusses the relevant theory which is followed by the details of our methodology. The methodology section has been subdivided into two major subsections: one to explain the methods to evaluate the brain-hyperthermia using an exergy-based approach and the other to elaborate on the development of the manikin. The experimentation section contains relevant computations and experiments. It is followed by a section on results and discussion including the case-study under induced-hyperthermia settings.
Section snippets
Human body exergy-balance equation (EBE)
The human body can be considered as a complex thermal system which accumulates as well as exhausts energies in addition to storing significant thermal energy inside itself to maintain core body-temperature and organelle functions. This research aims to explore the body's thermoregulation behaviour using an exergy-based approach (Tokunaga and Shukuya, 2011) which facilitates a breakdown of energy into exergy and entropy forms, as illustrated in Fig. 1. Exergy is a relatively new concept in the
Evaluation methods of brain hyperthermia
This section details the methodology to a compute human body's EBE under brain hyperthermia conditions. The solution of EBE requires information from several dependent parameters which we planned to extract from various studies (Awbi, 2003; Cheuvront et al., 2010; Gagge and Gonzalez, 2010; Hale, 2005; Kenefick et al., 2010; Kiyatkin, 2004; McPherson, 1993; Nybo, 2008; Prek, 2006; Rhoades and Bell, 2012; Shukuya, 2012; Tokunaga and Shukuya, 2011; Wang and Wang, 2000). The supplementary material (
Computation of exergy balance equation
The human exergy model is summarized in Fig. 2 with input/output variables. The direct sunlight exposure or indoor elevated temperatures may induce hyperthermia inside the body which is interpreted in terms of skin and core body-temperatures. The exergy consumption (EC) which is the energy with a potential to be dispersed is the output of the model, along with the entropy generation rate (). These two parameters are evaluated and validated experimentally in this study to suggest the
Case study
The computation of human-body thermometry at physiological-extreme is a pioneer attempt as compared to the existing physiological thermal comfort analysis in the literature. In this study, body thermal anomalies (EC, ) are computed under steady state conditions, considering typical stressed conditions and corresponding induced-hyperthermia hemodynamic. The environmental conditions are moderately worse (, ) with a relative humidity of 70% and body temperatures under
Conclusions
The effect of brain hyperthermia was investigated numerically to estimate a pathophysiological boundary under steady-state conditions. The acquired results set norms for controlled environment design, applicable in cooling therapies for similar heat-related illnesses.
- 1.
To define the pathological states more precisely, hyperthermia mechanics is modelled by considering thermal exergy, an important thermodynamic concept, with the addition of a metabolic heat storage factor.
- 2.
Metabolic rate, the most
The authors of this work have the same contribution
The University of Engineering and Technology Lahore Pakistan (postgraduate research funds) and College of Sciences, Massey University, New Zealand (#RM18150).
The authors gratefully acknowledge financial support from the College of Sciences, Massey University, New Zealand (#RM18150) and the University of Engineering and Technology Lahore Pakistan (postgraduate research funds).
Declaration of competing interest
The authors declare no conflict of interest.
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