Clinical Managing Heat Illness in Urgent Care Urgent message: The urgent care provider’s most critical role in heat illness is to identify risk factors and the cause, to cool and hydrate the patient, assess for complications, and educate the patient in the hope of preventing a more serious exposure. Bridget Dyer, MD, Samuel Keim, MD, and Peter Rosen, MD Introduction Physiology H w w w. j u c m . c o m As the core temperature ex- ceeds the hypothalamic set point, heat avoidance behav- ior is induced, and both sym- pathetic and parasympa- thetic tones are augmented. Increased sympathetic tone increases cardiac output, supporting cutaneous and skeletal muscle vasodilata- tion, allowing for radiation of heat, delivering plasma for sweat and oxygen for exer- tion, with contraction of splanchnic circulation. Parasympathetic tone mod- ulates sweating, increasing to 2.5 liters per hour in an accli- mated person during strenu- ous exercise. 3 Heat shock proteins (HSP) act as molecular chaperones that prevent denaturing of other proteins at higher temperatures. Ini- tial heat stress triggers increase HSP expression, which protects cells from a second heat exposure. 4 Any process or comorbidity that interferes with cardiac output, va- © Photoresearchers.com/iStockPhoto.com. Composite: Tom DePrenda eat illness occurs when external heat conditions and internal heat pro- duction overwhelm the ability of the body to dis- sipate heat. Evaporation of sweat is the most effective way to dissipate heat; when the humidity is high, evapo- ration is compromised. Cal- culations that are based on both temperature and hu- midity, such as the heat in- dex, are a more robust way of determining heat stress than ambient temperature alone. 1 Elevated humidity can cause even moderate temperatures to be dangerous, especially to persons at high risk. Internal heat production depends on both the level of exertion and the physiologic characteristics of the pa- tient. In the average adult, exertion can raise the basal metabolic rate from 100 kcal/hr to more than 1000 kcal/hr, 70% to 100% of which is released as heat. 2 JUCM T h e J o u r n a l o f U r g e n t C a r e M e d i c i n e | S e p t e m b e r 2 0 0 7 13