Hypothermia

An important facet of a forensic investigation into a death is the determination of the cause of death. In cases where outward signs of physical trauma (i.e., gunshot or stab wounds) are absent, a forensic investigator may be presented with more subtle indicators of death.

One example is hypothermia; the intentional or accidental reduction of core body temperature to below 95°F (35°C) which, in severe instances, is fatal. Humans are endothermic (warm-blooded) creatures, whose core body temperature is physiologically regulated at approximately 98.6°F (37°C), even in fluctuating environmental temperatures. An abnormal rise in this core temperature can cause heat stroke, with an abnormal decrease representing hypothermia.

Intentional hypothermia is used in medicine in both regional and total-body cooling. The body's metabolic rate (the rate at which cells provide energy for the body's vital functioning) decreases 8% with each 1.8°F (1°C) reduction in core body temperature, thus requiring reduced amounts of oxygen. Total-body hypothermia lowers the body temperature and slows the metabolic rate, protecting organs from reduced oxygen supply during the interruption of blood flow necessary in certain surgical procedures. In some procedures, like heart repair and organ transplantation, individual organs are preserved by intentional hypothermia of the organ involved. In open-heart surgery, blood supply to the chilled heart can be totally interrupted while the surgeon repairs the damaged organ. Organ and tissue destruction using extreme hypothermia -212 to -374°F (-100 to -190°C) is utilized in retinal and glaucoma surgery and to destroy pre-cancerous cells in some body tissue. This is called cryosurgery.

In contrast to these beneficial uses of intentional hypothermia, accidental hypothermia (i.e., falling into icy water, or exposure to cold weather without appropriate protective clothing) is potentially fatal and is of forensic interest.

Hypothermia is classified into four states. In mild cases, 95–89.6°F (35–32°C), symptoms include feeling cold, shivering (which helps raise body temperature), increased heart rate, and a desire to urinate, and some loss of coordination. Moderate hypothermia, 87.8–78.8°F (31–26°C) causes a decrease or inhibition of shivering, along with weakness, sleepiness, confusion, slurred speech, and lack of coordination. Deep hypothermia, 77–68°F (25–20°C) is extremely dangerous, as the body can no longer produce heat. Sufferers may behave irrationally, become comatose, lose the ability to see, and often cannot follow commands. In profound cases, 66–57°F (19–14°C), the sufferer will become rigid and may even appear dead, with dilated pupils, extremely low blood pressure, and barely perceptible heartbeat and breathing. This state usually requires complete, professional cardiopulmonary resuscitation for survival.

Normally, the body's core temperature represents a homeostatic balance between heat generation due to metabolic processes, and the loss of heat through conduction, convection, evaporation, and respiration, and radiation.

Conduction occurs when direct contact is made between the body and a cold object, and heat passes from the body to that object. Convection is when cold air or water makes contact with the body, becomes warm, and moves away to be replaced by another volume of cold air or water. The cooler the air or water, and the faster it moves, the faster the core body temperature drops.

Evaporation through perspiration and respiration provides almost 30% of the body's natural cooling mechanism. Because cold air contains little water and readily evaporates perspiration; and because physical exertion produces sweating, even in extreme cold, heat loss through evaporation takes place even at very low temperatures. When heat loss involves both evaporation and convection, for instance when someone is outdoors in wet clothes, body temperature can quickly plummet to dangerously low levels.