Hypothermia is the lowering of the body's temperature below normal levels. A dog's normal rectal temperature is 100.5 to 102.5 degrees Fahrenheit (37.7°C to 39.2°C). On the basis of body temperature, hypothermia can be classified as Mild (90-99° F or 32-35° C), Moderate (82-90°F or 28-32°C) and Severe (any temperature below 82°F or 28°C).
A lower than normal body temperature impairs normal functioning of the body and causes depression of central nervous system (CNS). The condition may also affect other vital organs and functions such as heart and blood flow (cardiovascular), breathing (respiratory), and the immune system. Hypothermic dogs may display conditions such as irregular heartbeat, trouble breathing, and impaired consciousness to the point of coma.
When the skin or blood is cooled enough to lower the body temperature in non-hibernating animals, the metabolic and physiological processes slow down. In the hypothermic state, the oxygen need of cells, particularly neurons is greatly reduced, and the circulation can be stopped for relatively long periods. At a rectal temperature of less than 28°C (82°F), the ability to regain normal temperature is lost, but animal will continue to survive if external heat is applied and the temperature returns to normal. Injuries to tissues due to decrease in the flow of oxygenated blood may occur. The extent of the injuries varies with the body temperature and duration of hypothermia. Hypothermia can be fatal.
Symptoms will vary according to the severity of condition. While mild hypothermia may be evident through weakness, shivering, and lack of mental alertness, moderate form of this condition will cause muscle stiffness, low blood pressure, a stupor-like state, and shallow, slow breathing. Meanwhile, severe hypothermia will be characterized by fixed and dilated pupils, inaudible heartbeat, difficulty breathing, and coma.