Ch 51 Hypothermia

CHAPTER 51

HYPOTHERMIA

Outline: Definition Temperature control Pathophysiology Causes of intra-operative heat loss Methods of prevention Methods of warming blood and IV fluids

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DEFINITION Hypothermia describes subnormal body temperatures and an inability to generate heat or compensate for heat loss. It may be categorised as mild with temperatures ranging between 32 - 35 degrees centigrade, moderate at 27 - 32 degrees centigrade or severe when the temperature is less than 27 degrees centigrade. Body heat is lost through: • Radiation (40%) • Convection (30%) • Evaporation (30%) − 20% is lost through the skin − 10% through respiration of which 8% is through evaporation and 2% through warming the inhaled air • Conduction (minimal) Induced hypothermia is sometimes used in neurosurgical, cardiac and vascular surgery to reduce damage to vital organs during periods of hypoxia. Accidental hypothermia is associated with • Prolonged surgery • Elderly and hypothyroid patients • Exposure • Near drowning Signs of hypothermia Signs of mild hypothermia are peripheral vasoconstriction, shivering, decreased motor activity and CNS depression resulting in apathy or amnesia. Moderate hypothermia leads to further depression of the CNS resulting in unconsciousness, cardiac arrhythmias and water diuresis with ensuing dehydration. Severe hypothermia decreases blood pressure, heart rate and may cause ventricular fibrillation. At these temperatures the person will be in a coma and without reflexes.

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TEMPERATURE CONTROL The temperature control mechanism is situated in the hypothalamus where temperature information from skin, central and neural tissues is processed. • Two types of thermostatic neurons are located in the hypothalamus. One type is affected by increased temperature, the other by decreased temperature of the blood. • Descending pathways connect mainly to cardiovascular and respiratory centres in the brain stem and initiate heat loss or heat production responses in an attempt to bring temperature back within normal levels. A rise in the temperature of the blood causes an increase in the respiratory rate, peripheral vasodilation (which increases radiation) and increased perspiration. Decreased temperature causes conservation of heat by vasoconstriction, which reduces loss from radiation and by stimulating shivering. Heat production is also raised by the increase of the thyrotropic function of the anterior pituitary and a resulting increase in thyroid activity. Temperature changes also cause obvious behavioural adjustments in conscious adults eg movement to or from a heat source and the adding or removing of clothing. PATHOPHYSIOLOGY OF HYPOTHERMIA Metabolic effects There is a decrease in oxygen consumption and carbon dioxide production. Serum glucose levels are increased as a result of catecholamine release and a decrease in insulin production. Metabolic acidosis occurs resulting in an increase in potassium to which the hypothermic heart is very sensitive. Central nervous system Cerebral blood flow is reduced, oxygen consumption is reduced, the victim becomes sleepy at 33o C and cold narcosis follows at 30o C. The minimum alveolar concentration of volatile agents is decreased therefore a smaller concentration is needed and emergence from sleep may be delayed. Blood There is a decrease in plasma volume with an increase in viscosity. Aggregation of platelets decreases their function and leads to thrombocytopenia.

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Respiratory system Shivering increases oxygen consumption 3-5 times and may hinder the reading of the pulse oximeter. There is reduced availability of oxygen to the tissues as a result of respiratory depression, a fall in cardiac output, peripheral vasoconstriction and increased blood viscosity. Increased solubility of carbon dioxide lowers the arterial pressure of carbon dioxide further decreasing ventilatory drive. The mechanism of hypoxic vasoconstriction is impaired resulting in an increase in ventilation/perfusion mismatch and hypoxaemia. The oxygen dissociation curve is shifted to the left decreasing oxygen delivery to the tissues. Cardiovascular system Hypothermia can induce ventricular ectopic beats leading to unresponsive ventricular fibrillation or to bradycardia leading to unresponsive asystole. Vasoconstriction increases systemic vascular resistance increasing after load and myocardial oxygen demand, causing tissue hypoxia and acidosis although coronary blood flow is well maintained initially. Urinary system Renal blood flow and the glomerular filtration rate are decreased. Decreased sodium reabsorption causes impairment of concentration of urine leading to cold diuresis and hypovolaemia. Liver The blood supply to the liver is diminished slowing down liver function and the metabolism of drugs. CAUSES OF INTRA-OPERATIVE HEAT LOSS For the anaesthetist, hypothermia as a result of prolonged surgery is the most important and most common cause. Patient factors There is loss of movement, a reduced capacity to shiver, exposure and an increased surface area with increased evaporation. The introduction of cold fluids, either intravenously, as peritoneal lavage or bladder washout, also contributes to heat loss. (For methods of warming infused fluids see below). Infants have an increased surface area to body mass ratio and therefore loose heat more rapidly than adults. Patients with burns, severe injuries and those who are hypothyroid are predisposed to greater heat loss, as are the elderly who have decreased sympathetic activity.

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Anaesthetic factors Dry anaesthetic gases cause extra heat and moisture loss. Many anaesthetic drugs, such as thiopentone and halothane, cause vasodilatation. Opiates decrease vasoconstriction and volatile agents interfere with thermoregulation in the hypothalamus. Subarachnoid blocks also cause vasodilatation, inactivate muscular movement and block sensory input to the thermoregulatory centre. Surgical factors Prolonged exposure of abdominal organs, abdominal lavage and bladder washouts all lead to a significant drop in body temperature. METHODS OF PREVENTION OF HEAT LOSS DURING SURGERY • • • •

The use of heat and moisture exchange filters (HME) helps to reduce heat loss from the respiratory tract. Warming the fluids given IV or used for lavage reduces further heat loss from exposed tissues and cold fluids. Some form of external body warming such as a hot air blanket, a covered hot water bottle or covering of head and exposed limbs by drapes or even woollen garments. Raising the ambient temperature of the operating room even if this may become uncomfortable for staff.

SIMPLE METHODS OF WARMING BLOOD AND IV FLUIDS • • •

Put the container of blood or plasma in lukewarm water before administering it. The infusion set tubing can be passed through a warm bath or under the warming blanket. Use a blood warmer. This consists of a very long plastic coil which is immersed in a water bath maintained at about 37oC. The aim is to deliver blood to the patient at near body temperature without causing blood damage. (See Chapter 47)

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