5. Temperature + Management

Heat is an energy form related to the activity, or kinetic energy, in the molecules of the particular substance. Temperature is a way of quantifying the thermal state of a substance.

Units of measurement: the SI unit is the Kelvin (K), which equals Celsius (C) plus 273.15. As 1 C is the same as 1 K, the unit is used universally in medicine.

Most of the body heat content (66%) lies within the central core compartment comprising the brain and the organs of the trunk, whose temperature is maintained between 36.5 and 37.5 C. The remaining 33% is accounted for by the periphery, the temperature of which can undergo much wider fluctuation. It is essential, therefore, to measure core temperature.

Electrical • Thermistor: • Thermocouple • Resistance thermometer:

a small bead of a semiconductor material, usually a metal oxide, is incorporated into a Wheatstone bridge circuit. The resistance of the bead decreases exponentially as the temperature rises

Thermocouple: if two dissimilar metals are joined, a small potential difference develops which is proportional to the temperature of the junction. (This is known as the Seebeck effect.)

Resistance thermometer: these are based on the principle that electrical resistance in metals (usually platinum) shows a linear increase with temperature. These systems are not used clinically.

• Mercury and alcohol thermometers • Dial thermometers:

Mercury and alcohol thermometers: volume increases with temperature. Like all thermometers, these are calibrated against fixed points, such as the triple point (at which water, water vapour and ice are in equilibrium) and boiling point of water.

Dial thermometers: these may use a coil comprising two metals with differential coefficients of expansion. As the temperature changes, the coil tightens and relaxes, and an attached lever moves across a calibrated dial.

— Tympanic membrane and non-contact (forehead) thermometers: body emits infrared radiation, whose intensity and wavelength vary with temperature.

• Cardiorespiratory effects: oxygen consumption increases and cardiac output decreases. Arrhythmias and myocardial ischaemia are more likely. The oxygen–haemoglobin dissociation curve shifts to the left and reduces oxygen delivery. Blood viscosity increases. • Coagulation: platelet function is impaired, intraoperative blood loss increases and transfusion requirements rise. • Metabolic effects and effects on drugs: within a few degrees of normal core temperature, metabolic rate decreases by 6–7% for each 1C fall Enzymatic reactions and intermediary metabolism are slower at core temperatures below 34 C. Drug actions are prolonged, especially those of muscle relaxants. Glucose utilization decreases and hyperglycaemia can result. Metabolic acidosis may supervene. Surgical outcome: hypothermia compromises immune function and increases postoperative infecion rates. Wound healing is adversely affected and hospital stay may be extended.

Radiation (50%): the body is an efficient radiator, transferring heat from a hot to cooler objects. Convection (30%): air in the layer close to the body is warmed by conduction, rises as its temperature increases and is carried away by convection currents. Evaporation (20–25%): moisture on the body’s surface evaporates, loses latent heat of vaporization and the body cools. Conduction (3–5%): this occurs only if the patient is lying unprotected on an efficient heat conductor Respiration (10%): heat loss is via evaporation and the need to heat inspired air.
Anaesthesia: this causes vasodilatation and also affects central thermoregulation.