5. Temperature + Management Flashcards
Heat
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 measuremen
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.
Where is temp distributed
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.
There are three main types of device for measuring temperature: electrical, nonelectrical and infrared.
Electrical
- Thermistor:
- Thermocouple
- Resistance thermometer:
Thermistor
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
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:
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.
Non-electrical
- Mercury and alcohol thermometers
- Dial thermometers:
Mercury and alcohol 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:
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.
Infrared
— Tympanic membrane and non-contact (forehead) thermometers:
body emits infrared radiation, whose intensity and wavelength vary with
temperature.
clinical consequences of hypothermia
- 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.
Heat Loss During Anaesthesia
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.
