By Dr. Ahmed Mostafa Assist. Prof. of anesthesia and ICU

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. Heat energy will flow from an object of a high temperature to an object of a lower temperature.

Units of measurement: 1- Kelvin (SI unit): One kelvin is equal to 1/ of the thermodynamic triple point of water. A change in temperature of 1 K is equal in magnitude to that of 1 ° C. 2- Celsius/centigrade: Is a common measure of temperature in which a change of 1°C is equal in magnitude to a change of 1 K To convert absolute temperatures given in degrees celsius to kelvin, you must add

Units of measurement: 3- Fahrenheit: Zero °F = freezing point of ammonium chloride = -17°C The degree of Celsius scale is different than the Fahrenheit; therefore, for conversion special equations are needed. Celsius scale °C = (°F-32) x 5/9 Fahrenheit scale °F = (°C x 9/5) + 32

Measurement of temperature Electrical: 1- 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. These beads are both robust and very small, and are used in the tips of pulmonary artery flotation catheters and in arterial lines for thermodilution measurements.

Measurement of temperature Electrical: 1- Thermistor:

Measurement of temperature Electrical: 2-Resistance thermometer: these are based on the principle that electrical resistance in metals shows a linear increase with temperature. it is incorporated into a Wheatstone bridge circuit. -Very accurate used for calibration. -Has long response time. -Expensive and fragile. -Not used clinically.

Measurement of temperature Electrical: 2-Resistance thermometer:

Measurement of temperature Electrical: 3-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.) Another junction between the metals is necessary to complete an electrical circuit, although another temperature dependent voltage will develop at this junction.

Measurement of temperature Electrical: 3-Thermocouple: The metals that are used are commonly copper and a copper/nickel alloy. When the thermocouple is used as a thermometer, one of the junctions forms the temperature probe, while the other is kept at a constant temperature and acts as a reference. Thermocouples are stable and accurate to ±0.1 ° C. The Seebeck effect: At the junction of two dissimilar metals, a voltage will be produced, the magnitude of which will be in proportion to the temperature difference between two such junctions.

Measurement of temperature Electrical: 3-Thermocouple:

Measurement of temperature Electrical: 4-Transistor thermometer. If a current is passed across transistor junctions, the voltage developed is temperature dependent. 5- Crystal resonators thermometers. The resonance frequency of a quartz crystal is temperature dependent.

Measurement of temperature Infrared(Optical thermometers): Objects emit electromagnetic radiations (infrared radiations) which are different in the wavelength and intensity according to the temperature of the objects. Types:-Infrared ear thermometer, Tympanic membrane thermometer, thermography.

Measurement of temperature Non-electrical: 1-Mercury and alcohol thermometers: Volume increases with temperature. Like all thermometers, these are calibrated against fixed points, such as the triple point and boiling point of water. 2-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.

Mechanisms of heat loss  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.

Dr. Ahmed Mostafa