REGULATION OF BODY TEMPERATURE AND TEMPERATURE MEASUREMENT (1).pptx

Body temperature regulation

1. WEST AFRICAN COLLEGE OF SURGEONS COLLEGE OUEST AFRICAN DES CHIRURGIENS Diploma in Anaesthesia Update Course

2. Diploma in Anaesthesia Update Course REGULATION OF BODY TEMPERATURE AND TEMPERATURE MEASUREMENT Dr OKOLO Eshemogie CONSULTANT ANAESTHESIOLOGIST MAITAMA DISTRICT HOSPITAL, MAITAMA, ABUJA

3. Introduction Temperature regulation Mechanisms of heat loss/gain Physiologic Mechanisms Regulatory Mechanisms Disturbances of thermoregulation Temperature Measurement Sites of Measurement Diploma in Anaesthesia Update Course COURSE OUTLINE Introduction Temperature regulation Mechanisms of heat loss/gain Physiologic Mechanisms Regulatory Mechanisms Disturbances of thermoregulation Temperature Measurement Sites of Measurement

4. INTRODUCTION Temperature is the property of a system which determines whether heat is transferred to or from other systems. The human body temperature is tightly regulated. Many enzyme and transport systems cannot tolerate excessive temperature changes.

5. Three temperature scales are recognised : a.) Kelvin scale b.) Celsius scale (formerly Centigrade) c.) Fahrenheit scale . The SI unit of temperature is the Kelvin . Normal body temperature is 37°C ± 0.7°C and thermoregulatory mechanisms maintain this.

6. TEMPERATURE REGULATION Humans are homeothermic , maintaining a constant body core temperature of 37°C ± 1°C. The core usually includes cranial, thoracic, abdominal and pelvic contents as well as deep portions of the limbs. Temperature is lowest at night and highest in mid-afternoon, also varying with the menstrual cycle, with a rise at ovulation.

7. Constant temperature is required for optimal enzyme activity. Denaturation of proteins occurs at 42°C Loss of consciousness occurs at hypothermia below 30°C. Body temperature may be altered by various factors including exercise, feeding, thyroid disease, infection and drugs.

8. MECHANISMS OF HEAT LOSS/GAIN HEAT GAIN From the environment From metabolism (mainly in the Brain, Liver and Kidney). Vigorous muscular activity may increase heat production by up to 20 times. In babies, Brown fat produces much heat.

9. HEAT LOSS Radiation from the skin. May account for 40% of total loss. Convection: Related to airflow. Accounts for up to 40% of total loss. Evaporation from the Respiratory tract and skin (increased by sweating, which normally accounts for 20% but may ↑ markedly).

10. Conduction: This is of little importance in air but significant in water.

11. PHYSIOLOGIC MECHANISMS Temperature sensitive cells are present in the Anterior Hypothalamus (thought to be the most important site), Brain Stem, Spinal Cord, skin, skeletal muscle and abdominal viscera. Peripheral temperature receptors are primary nerve endings and respond to cold and hot stimuli via A δ and C fibres respectively.

12. Central control of thermoregulation is by the Hypothalamus. Efferents pass via the sympathetic nervous system to blood vessels, sweat glands and piloerector muscles. Efferents also pass to somatic motor centres in the lower brainstem to cause shivering. Local reflexes are also involved.

13. THERMOREGULATORY MECHANISMS Behavioural : In a conscious adult, is the major regulator of heat loss e.g. Moving away from heat sources, modifying local temperature, adding or removing clothing, curling up in the cold etc. Skin Blood Flow: Vasodilation or Vasoconstriction of skin vessels and by opening or closing of arteriovenous anastomosis in the skin.

14. Shivering and Piloerection (reduced or absent in babies, Brown fat metabolism occuring instead). This can increase metabolic activity by up to 600% in adults. The effectiveness of this heat source is ↓ by the fact that muscle activity also ↑ blood flow to peripheral tissues, dissipating the heat produced.

15. Sweating: In stressed athletes, sweating can account for an ↑ in heat loss by a factor of up to 10 times.

16. DISTURBANCES OF THERMOREGULATION a.) FEVER One of the most common manifestations of disease. Due to the production of endogenous pyrogens. These cause the local release of prostaglandins in the hypothalamus.

17. b.) MALIGNANT HYPERTHERMIA A genetic condition that causes widespread persistent muscle contraction when triggered by stress or specific anaesthetic agents. Results in massive heat production and a rapid uncontrolled rise in body temperature with metabolic acidosis and myoglobinuria . Underlying lesion is a defect in the gene -

18. Contd. coding for the Ryanodine receptors in the sarcoplasmic triads which leads to excessive Ca²⁺ release. c.) HYPOTHERMIC EFFECTS OF ANAESTHESIA AND SURGERY Anaesthesia affects thermoregulation in a number of ways, especially as behavioural responses are abolished.

19. Also, cutaneous vasoconstriction is antagonised by vasodilator anaesthetics . Thermoregulatory responses to heat loss are impaired or abolished. Physical mechanisms for heat loss include: Conduction - due to contact e.g. cold operating table. Convection - heat loss due to movement away-

20. Contd. from exposed body surfaces and ↑ by imposed air flow currents eg in laminar flow operating theatres. Radiation -heat loss by infra red radiation from exposed portions of the body to neighbouring objects not in contact. Evaporation - of sweat from skin or body fluids from mucosal or tissue surfaces.

21. HYPOTHERMIA Hypothermia is said to be present when core body temp. is less than 36°C. May be associated with: Exposure, Near drowning, The Elderly, Hypothyroidism and Prolonged surgery. Normal metabolic and physiological processes are slowed down.

22. This results in Hypotension, Bradycardia and loss of consciousness. In severe cases, Pulmonary oedema and Ventricular fibrillation may occur.

23. TEMPERATURE MEASUREMENT Methods include: ELECTRICAL Thermocouple: Relies on the Seebeck effect i.e. the production of voltage at the junction of two different conductors, the magnitude of which is proportional to temperature. Junctions may be small and versatile. Low thermal capacity means rapid response times.

24. The voltage output per degree centigrade is small and requires signal amplification. THERMISTOR: Resistance falls exponentially with temperature. Consists of a metal oxide- semiconductor bead which may be small enough to be placed within body cavities. Calibration may be difficult. It has a rapid response time.

25. Thermistors from the same batch, show wide variation in their electrical resistance and as they age, their electrical resistance changes. Platinum Resistance Wire: Resistance increases proportionally with temperature. The change in resistance is detected using a Wheatstone bridge circuit and a Galvanometer.

26. Its very accurate but fragile. The size of the coil makes it difficult to produce a small probe. Also, the response time is slow.

27. NON-ELECTRICAL Liquid Thermometers: The liquid (usually Mercury), expands as temp. ↑, and moves out of it’s glass bulb and up the barrel of the instrument. Temp. is read from a scale along it’s length A constriction just above the bulb prevents the mercury from withdrawing back into the bulb.

28. It can thus be made in maximum reading form. The Mercury thermometer has some disadvantages in clinical practice viz : 2-3 min are required for complete thermal equilibrium. May be difficult to introduce into some orifices because of its rigidity and risk of its breaking.

29. Poor visual display of temperature, slow response time, limited temperature range, plus inability to be read remotely are some other disadvantages. Advantages include its simplicity and no power supply required for operation. Alcohol is sometimes used instead of mercury.

30. It is cheaper and more suitable for use at very low temperatures. It is however unsuitable for high temp. as alcohol boils at just 78.5°C. GAS EXPANSION THERMOMETERS e.g. an Anaeroid guage used for pressure measurement is calibrated in units of Temp. Accuracy is poor and calibration may be difficult.

31. BIMETALLIC STRIP: Here, 2 dissimilar metals each with a different coefficient of expansion are arranged in a coil. A pointer is moved by coiling or uncoiling the strip as temperature changes. INFRA-RED THERMOMETRY: Based on the fact that objects emit electromagnetic radiation over a range of wavelenghts . The intensity of this radiation and the wavelenght depend on the temperature of the object.

32. BIMETALLIC STRIP

33. DIGITAL INFRA-RED THERMOMETERS

34. Objects at body temp. primarily emit infrared radiation. The Infrared Ear Thermometer uses a tube inserted into the ear canal to direct radiation emitted by the ear drum and canal. A sensor converts the radiation into an electric signal. Two types of sensors are in use viz :

35. Pyroelectric Sensor and Thermopile Sensor CHEMICAL THERMOMETERS: Consist of a plastic strip containing a large number of cells each holding liquid crystals which melt and change colour according to temperature. Its accurate to 0.5°C.

36. SITES OF MEASUREMENT Oesophagus : Accurate if lower third is used. Placed above this level, it may under read due to the cooling effect of inspired gases. Correctly placed, provides a good estimate of cerebral blood temperature. Nasopharynx: Probe is positioned just behind the soft palate. Less accurate than an oesophageal probe for core temp. measurement.

37. Tympanic Membrane: Correlates closely with hypothalamic temp. Has a rapid response time. Risk of tympanic perforation if direct contact techniques are used. Blood: Blood temp. can be measured using thermistors placed into pulmonary artery catheters. Provides the best continuous measure of core temp.

38. Rectum: Rectal temp is influenced by the heat generated by gut flora, the cooling effect of blood returning from the lower limbs and insulation of the temperature probe by faeces . Rectal temp. is usually 0.5-1°C higher than core temp. Compared with other sites, the response time is slow.

39. CONCLUSION The maintenance of a constant body temperature is important for normal physiologic functions. This is achievable by a good understanding of normal physiologic mechanisms and quality monitoring.