Temperature & HumidityBy Dr. K. Krishna Kishore

Introduction

• Heat is a form of energy that can be transferred from a hotter substance to a colder substance , energy being in the form of kinetic energy of the molecules of the substance.

• Temperature is the thermal state of a substance which determines whether it will give heat to another substance or receive heat from it.

• Temperature scales-SI unit- Kelvin(K)-fraction 1/273.16 of the thermodynamic temperature of the triple point of water. Temp(K)=temp(C)+273.16

Temperature measurement

Non electrical technique-

1.Mercury thermometer

2.Bimetallic strip thermometer

3.Bourdon gauge thermometer.

Electrical techniques-

1.Resistance thermometer

2.Thermistor

3.Thermocouple.

Mercury thermometer

• Utilizes change in volume with temperature change.

• Reliable

• Disadvantages clinically-

2-3 minutes required for thermal equilibrium

Difficult to introduce in some orifices or in certain patients since it is rigid, with risk of breakage and consequent injury.

Dial thermometer

• A bimetallic strip of two dissimilar metals fixed together in a coil• A second type is a Bourdon-type in which a small tube of Hg expands or contracts exerting lesser or greater pressure on a needle.

Resistance thermometer

• Principle: Electrical resistance of a metal increases linearly with increased temperature.

• Consists of a platinum wire resistor, battery to supply current, and ammeter to measure resistance. Usually incorporates a Wheatstone bridge to increase the device's sensitivity.

Thermistor

• Principle: A bead of metal oxide, the electrical resistance of which falls exponentially as temperature rises. Often also used with a Wheatstone bridge.• Advantage: Smaller and cheaper than a resistance thermometer.• Disadvantage: Calibration will change if device is subjected to severe changes in temperature (e.g., gas sterilization).

Measurment of body temp

• Recommended-core temp >36⁰ C.• 1.Temp in lower 25% of esophagus• 2.Nasopharyngeal temp • 3.Rectal temp• 4.Bladder temp• 5.Tympanic membrane and aural canal temp• 6.Infrared thermometers• 7.Thermistors• 8.Skin temp.

• Body temp is determined by the relationship between heat production and heat dissipation.

• Normal core body temperature -36⁰ to 37.5⁰ C. Lowest-in morning due to 10%-15% decrease

in BMI during sleep. Highest- in evening.• Heat loss- through skin(the most important

route) and lung.• Types of heat loss:Radiation (60%)conduction(<5%)convection(15%)evaporation(20%).

Radiation

• 60% of heat loss.

• Warm object emits energy in the form of infrared radiation. Infrared radiation allows heat transfer independent of the intervening air.

• Space blankets.

• Radiant warmers.

Conduction

• By direct contact with a cooler object.

• Area of conducting surfaces, temp difference, presence/absence of insulation affects conduction.

• Reduction in core temp after administering i v fluids is due to conduction.

• Convection-conducting heat loss to air greatly facilitated by air movement is known as convection. Depends on air temperature and velocity.

• Evaporation-20% of heat loss. Mostly through skin. Depends on environmental humidity, exposed skin surface area, presence of diaphoresis, wound and bowel exposure, prep solutions. Only mechanism by which body can eliminate excess heat when the surrounding temp is high.

• Normal unacclimatized individual max sweat -700ml/hr, continued exposure-1500ml/hr

Thermoregulation

• Preoptic nucleus of anterior hypothalamus.

• Afferents-thermoreceptors in skin, deep tissues ,spinal cord. Also contains heat sensitive neurons and receives additional thermal input from extra hypothalamic areas.

• Reflex response to cold:

• Vasoconstriction• shivering, • piloerection,• nonshivering

thermogenesis• From posterior

hypothalamus.

• Reflex response to heat:

• vasodilatation

• Sweating

• From anterior hypothalamus.

• In awake individual behavioural responses occur before core temp reaches new set points.

• Vasoconstriction-at 36.5⁰C

• shivering-at 36.2⁰C.

• During general anesthesia-threshold temp for activation of responses to cold is decreased.

• Maintenance of body temp at close to optimum for enzyme activity assures constant rate of metabolism, optimal nerve conduction, skeletal muscle contraction.

Hyperthermia

• In this condition hypothalamic set point is normal but peripheral mechanisms are unable to maintain body temp that matches set point.

CausesDisorders associated with excess heat

production:• Malignant hyperthermia• NMS• Thyrotoxicosis• Delirium tremens• Pheochromocytoma• Salicylate intoxication • Drug abuse• Status epilepticus• Exertional hyperthermia

Causes

Disorders associated with decreased heat loss:

• ANS dysfunction

• Anticholinergics

• Drug abuse (cocaine)

• Dehydration

• Occlusive dressings

• Heat stroke.

Causes

Disorders associated with dysfunction of hypothalamus:

• Trauma

• Tumour

• Idiopathic hypothalamic dysfunction

• Cerebrovascular accidents

• Encephalitis

• NMS.

• Fever-pyrogens cause the setting point of the hypothalamic thermostat to increase. Pyrogens are polypeptides ,unlikely to cross BBB. But they act on the organum vasculosum of lamina terminalis (OVLT) leading to release of PG’s leading to stimulation of preoptic nucleus and generation of febrile response.

• Chills• Cutaneous blood flow-.Largely regulated by

symp. Nerves. Subcutaneous venous plexus is the major vascular structure. Fingers, palms,toes, earlobes-richly innervated with AV anastomoses.

• Normal blood flow-400ml/min.In severe cold –upto50ml/min,in severe heat –upto 2800 ml/min.

Perioperative temp changes

• Thermoregulatory system consists of afferent input, central processing, efferent response.

• General anesthesia affects all three elements.• Regional anesthesia affects afferent and efferent

component. • Both widen the interthreshold range to 4⁰C.

Threshold for sweating increases by abt 1⁰C and for vasoconstriction and shivering decreases by abt3⁰C. Anesthetics inhibit thermoregulation in a dose dependent manner .

• Inhibit vasoconstriction and shivering about 3times as much they restrict shivering.

Sequence of temp changes

• Body heat is unevenly distributed. Bcoz of vasoconstriction temp gradient betn core temp and periphery is 2 to 4⁰C.

• Core compartment: major viscera.• For general anesthesia patients :• 1st hr-heat will move from core to periphery responsible for 1

to 5 ⁰C decrease in core temp.• After 1st hr-core temp decreases at slower rate. Nearly linear.

Continuing heat loss exceeds heat production.• After 3 to 5hrs-plateau phase. heat loss equals heat

production.• If patient becomes severly hypothermic then activation of

thermoregulatory vasoconstriction will occur. • Regional anesthesia:• initial redistributive temp may be less, plateau phase may not

be seen. Core temp may decrease sufficiently.

Adverse consequences

• 1 ⁰C decrease in temp -5%reduction in MAC, increase in volatile anesthetic blood/gas solubility.

• Drug metabolism-decreased, particularly of non depolarizing agents.

• Core temp decrease by 1.5⁰ C triples incidence of VT, morbid cardiac events.

Beneficial effects

• Oxygen consumption-decreases by 5% to 7%.• Decrease in core temp bet 1 to 3⁰ C protects against

cerebral ischemia and arterial hypoxemia.• During cardiopulmonary bypass, • carotid endarterectomy,• aneurysm clipping,• cardiac surgery,• aortic cross-clamping,• malignant hyperthermia.• Main advantage-reduction in metabolic demand.

Prevention of hypothermia

HumidityAbsolute humidity (AH):• The amount of water vapor which gas can contain at a

specified temperature.• Increasing the temperature of a gas increases the amount of

water vapor it can carry. Decreasing the temperature lowers the kinetic energy of the vapor molecules to the point where they rain out or condense. Thus cold air holds less moisture, when fully saturated, than warmer air.

• Room temperature air (21ºC) when 100% humidified holds 18 mg H2O/L of gas. Tracheal air at the carina (37ºC) holds 44 mg H2O/L.

Relative humidity (RH):• Ratio of the mass of water vapor present in a given volume of

gas, compared to the mass required to saturate that volume at the same temperature. Usually expressed as a percentage.

Hair hygrometer

• Principle-hair attached to a calibrated scale.

• The hair will lengthen with increases in RH.

Wet and dry bulb hygrometer• Consists of two thermometers, one dry reading ambient temperature, theother wetted, which reads a lower temperature because of the cooling effect ofthe evaporating water it is moistened with (loss of the latent heat ofvaporization).• Difference between the two temperatures is related to the rate of evaporationwhich is related to ambient RH. Tables are used to read the RH.

Clinical aspects

• Normally when pt beathes through nose , inspired air is warmed and saturated with water vapour.

• If nose is bypassed , dry air enters tracheasecretions bcum dried, tenacious, form mucous plugs.

• Cilia become inhibited or damaged by dry gases on long exposure, cilia disappear, epithelium keratinised.

• Normally air entering trachea is saturated with water vapour to a humidity of 34 g/m3

• Methods of increasing the inspired humidity:

• 1.Humidifying the env.

• 2.Humidifying the inspired gases alone.

HME

•Inlet and outlet•Disposable element of paper, sponge, foam impregnated with hygroscopic substance such as calcium chloride, lithium chloride, silica gel.•Also known as “Artificial nose”.•During expiration and inspiration•Disadvantages•HMEF

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