temperature and humidity play a key role in patient recovery from anesthesia...............

1. Temperature & Humidity
By Dr. K. Krishna Kishore

2. 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

3. 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.

4. 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.

5. 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.

6. 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.

7. 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).

8. 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.

9. • 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%).

10. 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.

11. 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.

12. • 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

13. 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.

14. • Reflex response to
cold:
• Vasoconstriction
• shivering,
• piloerection,
• nonshivering
thermogenesis
• From posterior
hypothalamus.
• Reflex response to
heat:
• vasodilatation
• Sweating
• From anterior
hypothalamus.

15. • In awake individual behavioural responses occur
before core temp reaches new set points.
• Vasoconstriction-at 36.5⁰C
• shivering-at 36.2⁰C.

16. • 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.

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

18. Causes
Disorders associated with excess heat
production:
• Malignant hyperthermia
• NMS
• Thyrotoxicosis
• Delirium tremens
• Pheochromocytoma
• Salicylate intoxication
• Drug abuse
• Status epilepticus
• Exertional hyperthermia

19. Causes
Disorders associated with decreased heat
loss:
• ANS dysfunction
• Anticholinergics
• Drug abuse (cocaine)
• Dehydration
• Occlusive dressings
• Heat stroke.

20. Causes
Disorders associated with dysfunction of
hypothalamus:
• Trauma
• Tumour
• Idiopathic hypothalamic dysfunction
• Cerebrovascular accidents
• Encephalitis
• NMS.

21. • 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 –upto
50ml/min,in severe heat –upto 2800 ml/min.

22. 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 abt
3⁰C. Anesthetics inhibit thermoregulation in a dose
dependent manner .
• Inhibit vasoconstriction and shivering about
3times as much they restrict shivering.

24. 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.

26. 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.

28. 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.

29. Prevention of hypothermia

30. Humidity
Absolute 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.

32. Hair hygrometer
• Principle-hair attached to a calibrated scale.
• The hair will lengthen with increases
in RH.

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

34. Clinical aspects
• Normally when pt beathes through nose ,
inspired air is warmed and saturated with
water vapour.
• If nose is bypassed , dry air enters
tracheasecretions 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

35. • Methods of increasing the inspired humidity:
• 1.Humidifying the env.
• 2.Humidifying the inspired gases alone.

36. 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

37. THANK YOU