1. Humans tightly regulate their core body temperature around 37°C through feedback mechanisms involving thermal sensing receptors, central processing in the hypothalamus, and efferent responses like vasoconstriction and shivering. 2. During general anesthesia, thermoregulation is impaired as behavioral responses are removed and autonomic responses like shivering and vasoconstriction are decreased. This commonly leads to unintended hypothermia from heat loss exceeding the body's ability to generate heat. 3. Regional anesthesia also impairs thermoregulation by blocking cutaneous vasoconstriction and decreasing the shivering threshold, potentially resulting in hypothermia if not monitored. Active warming and other

1. THERMOREGULATION
Physiology & Monitoring
BY- Dr. Abhinav Shreeram
DNB Trainee
Dept. of Anaesthesiology

2. INTRODUCTION
• Humans are homeothermic.
• Any deviation ----> derraned metabolic function-----> Death
• Normal Body Temp ≈370 C
• Decrese of core temp 1-20 C will lead to:
:Raised incidence of cardiac mrbidity
:3 times more chance of wound infection
:Increased surgical blood loss
:20% increased chance of blood transfusion.
:Prolonged post anaesthesia recovery

3. NORMAL THERMOREGULATION
• +/- Feedback system.
• Multiple signal from nearly every
tissue.
• Processing in 3 phases:
1) Afferent thermal sensing.
2) Central regulation.
3) Efferent response.

4. AFFERENT INPUT
• Thermal sensing receptors all over the body.
• 80% by skin & 20% by others (spinal cord, thoracic tissue,
abdominal tissue etc.)
• Different for cold & warm.
:Cutaneous warm receptor
:TRP Receptor
• Pathways :
:C fibres : Warm + Pain
:Aδ fibres : cold
• Ascending lateral Spinothalamic Tract mainly but overlaps.

5. CENTRAL CONTROL
• 10 by Hypothalamus : Acts as Integrated Circuit.
• Preprocessed : In spinal cord & other brain areas.
• So in High spinal transection : Intact Thermoregulation.
• Few terms:
1.Gain of thermoregulation.
2.Threshold temp.
3.Inter threshold Range.
4.Maximum Intensity.

6. • Gain : Slope of response Intensity Vs Core temp.
• Threshold: Temp at which the regulatory response occur.
• Inter Threshold Range: Core temp not triggering response.
Tight control in humans.
• Maximum intensity: Mximum achievable response.

7. • Determinants of threshold : Not well understood but mediated
by : NE, Dopamine, 5HT, Ach, PGE1 & Neuropeptides.
• Threshold follows cicardian rythm.
• Alteration of threshold temp :
food intake, infection, hypothyroidism, hyperthyroidism,
Anaesthetic drugs, Alcohol, Thermal adaption.
• Central regulation intact in infants even premature ones.
• Sweating and vasocontriction both more in females.

8. EFFERENT RSPONSE
• Increased metabolic heat production.
• Alteration of Environmental Loss (Vasoconstriction/dilation)
• All response has own gain, threhold & Maximum.
• For hypothermia vasocontrion is 1st response: Energy efficient.
• 2nd is Increased production : Active process.
• Behavioural regulation : Most important effector.

9. 1. VASOCONSTRICTION
• Cutaneous response.
• Most effective autonomic response.
• Controlled by a adrenergic symp. nerves.
• Numerous in palm & digits.
• Capillaries and AV shunts.

10. 2. METABOLIC HEAT
• Increases whole body oxygen consumption.
• No mechanical work involved.
• Infants have two times more efficiency.
• Not that much effective in adults.
• Occurs in skeletal muscle & brown fat.
• Mediated by NE release.
• Through uncoupling protein.
• Increases only core temp.

11. 3. SUSTAINED SHIVERING
• Augments metabolic heat production.
• Increases the O2 consumption by 50-100%
• Not so effective than exercise (500%
increased O2 consumption)
• ≤250 Hz rapid, unsyncronised, not
centrally controlled.
• Superimposed by waxing and waning (4-8
times), Centrally controlled.
• Less muscle mass, NM diseases & Muscle
relaxants reduces shivering.

12. 4. SWEATING
• Only mechanism for raised body
temperature.
• Mediated by post ganglionic cholinergic
nerves.
• Active process.
• Inhibited by nerve block & atropin
administration.
• Normal : 1L/H, Athletes: 2L/H.
• 0.58 Kcal/g energy lost in evaporated sweat.
• With sweating vasodilation too occur.
• During extreme heat stress blood flow from
cutaneous vessels : 7.5L/min.

13. REGULATION DURING GENERAL ANAESTHESIA
• No behavioral regulation : Paralysed, uncouncious.
• Impared autonomic thermal control.
• Warm response increases.
• Cold response dcreases too much.
• So hypothermia is more common.
• Gain: Normal.
• Interthreshold range : 2-40 C (N=0.20 C)
• Maximum intensity : Normal.

14. RESPONSE THRESHOLD IN GA
• Drugs which increases the sweating threshold & decreses the
shivering & vasoconstriction threshold:
Propofol
Alfentanyl
Dexmed
Isoflurane
Desflurane
Sevoflurane
Halothane
N2O+ Fentanyl Combination
Clonidine
• No change in gain & Max intensity

15. HYPOTHERMIA DURING GA
• Most common thermoregulation defect.
• Due to combination of Anaesthetic drugs & Low OT temp.
• Heat loss in OT: by radiation, conduction, convection, evaporation
& perioperative.
• If still air then less loss.
• Heat lost by evaporation is least.
• Infants tends to loose more heat because of thin skin.
• Respiratory heat loss is almost nil.

16. PATTERN OF HEAT LOSS
• Initial rapid decrease in core
temp.
• Almost 0.5 to 1.50 C during 1st 4
hours.
• Volatiles leads to heat loss by
direct peripheral vasodilation.
• Core temp is generally 3-40 C
more than the peripheral ones.

18. REGULATION IN NEURAXIAL ANAESTHESIA
• Always impared.
• Intra -op decrease in core temp.
• Not perceived conciously.
• Triggers shivering.
• Paradox : Hypothermia but not feeling cold.
• Decreas in VC & Shivering threshold above the level of block.
• Alteration in central control.
• No perception of cold below the level of block.

19. • Epidural block even decreases the gain and maximum intensity.
• Sedatives & analgesics used with block and advanced age will
agment the impairment.
• Decrese in core temp may not trigger response because it depends
mainly on skin temp.

20. Regional Block
Relative warm perception of skin temp
Shivering & VC threshold decrese
Neglecting monitoring
Decrease in Core temp
Hypothermia

21. • Heat loss after regional anaesthesia almost same as during GA.
• 0.5 to 1 C decrease in temp. shortly after induction.
• Core temp. does not plateau like GA.
• So decresing temp will lead to hypothermia.
• VC in leg is directly prevented by block.
• With combined RA + GA 1C more dip in response trigger.
• Shivering in RA is common occuring due to core hypothermia.
• Temp of injected LA doesn't affect.

22. MILD INTRA-OP HYPOTHERMIA
BENEFITS
• Protection against cerebral ischemia & hypoxia.
• Far better protection than high dose isoflurane or barbiturate
coma.
• Can be used in asphaxiated neonates.
• Under research : role in raised ICT, CVA, SAH, AMI.
• Can be used in post cardiac arrest patients.

23. ADVERSE EFFECTS:
• Impairement of coagulation by platelet function defect.
• May be directly imparing the enzymes of coagulation.
• Increases tranfusion requirement.
• 1 C decrease in temp increases chance of complication by 20%.
• Decrease in drug metabolism. So will raise the plasma conc of all
the anaesthetics.
• if Core temp goes below 20 C we do not require any anaesthesia.
• MAC decrese by 5% with dip of every 1 C.
• Delayed wound healing.
• Worst experience of patients life.

24. POST ANAETHESIA SHIVERING
• Almost 40% incidence.
• But now is decreasing.
• Increses O2 consumption by 100%.
• Poorly corelated with MI.
• IOP & ICT increases.
• Wound pain also increases.
• Happens more in young age.

25. • Possible etiology:
1.Uninhibited spinal reflex.
2.Pain.
3.Decreased symp activity.
4.Pyrogen release.
5.Adrenal suppresion.
6.Respiratory alkalosis.
7.Intra- Op hypothermia.
• Characterised by tonic and phasic pattern.
• Tonic : Waxing & Waning (4/8 cycles per min).
• Phasic: Bursting pattern (5-7 Hz)
• Clonic pattern if seen will always be pathologic.

26. • Treatment:
: Skin surface warming : Augments Core temp.
: Cutaneus warmer machine.
: Drugs:
1. Meperidine
2. Clonidine (75ug iv)
3. Ketanserin(10mg iv)
4. Tramadol(50mg iv)
5. Physostigmine(0.04 mg/kg iv)
6. Nefopam(0.15mg/kg)
7. Dexmed
8. MgSO4 (30mg/kg iv)

27. • No specific drug mechanism.
• Clonidin & dexmed acts centrally.
• Alfentanyl is u receptor agonist.
• Meperidine is best which reduces the shivering and VC
threshold by almost half keeping gain & max intensity
normal.

28. PERIOPERATIVE THERMAL MANIPULATIONS
1. Increasing Vasomotor Tone:
: Reaching plateau early.
: Transfers heat from periphery to core.
2. Prevention of redistribution hypothermia:
: prewarming the periphery.
3. Airway heating and humidification:
: More effective in infants & children.
: Actively heating the airway much better.

29. 4. I.V. Fluids:
: Not effective to treat but good for prevention
: Can't greatly exceeds body temp.
: 1U refridgerated blood/ 1L of crystalloids at room
temp will decrease the body temp by 0.25 C.
: Fluid warmers are used in prolonged Surgeries.

30. 5.CUTANEOUS WARMING
• Room temp 23 C for adults & 26 C for infants is ideal but not
practical.
• Passive insulators to be used : Cotton blankets, surgical drapes,
plastic sheatings, reflective composits(space blanket)
• Warming the blanket is of not that much use.
• Prevention is better & more effective.
• Depends on surface area.
• Common false perception that heat loss from head is more.

31. • Wrong to only cover the head & leave the arm.
• Active warming advised for prolonged surgeries.
• Circulating water matress & forced air is commonly used.
• No or very little heat loss from the back.
• If No cutaneous heat loss the metabolic heat will raise the
body temp by 1C/hour.

32. MILD THERAPEUTIC HYPOTHERMIA
• Target temp: 32 to 34 C but have to reach very quickly.
• Generaly useful in neuro surgeries.
• Immersion of patient in cold water will be ideal but its not practical.
• Refrigerated IVF : reduction by 0.5C/L but fluid restriction in neuro
Sx.
• Forced air cooling : Slow (2.5 Hrs)
• Energy exchange pads.
• Best is endovascular cooling : heat exchanging servo catheters in
IVC. Decreases temp by 4C/hour.

33. • Body has to tolerate that hypothermia.
• So we will augment it with drugs.
• Buspiron+ Mepiridine : Synergisticaly reduces shivering
threshold to 34 C.
• Ondansetron + Doxapram + MgSO4 can be used but its not
that effective.

34. SEVERE DELIBERATE HYPOTHERMIA
• Used in cardiac surgeries mostly.
• Earlier 28C was used for bypass surgeries but it shows no benefit.
• Can be lethal.
• Procedure at normothermia or 33C shows much better prognosis.
• But its still a routine for causing intentional circulatory arrest.

35. MONITORING GUIDELINES
• Core body temp should be measured in most patients given general
anaesthesia for more than 30 mins.
• Temp should also be measured during regional anaesthesia when
changes in body temp are intended, anticipated or suspected.
• Unless hypothermia is specifically indicated (for protection against
ischemia), efforts sould be made to maintain intraoperative core
temp higher than 36C.

36. TEMP MONITORING SITES
• Core temp measuring sites :
:Pulmonary artery
:Distal esophagus
:Tympanic Membrane
:Nasopharynx.
• Can be estimated with
reasonable accuracy using oral,
axillary, rectal & bladder temp.

37. THANK YOU!