Temperature Control Flashcards
Thermoregulation
mechanism by which the body maintains a stable temperature (mainly regulated by the hypothalamus)
Normal Body temp = 36.5 - 37.5
mean 36.8-37
Normal THermoregulation process
- Input
- thermoreceptors at tissue, brain, hypothalamus, spinal cord, thorax, skin - Integration
- hypothalamus set point - Output (if body registers the body is outside its set point)
- Behavioral: movement, dressing, position change
- Autonomic: shivering, vasoconstriction, vasodilation, sweating
What is non-shivering thermogenesis
Doubles heat production in infants
B3 adrenergic receptors located in brown fat
Brown fat = abundant in infants between scapula nape of neck and along great vessels
Contains many mitochondria increasing lipid oxidation to generate ATP production & heat
Which factors can influence the set point at the hypothalamus
anaesthesia
age
circadian rhythm
drugs
alcohol
ovulation
pyrogens
Describe the process of Afferent input:
Cold signals travel in the Aδfibres. Warmth signals travel via C-fibres & C-fibres are responsible for detection and transmission of pain.
The skin contains both cold & warmth receptors. The afferent information ascends in the spinothalamic tracts in the anterior spinal cord.
Describe the integration part of thermoregulation
Central structures (pre-optic area of the anterior hypothalamus) are responsible for the integration and regulation of thermal input
anterior hypothalamus governs vasodilatation and integrates afferent responses.
posterior hypothalamus that integrates cold afferent signals from the periphery
Threshold temperature
within 0.2 degrees
Cold defences
o Cutaneous vasoconstriction of arterio-venous shunts (and piloerection) at 36.8°C
o Non-shivering thermogenesis at 36°C
o Shivering at 35.7°C
Heat defences
o Sweating at 37°C
o Active vasodilatation (precapillary) at 37°C
General anaesthesia & thermoregulation
Impairs normal control, depresses thermoregulatory centre, and inactivates behavioural effector
Cold response threshold is decreased, whereas warm responses are better preserved interthreshold range widens from 0.2°C to 2 – 4°C.
Methods of temperature changes:
- Lose heat evaporation, conduction, convection, radiation
- Produce heat metabolic, conductive, convective, radiation
Radiation
The loss of energy through radiant electromagnetic waves, no direct contact between the two objects. Energy transfers / radiates from a warmer object to a colder one. This is exacerbated by the vasodilation of anaesthesia and therefor the most common during surgery.
Counteract radiation by covering patient with something.
Convection
The heat transfer through a fluid medium such as air / water which is in contact with the body’s surface and then the subsequent movement of the heated material away from the body. Happens in theatre due to laminar flow.
Counteract convection with forced air warmer / blanket
Conduction
The transfer of heat energy through direct contact between two objects.
Counteract conduction with insulated foam under patient.
Evaporation
The loss of heat due to vaporisation of moisture (includes sweating & open wounds).
Neuraxial anaesthesia and thermoregulation:
First, hypothermia does not provoke as much thermal discomfort. Consequently, patients do not complain of feeling cold
Second, hypothermia impairs thermoregulatory control centrally, reducing the vasoconstriction and shivering thresholds augmenting the interthreshold range
Third, all autonomic thermoregulatory defences are primarily neurally mediated. Thus, active vasodilation, sweating, vasoconstriction, and shivering all require intact nerves.
Redistribution is still seen peripheral inhibition of vasoconstriction
Consequences of hypothermia:
Cardiovascular
Arrhythmias
Decreased cardiac output/heart rate
Increased blood pressure
Coagulation
Impaired platelet function
Impaired coagulation (enzyme)
Increased viscosity
Enhance fibrinolysis
Immune
Immunosuppression
Poor wound healing
Increased risk of wound infection
Pharmacological
Decreased drug metabolism
Prolonged neuromuscular blockade
Delayed emergence
Endocrine
Activation of sympathetic nervous system
Increased TSH & T4
Post-operative shivering (increased VO₂)
Respiratory
Lack of hypoxic drive
Blunted ventilatory response to CO₂
Increased affinity of Hb for O₂
Renal
Decreased blood flow (GFR)
Gastrointestinal tract
Decreased blood flow
Decreased metabolic function (liver)
Central nervous system
Decreased metabolic rate (CMRO₂)
Decreased cerebral blood flow
DDx for Hyperthermia
o Malignant hyperthermia
o Thyrotoxicosis
o Cholinergic crises
o Neuroleptic malignant syndrome
o Infection
o Blood transfusion reaction
o SIRS
Hyperthermia
elevated body temperature due to failed thermoregulation
Fever temperature > 38.3°C
treatment of hyperthermia
- Monitoring = ECG, sats, BP, urine output, GCS
- Investigations = U&E, coagulation, ABG
- Supportive management = ice packs, cooling blanket, cold IV fluids, bypass
- Pharmacological = paracetamol, NSAIDS, Dantrolene (malignant hyperthermia)
Pre-operatively maintaining normothermia
- Warm induction room
- Prewarm patient (1 hour pre-op)
- Insulate patient
Intraoperatively maintaining normothermia
- Maintain ambient temperature at 21 – 24°C
- Neonates ambient temperature at 26°C
- Insulate patient
- Forced air warmer (early)
- Humidify anaesthetic gases
- Warm IV fluids / blood
- Warm fluid irrigation
- Cardiopulmonary bypass
- Limit skin exposure
Post-operatively maintaining normothermia
- Forced air warmer
- Pethidine 0.3 mg/kg IV / IM for shivering
