Core and Peripheral Temperature
Core and Peripheral Temperature
•Core Temperature Range: 36.5°C – 37.5°C
•Peripheral Temperature: 2°C-4°C lower than core temperature
Sites for Temperature Measurement
Sites for Temperature Measurement
1. Pulmonary Artery
• Considered the gold standard for core temperature monitoring.
• Requires a pulmonary artery catheter, which is usually reserved for critically ill patients.
2. Lower 1/3 Esophagus
• Ideal site for core temperature measurement in anesthetized patients.
• Located 24 cm below the larynx, at the point of maximum heart sound.
• Less affected by external temperature changes.
3. Nasopharynx
• Reliable for core temperature monitoring.
•Risk: Can cause epistaxis (nosebleeds) in some patients.
4. Oral Temperature
•0.5-1°C lower than rectal temperature.
• Affected by oral intake, breathing patterns, and environmental factors.
5. Axillary Temperature
•0.5-1°C lower than oral temperature.
• Least reliable site due to exposure to ambient temperature changes.
6. Rectal Temperature
• Considered a reliable core temperature measurement.
•Limitations: Can be affected by stool or rectal lavage.
7. Urinary Bladder
• Temperature measured via a Foley catheter with a temperature probe.
•Affected by urine temperature, which can fluctuate with diuresis or fluid administration.
8. Tympanic Membrane
• Closely reflects core temperature.
•Risk: Potential tympanic membrane perforation if not inserted properly.
9. Tracheal Temperature
• Measured using a probe in the endotracheal tube.
•Limitations:
•1-minute lag time before temperature readings stabilize.
• Affected by fresh gas flow from the anesthesia machine.
Thermoregulation in Humans
Thermoregulation in Humans
Most Effective Thermoregulatory Responses
1.Behavioral Responses (e.g., seeking warmth, putting on clothes)
2.Vasoconstriction (reduces heat loss)
3.Shivering (generates heat after vasoconstriction fails)
Efferent Responses to Temperature Changes
Efferent Responses to Temperature Changes
1.Basal Metabolic Rate (BMR) – Heat production from normal body metabolism.
2.Sympathetic Activity – Increases heart rate and vasoconstriction.
3.Hormonal Activity – Involves thyroid hormones and catecholamines.
4.Muscular Activity – Includes shivering and non-shivering thermogenesis.
5.Vascular Tone – Adjusts blood flow to regulate heat loss or conservation.
Inter-Threshold Range
Inter-Threshold Range
• The range within which no autonomic thermoregulatory responses occur.
•Normal Range: ±0.2°C.
•Under General Anesthesia (GA): The range widens to ±2-4°C, causing blunted thermoregulatory responses.
Thermoregulatory Thresholds
•36.5°C: Vasoconstriction threshold.
•36.0°C: Shivering threshold.
Maintained by:
• Neurotransmitters such as noradrenaline, dopamine, serotonin, acetylcholine, and prostaglandins.
Thermoregulation in Regional Anesthesia (RA)
Thermoregulation in Regional Anesthesia (RA)
•Vasodilation occurs → Leads to core temperature drop and skin temperature increase.
•Behavioral regulation impaired as the patient doesn’t feel cold.
•Direct impairment of the thermoregulatory center.
Phases of Hypothermia in Anesthesia
Phase I (Redistribution Phase)
•Occurs in the first hour of anesthesia.
•Rapid temperature drop (0.5°C-1.5°C) due to peripheral vasodilation.
• Heat moves from the core to the periphery.
Phase II (Linear Phase)
•Occurs over the next 2 hours.
•Gradual heat loss via:
•Radiation (major mechanism).
•Convection, conduction, and evaporation.
Phase III (Plateau Phase)
•Occurs after 3-4 hours.
•Vasoconstriction sets in (except in RA, where it remains impaired).
• Temperature stabilizes.
Shivering in Anesthesia
Definition
•Involuntary fasciculations of the face, jaw, and head, or muscle hyperactivity lasting ≥15 seconds.
Risk Factors
• Younger males.
•General Anesthesia (GA) > Regional Anesthesia (RA).
• Use of certain anesthetic agents (Propofol > Thiopentone).
Mechanism
• Occurs due to cooling of the pre-optic region of the hypothalamus.
• Mediated by K-opioid, NMDA, and 5-HT receptors.
Characteristics
•Frequency: ~200 Hz.
•Slow cycles (4-8 per minute).
•Tonic patterns (seen in hypothermia).
•Clonic patterns (seen during recovery from volatile anesthetics).
Effects of Hypothermia & Shivering
CNS Effects
•Altered mental status.
•Decreased MAC (Minimum Alveolar Concentration).
•Decreased CMRO₂ (Cerebral Metabolic Rate of Oxygen).
•Increased ICP (Intracranial Pressure) and IOP (Intraocular Pressure).
Cardiovascular Effects
•Increased Nor-Adrenaline & Adrenaline release → Vasoconstriction.
•Decreased cardiac output (CO).
•Arrhythmias (<28°C).
Coagulation Effects
•Platelet sequestration.
•Reduced platelet function and clotting factor activity.
Other Effects
•Interference with anesthesia monitoring (SpO₂, ECG, and IBP become unreliable).
•Decreased drug metabolism.
Prevention of Hypothermia
1.Maintain Operating Theatre (OT) temperature at 21-23°C.
2.Pre-warm patients for 30 minutes before regional anesthesia.
3.Administer Ketamine (0.5 mg/kg IV) before GA/RA.
4.Use Ondansetron (8 mg IV) before induction.
5.Pre-warm IV fluids and irrigation fluids.
Treatment of Hypothermia
1. Rewarming Methods
•Active Rewarming:
• Circulating water mattresses.
•Forced air warmers (40-43°C).
•Passive Rewarming:
• Maintain OT temperature (21-23°C).
•Use cotton blankets.
2. Humidification
•Heat Moisture Exchanger (HME) in the respiratory circuit.
3. Supplemental Oxygen
• Helps compensate for increased oxygen consumption due to shivering.
4. Pharmacological Management
Drugs for Shivering
1.Opioids (μ & κ receptor activation)
•Meperidine (0.35-0.4 mg/kg IV) → Most effective.
• Tramadol, Butorphanol, Fentanyl, Alfentanil.
2.5-HT Agonists
• Ondansetron.
3.Alpha-2 Agonists
• Clonidine (2.5 µg/kg).
• Dexmedetomidine (0.5 µg/kg/hour).
4.Anti-Cholinergics
• Physostigmine (0.04 mg/kg).
• Chlorpromazine (10-25 mg IV).
5.NMDA Antagonists
• Ketamine (0.5 mg/kg).
6.Miscellaneous
• Propofol (10-70 µg/kg/min infusion for refractory shivering).
