Temperature Monitoring

Temperature Monitoring - Why We Sweat It

• Why Monitor? Prevents perioperative hypothermia complications (e.g., coagulopathy, ↑infection risk, delayed recovery, cardiac events).
• Normal Core Temp: 36.5-37.5°C.
• Core vs. Peripheral:
  • Core (e.g., pulmonary artery, distal esophagus, nasopharynx, tympanic membrane): Reflects true body temperature.
  • Peripheral (e.g., skin, axilla): Variable; influenced by environment & vasoconstriction.
• Thermoregulation & Anesthesia:
  • Normal responses: Vasoconstriction, shivering, non-shivering thermogenesis (NST), sweating.
  • Anesthesia: Impairs central regulation, ↓thresholds for vasoconstriction & shivering; abolishes behavioral responses.

  ⭐ Anesthesia abolishes behavioral responses and impairs autonomic thermoregulation, leading to a core-to-peripheral redistribution of heat.

• Mechanisms of Heat Loss (R>Cv>Ev>Cd):
  • Radiation (~60%): To cooler objects not in direct contact.
  • Convection (~15-30%): To moving air currents.
  • Evaporation (~20%): From skin, open wounds, respiratory tract.
  • Conduction (~5%): To cooler surfaces in direct contact (e.g., OR table).

Temperature Monitoring - Probing the Degrees

Core temperature: 36.5-37.5°C. Perioperative hypothermia (< 36°C) is common.

Monitoring Sites:

Device Types:

• Thermistors: Semiconductor; resistance ↓ with ↑ temp. Most common.
• Thermocouples: Voltage at junction of two dissimilar metals; proportional to temp.
• Liquid Crystal Devices: Change color at specific temps. Skin patches.
• Infrared Thermometers: Detect thermal radiation. Tympanic, temporal artery.

Temperature Monitoring - The Big Chill

Perioperative hypothermia: Core temp < 36°C.

⭐ Mild perioperative hypothermia (core temperature 34-36°C) significantly increases risk of surgical site infections, adverse myocardial outcomes, and impaired coagulation.

• Phases (Intraop Hypothermia):
  • Redistribution: Rapid initial ↓ (1st hr)
  • Linear Decline: Heat loss > production
  • Plateau: Vasoconstriction / warming balances
• Risk Factors: Anesthetics, cold OR/fluids, large incisions, long surgery, age extremes, low BMI, ASA >II.
• Adverse Effects:
  • Cardiac: Arrhythmias, ischemia
  • Coagulopathy: Platelet dysfunction, ↓enzyme activity
  • SSI: Impaired immunity, vasoconstriction (↓tissue O2)
  • Delayed drug metabolism
  • Shivering: ↑O2 consumption (400-500%), ↑CO2, ↑ICP/IOP
• Prevention & Management:
  • Pre-warming (30-60 min); Passive insulation (blankets)
  • Active warming: Forced air, fluid warmers, ↑OR temp (>21°C)
  • Monitor core temp (esophagus, nasopharynx, bladder, tympanic)

Temperature Monitoring - Fever Pitch

• Perioperative Hyperthermia: Core temp > 38°C (not due to warming).
• Causes: Malignant Hyperthermia (MH), sepsis, drugs (atropine), thyrotoxicosis, transfusion reactions, NMS, iatrogenic.
  • 
• Malignant Hyperthermia (MH):
  • Genetic; Triggers: Volatiles, succinylcholine.
  • Signs: ↑ETCO2 (early!), tachycardia, muscle rigidity, rhabdomyolysis, ↑temp (late).

  ⭐ Unexplained, persistent increase in end-tidal CO2 (hypercarbia) is often the earliest and most sensitive sign of Malignant Hyperthermia.

  • Rx: Stop triggers, dantrolene 2.5 mg/kg IV, 100% O2, cooling.
• Consequences (severe): CNS dysfunction, multi-organ failure.
• Management: Treat cause, active cooling, support.

High‑Yield Points - ⚡ Biggest Takeaways

• Core temperature monitoring is vital; pulmonary artery (gold standard), distal esophagus, nasopharynx are key sites.
• Peripheral sites (skin, axilla) lag behind core changes, less reliable.
• Hypothermia (<36°C) under anesthesia risks coagulopathy, infection, shivering, delayed recovery.
• Malignant Hyperthermia (MH): early sign is unexplained ↑ETCO2, late sign is rapid ↑temperature.
• Shivering dramatically increases oxygen consumption (↑VO2).
• Radiation is the main mechanism of intraoperative heat loss.
• Thermistors are common in temperature probes.