Perioperative Temperature Management
🔹 1. Physiology of
Thermoregulation
🧠 Core principles
- Core
temp ~37°C (tight control ±0.2–0.4°C)
- Peripheral
tissue cooler 2–4°C → ทำหน้าที่ buffer
- Defense
mechanisms:
- Cold:
vasoconstriction + shivering
- Heat:
vasodilation + sweating
🔻 2. Effect of Anesthesia
🧪 General anesthesia
- ↓ vasoconstriction threshold
→ ~34.5°C
- ↓ shivering threshold
- ↑ inter-threshold range
(~10x wider)
👉 ผล:
- ผู้ป่วย ไม่ activate defense จน temp
ต่ำมาก
🧪 Neuraxial anesthesia
- ↓ central thermoregulation
- ↓ shivering awareness (คนไข้ไม่รู้สึกหนาว)
- ↓ autonomic response
⚠️ Combined GA + neuraxial
👉 เสี่ยง
hypothermia สูงสุด
🔻 3. Intraoperative
Hypothermia
🔑 เกิดเกือบทุกรายถ้าไม่
warming
📉 3 phases
1.
Redistribution phase (1st hour)
o core
→ peripheral
o ↓ temp เร็ว
(สำคัญที่สุด)
2.
Linear phase
o heat
loss > production
3.
Plateau
o vasoconstriction
activate (~34.5°C)
🔥 Mechanism heat loss
- Radiation
(หลัก)
- Convection
- Evaporation
/ conduction (น้อย)
⚠️ 4. Consequences of Hypothermia
🩸 Coagulopathy
- ↓ platelet function + ↓ coagulation enzymes
- ↑ bleeding / transfusion
🦠 Infection
- ↓ tissue perfusion
- ↓ immune cell function
- ↑ wound infection
💊 Prolong drug effect
- ↑ duration NMBA
- ↑ propofol level
- → Delayed emergence
❤️ Cardiovascular
- ↑ sympathetic → ↑ O2 demand
- risk
ischemia / arrhythmia
❄️ Shivering
- ↑ metabolic rate มาก
- ↑ myocardial stress
🎯 Target temperature
- ≥35.5°C
(minimum goal)
- Optimal
~36–37°C
🔧 5. Prevention &
Management
🔹 Prewarming
- ↑ peripheral temp → ↓ redistribution
- effect
เล็ก (~0.4°C)
🔹 Passive insulation
- ลด heat loss ~30%
- ไม่พอ → still hypothermic
🔹 Active warming
(standard of care)
✔️ Best method
- Forced-air
warming (นิยมที่สุด)
- Resistive
/ circulating water (เทียบเท่า)
👉 ลด:
- shivering
- infection
- transfusion
🔹 Fluid warming
- ใช้เมื่อ >1 L/hr
- ป้องกัน cooling แต่ ไม่ช่วย warming
จริง
🔹 Respiratory gas warming
- ไม่มี clinical impact
🔹 Endovascular warming
- effective
มาก แต่ invasive →
limited use
🔺 6. Hyperthermia
🔥 Causes
- Excess
warming
- Malignant
hyperthermia
- Infection
/ fever
- Drug
reaction (serotonin syndrome, NMS)
⚠️ Consequences
- ↑ metabolic rate
- ↑ myocardial O2 demand
🛠️ Management
- remove
heat source
- antipyretic
(paracetamol)
- treat
underlying cause
🔻 7. Postoperative
Temperature
❄️ Hypothermia
- common
→ ต้อง:
- warm
to ≥36°C
- forced-air
warming
👉 complication:
- delayed
drug clearance
- respiratory
depression
- ischemia
- discomfort
🌡️ Fever
- ≥38°C
- cause:
- infection
- transfusion
reaction
- drug
→ treat:
- paracetamol
- evaluate
cause
❄️ Shivering (important)
- ↑ O2 consumption
- ↑ cardiac stress
Treatment
- Meperidine
12.5–25 mg IV (first line)
- alternative:
- clonidine
- dexmedetomidine
🚨 8. Clinical Pearls (ใช้จริง)
🧭 Checklist PACU/OR
- monitor
temp ทุก case >30 min GA
- target
≥35.5°C
- active
warming = standard
💡 Key insights
- Redistribution
= main cause early hypothermia
- Fluid
warming alone “ไม่พอ”
- Hypothermia
→ delayed emergence
- Shivering
= harmful (not just discomfort)
🔴 Red flags
- Temp
<35°C
- unexplained
tachycardia + rigidity →
think malignant hyperthermia
- persistent
hypothermia →
metabolic / sepsis
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