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INTRAOPERATIVE HYPOTHERMIA IN NEONATES (10 Marks)

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Definition

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Why Neonates Are Prone to Hypothermia - Physiological Basis

1. High surface area-to-body weight ratio - greater relative surface for heat dissipation
2. Thin skin - increased evaporative losses (particularly premature neonates)
3. Lack of shivering thermogenesis - cannot generate heat by shivering
4. Limited subcutaneous fat - poor insulation
5. Inability to vasoconstrice effectively - immature autonomic responses
6. Dependence on non-shivering thermogenesis (NST) - relies on brown adipose tissue (BAT) breakdown; premature infants have significantly less brown fat
7. Anaesthetic agents blunt thermoregulatory responses - vasodilation, inhibition of NST, and impaired hypothalamic control

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Mechanisms of Heat Loss in the OR

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Anaesthetic Implications

1. Cardiovascular Effects

• Bradycardia (primary cause of cardiac output reduction in neonates - rate dependent)
• Decreased myocardial contractility
• Ventricular arrhythmias at temperatures <28°C
• Increased blood viscosity - risk of thrombosis

2. Respiratory Effects

• Respiratory depression, apnoea
• Shift of oxygen-dissociation curve to LEFT (Hb holds O2 tighter, impaired tissue O2 delivery)
• Increased pulmonary vascular resistance (PVR) - may trigger return to fetal circulation (persistent pulmonary hypertension of newborn - PPHN)

3. Metabolic Effects

• Metabolic acidosis (lactic acidosis from poor perfusion)
• Hypoglycaemia - neonates have limited glycogen stores, and hypothermia increases glucose consumption
• Coagulopathy - impaired clotting factor function, platelet dysfunction
• Increased oxygen consumption (initially, as body tries to compensate)

4. Pharmacological Implications

• Prolonged drug action: hypothermia reduces hepatic metabolism and renal elimination
• MAC (minimum alveolar concentration) decreases ~5% per 1°C drop in temperature - risk of overdose of volatile agents
• Neuromuscular blocking agents have prolonged duration
• Opioids have prolonged effect - post-op respiratory depression risk

5. Other Effects

• Impaired wound healing and increased surgical site infection risk
• Delayed awakening and prolonged recovery
• Postoperative shivering (in older neonates/infants) - increases O2 demand
• Hypoventilation and apnoea in premature neonates

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Management of Intraoperative Hypothermia

A. Prevention (most important)

• Increase operating room temperature to 26-28°C before bringing neonate in
• Warm all IV fluids and blood products to body temperature
• Warm and humidify anaesthetic gases (heated humidifier circuits)
• Minimize time of exposure - limit uncovered periods

• Forced-air warming blankets (Bair Hugger) - most effective
• Radiant overhead warmers during induction and positioning
• Warming mattress/underbody heating pads (avoid direct contact burns)
• Chemical warming wraps for limbs and head (head = large surface area in neonates)
• Cover non-surgical areas with warm cotton wool, plastic wrap, or aluminium foil

• Warm irrigation fluids during open abdominal/thoracic procedures
• Minimize evaporative loss from open body cavities

B. Monitoring

• Continuous core temperature monitoring is mandatory
  • Nasopharyngeal/oesophageal probe - gold standard for core temperature
  • Rectal thermistor (lags behind core ~15 min)
  • Axillary temperature (less accurate)
• Monitor blood glucose, ABG, electrolytes intraoperatively

C. Treatment of Established Hypothermia

• Passive rewarming - insulating blankets, covering all exposed areas
• Active external rewarming - forced warm air, warm water mattress
• Active internal rewarming - warmed IV fluids, warmed humidified gases
• For severe hypothermia: warmed irrigation (peritoneal, pleural) or ECMO in extreme cases
• Correct metabolic derangements: glucose, bicarbonate (cautiously), Ca²⁺
• Treat coagulopathy with FFP, platelets, cryoprecipitate as needed

D. Target Temperature

• Maintain core temperature 36.5-37.5°C throughout surgery
• Exception: deliberate mild hypothermia may be induced for certain cardiac/neurosurgical procedures (neuroprotection)

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PRONE POSITION DURING SURGERY (10 Marks)

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Introduction

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Physiological Effects

Cardiovascular

• Decreased cardiac output - compression of the inferior vena cava (IVC) reduces venous return; degree depends on adequacy of chest roll support
• Properly supported prone position (chest rolls/Wilson frame) - IVC compression minimised
• Unsupported prone position - IVC compression causes decreased venous return, hypotension
• Increased abdominal pressure when abdomen not free - raises epidural venous pressure, worsens surgical field bleeding
• Heart rate and BP may decrease on turning prone, especially in hypovolaemic patients

Respiratory

• Improved ventilation-perfusion (V/Q) matching - with free abdomen, diaphragmatic excursion is improved
• Functional residual capacity (FRC) increases compared to supine - less atelectasis
• Compliance may decrease initially if abdomen compressed; improves if abdomen is free
• In ARDS patients: recruitment of dorsal alveoli (used therapeutically)
• Peak airway pressures may rise if abdomen restricted

Central Nervous System

• Increased intraocular pressure (IOP) - due to venous congestion and Trendelenburg component
• Increased intracranial pressure (ICP) - venous drainage from head impaired
• Cerebral perfusion pressure may be compromised in prolonged procedures

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Advantages of Prone Position

1. Excellent surgical access to posterior spine, posterior fossa, posterior soft tissues
2. Improved V/Q ratio in spontaneously breathing patients - reduces atelectasis
3. Diaphragm moves caudally when abdomen is free - better diaphragmatic excursion
4. Used therapeutically in ARDS to improve oxygenation (dorsal alveolar recruitment)
5. Better access for posterior approaches to hip, knee, ankle
6. Useful for rectal and perineal surgery (lithotomy variant)

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Disadvantages of Prone Position

1. Difficult airway access once prone - re-intubation is challenging
2. ETT kinking/displacement - must be secured very well before turning
3. IV line/arterial line access is impaired
4. Risk of accidental extubation during turning
5. Cardiovascular instability during turning - transient hypotension, arrhythmias
6. Difficult CPR if cardiac arrest occurs while prone
7. Prolonged setup time - positioning requires a team
8. Restricted chest compliance if not properly supported

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Complications and Nerve Injuries

Pressure-Related Complications

Ocular Complications

• Postoperative Vision Loss (POVL) - most devastating; incidence ~0.017% in spine surgery
  • Ischaemic Optic Neuropathy (ION) - most common cause of POVL in prone spinal surgery
    • Posterior ION: due to venous congestion, anaemia, hypotension, raised IOP
    • Risk factors: prolonged duration, high blood loss, anaemia, hypotension, prone Trendelenburg
  • Central Retinal Artery Occlusion (CRAO) - from direct globe pressure
  • Cortical blindness - from posterior cerebral artery ischaemia (Barash Clinical Anaesthesia, 9e)

Nerve Injuries

Airway/ETT Complications

• ETT dislodgement/kinking - facial and neck oedema from prolonged head-down prone position
• Post-extubation airway oedema - especially with prolonged (>6 h) procedures
• Delayed extubation may be warranted; consider flexion/extension neck check before extubation

Vascular Complications

• Venous thrombosis (stasis)
• Air embolism (posterior fossa surgery - head elevated)

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Anaesthesia Considerations

Preoperative

• Airway assessment is paramount - the secured airway is inaccessible once prone
• Discuss with surgeon: estimated duration, blood loss, need for neuromonitoring (SSEP, MEP)
• Eye assessment in patients with pre-existing glaucoma or visual impairment (high POVL risk)
• Adequate IV access (at least 2 large-bore IVs) and arterial line placement before turning

Induction and Intubation

• Intubate in supine position; use reinforced/armoured ETT to prevent kinking in prone position
• Secure ETT meticulously with adhesive tape + tie - risk of dislodgement during turning
• Use Total IV Anaesthesia (TIVA) if intraoperative neurophysiological monitoring (IOM) required (volatile agents suppress MEP/SSEP signals)
• Supraglottic airway devices (LMA) can be used in selected cases (e.g., short procedures)

Turning the Patient

• Coordinated team turn - minimum 4 persons (anaesthesiologist, surgeon, 2 assistants)
• Protect all lines, tubes, catheters before turning
• Anaesthesiologist guards the airway and head during the turn
• Post-turn: immediately check ETT position (auscultation + ETCO2), check all IV lines, check BP

Positioning

• Chest rolls/Wilson frame/Jackson table - support chest and pelvis, abdomen must hang freely
• Head: neutral position, foam headrest or pinions (Mayfield) for cranial surgery; avoid neck rotation/extreme flexion
• Eyes: must be free of pressure at all times - use padded horseshoe headrest or Mayfield pins
• Arms: tucked at sides OR on arm boards (abducted <90°, avoid hyperextension)
• Elbows: padded to protect ulnar nerve
• Knees: padded; common peroneal nerve at fibular head must be free

Intraoperative Monitoring

• Continuous BP monitoring (arterial line preferred for long/major cases)
• ICP-aware positioning - avoid extreme head-down tilt
• Regularly check and document eye position - avoid globe compression
• Neuromonitoring: SSEP and MEP if spinal cord at risk
• Foley catheter to monitor urine output

Cardiovascular Management

• Anticipate hypotension on turning - have vasopressors ready
• Maintain normovolaemia - hypervolaemia worsens venous congestion; hypovolaemia worsens ION risk
• Target MAP >65 mmHg (or higher as directed by neuromonitoring)
• Judicious use of vasopressors to maintain CPP

Airway Management During Surgery

• Monitor ETCO2 and airway pressures continuously
• Re-evaluate ETT position after position changes (Trendelenburg, lateral tilt)
• In case of accidental extubation: immediately turn supine, re-intubate, stabilize, then re-position

Turning Back to Supine

• Coordinated turn as before
• Extubation in supine unless prolonged oedema risk (delayed extubation in ICU if needed)
• Post-op: check vision on awakening (ask patient to report any visual disturbance)

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Summary Table

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