Prone Position

The prone position is:

• Required to facilitate certain operations
  • Spinal surgery
  • Posterior fossa craniotomy
  • Certain ankle operations
• Associated with many physiological changes
  • Broadly positive respiratory effects
    • Unchanged lung and chest wall compliance
    • ↑ FRC provided abdomen is not compressed
    • Improved V/Q matching via better distribution of pulmonary blood flow
      Leads to ↑ in PaO₂.
  • Broadly negative CVS effects
    These are predominantly due to abdominal and subsequent IVC compression, and can be reduced by ensuring the abdomen is not compressed.
    • Effects include
      • ↓ preload
        Associated with ↓ in CI by ~25%.
      • ↑ venous pressure
        Leads to ↑ bleeding from vertebral wall venous plexuses.
      • ↑ PPV and SVV
    • Effects vary depending on the table being used
      • The Jackson table has the least effect on CVS parameters
      • The Wilson table and Andrew’s support ↓ CI
      • The jack-knife position leads to ↓ CI, HR, and ↑ MAP
  • ↓ CBF
    May occur if the head is rotated due to partial occlusion of the ICA, VA; and compression of venous drainage.

Risks

Risks of prone position include:

• Blindness
  Risk of~1:60,000, or 1:30,000 in spinal surgery. Via two mechanisms:
  • Direct pressure on the globe
    Lead to retinal artery occlusion.
  • Impaired ocular perfusion
    Mechanism of blindness in absence of ocular pressure (e.g. use of Mayfield clamp).
    • \[Ocular \ Perfusion \ Pressure = MAP - IOP\]
      A MAP target >70mmHg is reasonable to preserve ocular perfusion during anaesthesia.
      • IOP will ↑ with:
        • Venous occlusion
        • Rise in CVP
        • Head-down position
    • Key factors:
      • Lengthy (>6 hour) operation
      • Blood loss >1L
      • Atherosclerosis
      • DM
      • HTN
      • Male
    • Notable factors not associated with blindness:
      • Glaucoma
      • Intraoperative blood pressure management
      • Anaemia
      • Hypothermia
      • Anaesthetic technique
  • Corneal abrasion
• Neuropathy
  Via compression or stretching of superficial nerves.
  • Risk is reduced with careful positioning
• Pressure injury
  • Directly, via pressure on:
    • Vocal cords
    • Forehead
    • Nose
    • Chest
    • Arms
    • Superior iliac crests
    • Knees
    • Feet
  • Indirectly, via compression of arterial supply or venous drainage:
    • Intraoral swelling May lead to airway obstruction and delayed extubation, attributed to excessive neck flexion.
    • Hepatic and pancreatic infarction Attributed to visceral ischaemia.
    • Lower limbs
      At risk of compartment syndrome when ‘tucked’; i.e. with hip and knee flexion.

Technique

Can be divided into:

• Assessment
• Preparation
• Induction
• Positioning
• Emergence
• Emergency Management

Preoperative Assessment

• Consent
  Must cover risk of blindness.
• Position required by surgeons
• Length of procedure
• C-spine assessment
  Will complicate airway management.
• Limb mobility
  Will affect intra-operative positioning.
• Peripheral neuropathy
  • Risk factors
  • Severity of pre-existing disease

Preparation

• Avoid the ACF for venous access
• Place ECG electrodes on the back
• Ensure the correct operating table is available

Induction

• Secure the airway
  • Consider use a reinforced ETT with a bite block
    Less likely to become kinked during position changes.
  • Secure the ETT well with tape
    Ties may occlude venous drainage.
  • Recommend against using an LMA
    Difficult to access the airway if airway difficulties remove.
• Protect the eyes
  • Tape shut
  • Place extra protective padding over the lids
    Tape the padding in.

Positioning

• Summon a team of at least 5 trained people, including the surgeon
• Disconnect the ETT
• Slowly turn the patient onto the operating table
• Use a foam head holder to support the patients head
• Reconnect the ETT and complete an intraoperative cross-check
  • A sustained or ↑ a Pip >5cmH₂O should prompt review for endobronchial intubation or bronchospasm
• Assess the pressure areas and position
  • Ensure that the eyes are free of pressure
    Use a mirror to confirm the eyes are not compressed.
  • Ensure spine is not excessively flexed or extended
    Common cause of injury.
  • Ensure abdomen is not compressed
    Will lead to pressure injury, abdominal organ compression with potential intra-abdominal hypertension, and impair respiratory excursion.
  • Ensure that axilla are not under tension
    May lead to brachial plexus palsy.
  • Check other pressure points
  • Place padding liberally on potential pressure areas and bony prominences
  • Ensure lines and cables are not compressed into patient
    e.g. IV/arterial lines, ECG cables, SpO₂ cable, IDC, NGT.

Emergence

• Maintain anaesthesia until the patient is repositioned supine
• Check for a cuff leak prior to extubation

Emergency Management

Accidental Extubation:

• Should be anticipated and prepared for

• Ensure the airway is secure and patent after turning, and prior to removing the bed from the operating theatre

• Ensure the ETT and breathing circuit is secured and not hanging freely

• LMA is the first-choice for airway rescue
  87.5% first-pass and 100% second-pass success.

  • Once placed, a decision can be made to continue the case using the LMA or to replace with an ETT
    The LMA can be used as a conduit to replace the ETT over a fibrescope in the prone position.

• Fibreoptic intubation in the prone position can also be performed

  • Anatomically favourable as the tongue falls forwards
  • Requires the head to be accessible

Cardiac arrest:

• Chest compressions can be performed
  Place hands over each scapula.
• Defibrillation be performed in the prone position
  Pads can be placed posterolaterally under each axilla.
• If in Mayfield clamp
  Remove mayfield clamp for cardiac compressions and CPR.

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References

1. Edgcombe H, Carter K, Yarrow S. Anaesthesia in the prone position. Br J Anaesth. 2008 Feb;100(2):165-83.
2. Feix B, Sturgess J. Anaesthesia in the prone position. Continuing Education in Anaesthesia Critical Care & Pain. 2014 Dec. Pages 291–297.