FRCA Notes

Sitting Position

Resources

  • The sitting position is used for:
    • Posterior fossa operation
    • Foramen magnum decompression
    • (Rarely) cervical spine operations

  • It improves surgical access to the posterior fossa by:
    • Gravity-assisted drainage of blood & CSF, reducing ICP
    • Improving surgical orientation and access to midline structures
    • Decreases amount of retraction needed to gain access to deeper structures

  • Its use has declined due to risks (especially that of venous air embolism) and use of other positions e.g. park bench
  • Abnormal ventriculo-atrial connections e.g. PFO is an absolute contraindication due to venous air embolism risk
  • Relative contraindications include autonomic dysfunction, severe hypertension or severe COPD

Venous air embolism

  • Incidence is 25 - 75% of sitting surgeries
  • The sitting position should be avoided, if other suitable alternatives are acceptable, owing to the possibility of life-threatening venous air embolism
  • If the sitting position is used, there should be close monitoring for venous air embolus as early detection & treatment reduces morbidity and mortality

  • Mechanism:
    • As surgical site is above the level of the heart there is negative venous pressure at the surgical wound
    • Open veins will entrain atmospheric air into the circulation
    • Typically the venous sinuses or diploic veins, but can occur through head-pin sites

Cardiovascular collapse

  • Venous pooling in the legs can result in significant hypotension
  • In posterior fossa surgery, stimulation of the lower pons, upper medulla, 4th ventricular floor or cranial nerve nuclei can result in variable cardiovascular responses (i.e. tachycardia or bradycardia, hypertension or hypotension)
  • Rapid return to the supine position should occur for resuscitative measures

Pneumocephalus

  • Intra-operative techniques to reduce the volume of the brain encourage air to enter the intracranial space
  • When the cranial cavity is closed and the brain expands, the air is compressed
  • It can cause a mass effect with elevated ICP
  • Presents as:
    • Delayed recovery
    • Neurological deficit
    • Headache
    • Confusion/agitation
    • Seizures
    • Cardiac arrest from brain herniation if tension pneumocephalus

  • Management is conservative, or with high-flow oxygen ± burr hole craniotomy & aspiration of air

Macroglossia

  • Obstructed venous (EJV) and lymphatic drainage of the tongue due to prolonged neck flexion
  • Can cause post-operative respiratory obstruction (especially in children)

Nerve injuries

  • Subgluteal padding to protect the sciatic nerve
  • Pressure on the common peroneal nerve is of particular concern

Quadriplegia

  • A rare but devastating complication of the sitting position
  • Prolonged, focal pressure on the spinal cord from head flexion can occur
  • Regional cord flow may be further compromised by intra-operative hypotension, resulting in ischaemic damage to the cord