Venous Air Embolism in Neuro Surgery (VAE)

Rate of occurrence of VAE depand on

1. Procedure
2. Intra-operative Position
3. Method of detection

Associated – principally hazard of post. fossa procedures

Surgery → performed in sitting position and upper cervical spine

• Can occur with supratentorial procedures like facine menigiomas encroaching post. half of sagital sinus, craniosynostosis.
• Pin sites & trapped gas under pressure can also lead.

Common sources of VAE →

• Major cerebral venous sinuses particularly transverse sigmoid & post half of sagital sinus.
• Emissary veins from suboccipital musculature
• Diploic space of skull
• Cervical epidural veins
• Can occur when pressure within an open vein is subatmospheric
• Any position in which wound is above level of heart.
• Physiological consequences depend up on volume and rate of air entry & presence of patent foramen ovale. (present in 25% adults) → paradoxical air embolism.
• N₂O, by increasing volume of entrained air ↑ even small amount of air.

Pathophysiology of VAE →

Small bubbles in venous system
↓
lodges in pulmonary circulation
↓
Diffuse into alveoli & exhaled

Large air bubbles
↓
Pulmonary clearance rate decreses
↓
PAP rises progressively
↓
RV Afterload
↓
↓ed COP
↓
Hypotension, desaturation due to ↑ed alveolar dead space, tachycardia.

Clinical Signs ↓ anaesthesia →

• ↓ ETCO2
• ↓ Sao2
• ABG – ↑ Paco2
• Hemodynamic compromise- hypotension
• Sudden circulatory arrest with large air
• Incompetence of pulmonary & tricuspid valve
• Mill-wheel murmur late.
• Paradoxical air embolism stroke

Coronay occlusion

• When left to Right pressure gradient is reversible
• Reversal of shunt forward by PEEP

Role of central venous catheterisation

• Mandatory in sitting craniotomies & high risk surgery.
• Allows aspiration of air.
• Placement of catheter tip confirmed by radiograph, TEE or intravascular echo.
• Multiorifice catheter → catheter tip 2cm below SVC-Atrial Junction
• Single orifice catheter → 3am above SVC – RA Junction

Monitoring for VAE →

Monitors used for detection of VAE should be

• Sensitive
• Specific
• A rapid response
• Quantitative measurement of VAE
• Indicate course of recovery from VAE

1. Precordial Doppler & CO₂ monitor – standard
2. Most sensitive → TEE & precordial Doppler

• Can detect 0. 25 ml of air bubble
• Doppler probe placed over Right atrium
• Interruption of regular Swishing sound by sporadic roaring sound indicates VAE
• ETW2 & PAP are late and less sensitive but important
• Sudden ↓ in Etco2 due to ↑ pulmonary Dead space.

1. Measurement of N₂ in expired air

Treatment →

• Notify surgeon
• Flood surgical field with saline or pack bone wax applied to skull edges till entry site identified
• Discontinue N₂O, give 100% O₂
• Head low position with Right side up
• Aspiration of air from central venous catheter
• Infuse volume
• start Vasopresor drug.
• B/L jugular venous compression may slow air entrainment & might help surgeon to identify source of embolus
• Wound closure if hemodynamic do not stable
• Resuscitation in supine position if circulatory arrest.