Venous Air Embolism in Neuro Surgery (VAE)
Rate of occurrence of VAE depand on
- Intra-operative Position
- 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
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
- 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
- A rapid response
- Quantitative measurement of VAE
- Indicate course of recovery from VAE
- Precordial Doppler & CO2 monitor – standard
- 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.
- Measurement of N2 in expired air
- 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.