Venous Air Embolism

· Pressure in an open vein is sub-atmospheric leading to introduction of air into venous system

o Paradoxical air embolism – air entered into venous system passes from right to left into arterial circulation and may embolize in cardiac or cerebral circulation; usually intracardiac defect such as ASD or PFO, but can also occur with intrapulmonary shunts or if pulmonary circulation is overwhelmed with quantity of air

• Entry of air into the venous system through tears in cortical veins, dural sinuses, or diploic veins with collection in the right atrium blocking pulmonary outflow tract (“air lock”) leading to circulatory arrest
• If patent foramen ovale present, air can cross to left heart and lead to paradoxical pulmonary infarction and stroke

· Risk factors: operative site 5 cm above right atrium; most commonly sitting neurosurgical procedures

· Incidence:

o Sitting neurosurgical procedures

§ Posterior fossa craniectomy 25-45%

§ Cervical laminectomy 10%

o Prone posterior fossa craniectomy 10-12%

o Lateral craniectomy 8%

o Supine craniotomy 11-12%

· Pathophysiology – morbidity/mortality related to rate of air entry; symptoms at 50 ml (or 1 ml/kg); Lethal dose 300 ml (4-5 ml/kg)

o Bolus injection of air may result in airlock within right heart blocking right ventricular outflow tract and obstructing venous return

o Slow entrainment of air can be tolerated until a critical volume accumulates in the peripheral pulmonary circulation

· Physiological effects

o Increased CVP

o Increased PAP (may initially be decreased)

o Decreased CO

o Decreased systemic blood pressure

o Decreased PO₂, Increased PCO₂ (earliest change is increased end-title CO₂)

o Myocardial and cerebral ischemia

· Monitoring for Venous Air Embolism

o Trans-esophageal echocardiography (most sensitive)

o Precordial Doppler (check position by injecting 5 cc agitated saline through CVP)

o Pulmonary artery catheter (more sensitive than CVP catheter but not as good at aspirating air)

o Capnography with measurement of end expiratory CO₂

o CVP catheter (holes should traverse junction of SVC and right atrium)

o Transcranial Doppler

· Treatment of Venous Air Embolism

o Rx:  Prevention by meticulous attention to all bleeding points is essential

o Flood surgical field with saline, wax bone edges, identify and coagulate all bleeding points

o Attempt to identify source of air entry (consider 15-second compression of jugular veins to raise jugular venous pressure enough so that open vessels will back bleed)

o Administer 100% O₂

o Modify anesthetic – discontinue N₂O (diffuses into bubbles faster than nitrogen diffuses out); N₂O controversial for sitting position, but prospective randomized study showed no change in incidence or hemodynamic changes as long as stopped as soon as venous embolism detected

o Attempt aspiration of air via CVP

o Change patient position (level, left lateral decubitus)

o Cardiovascular support, hydration

o PEEP NOT recommended due to adverse hemodynamic effects in sitting position and opening of right-to-left shunt by increasing CVP and right atrial pressure