VENOUS AIR EMBOLUS (VAE)

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-occurs when the pressure within an open vein is subatmospheric

VAE may occur when:
-surgical wound is above the level of the heart
-sitting position increases occurrence approximately 20 – 40%
-increased risk with large cerebral venous sinus and low pressure vein

Physiological consequences of a VAE depends on:
-rate of air entry
-volume of air entry
-presence/absence of probe-patent foramen ovale

Physiological effects of a VAE include:
-air bubbles which enter the venous circulation usually lodge in the pulmonary circulation
-air bubbles entering the pulmonary circulation generally diffuse within the alveoli and are eventually exhaled out

Generally well tolerated by most patients unless:
-the rate of absorption of air bubbles entering the pulmonary circulation exceeds the pulmonary clearance
-resultant progressive rise in pulmonary artery pressure: ↑PAP
-increased RV afterload leads to decreased left ventricle end diastolic volume (preload)
-decreased preload results in decreased cardiac output

Enhanced detrimental effects of VAE may occur with:
-preexisting cardiac disease
-preexisting pulmonary disease
-administration of nitrous oxide: may increase the volume of entrained air

Clinical signs of venous air embolus:
-decrease in ETC02
-decrease in arterial oxygen saturation
-PaC02 may slightly be increased (due to pulmonary dead space)
-hypotension
-hypoxemia
-circulatory collapse with large air entrainment secondary to right ventricular outflow tract obstruction

Paradoxical air embolism:
the passage of an air embolus into arterial circulation through a patent foramen ovale

May result in:
-stroke
-coronary artery occlusion
-probe-patent foramen ovale
-patent foramen ovale especially when normal trans-arterial pressure gradient is reversed (ex. hypovolemia, PEEP)

Monitoring for venous air embolus (VAE)
Central venous cathetization
Transesophageal echocardiogram
Precordial Doppler Sonography
End tidal gas tensions (ETCO2 ETNO)
Pulmonary artery pressure
Blood pressure and heart sounds

Central venous catherization
-may allow for aspiration of entrained air

Correct positioning of the multi-orificed catheter:
-at the junction of the SVC and RA
-allows for optimal recovery or air following VAE
-confirmed by intravenous electrocardiogram or transechocardiogram

High atrial positioning of CVC associated with:
-biphasic P wave on intravenous ECG

CVC advanced too far associated with:
-P wave changes from a negative deflection to a positive deflection
-right ventricular waveform may occur when the pressure is transduced

Most sensitive intraoperative monitor for detecting a venous air embolus:
-transechocardiography (TEE)
-precordial Doppler sonography
-may dectect air bubbles small as 0.25 ml

Transesophgeal echocardfiography of detecting a VAE:
-detecting the amount of air bubbles
-trans-arterial passage of air bubbles
-assess cardiac function

Precordial Doppler sonography for detecting aVAE
-probe over the right atrium
-interruption of regular predicted fluid motion of the Doppler signal may indicate presence of a venous air embolus

Other useful but less sensitive monitors for detecting a VAE:
end tidal respiratory gas tensions:
-sudden decrease in ETC02

Increased pulmonary artery dead space
-sudden increase in expired nitrogen

Pulmonary artery pressure changes:
-increase in direct proportion of the amount of entrained air

Blood pressure and heart sounds:
ex. mill wheel mumor is a late characteristic of a VAE

TREATMENT OF VENOUS AIR EMBOLUS

-notify the surgeon in order to flood the surgical area with saline and pack the skull edges with bone wax
-identify the site of air entry
-nitrous oxide should be discontinued immediately
-deliver an inhalational agent with 100% oxygen
-aspirate the CVC in order to aspirate the entrained air
-attempt to increase the CVP (ex. intravenous fluids, bilateral jugular vein compression, PEEP)
-all mentioned attempts fail, place the patient in head down position and close rapidly close the surgical wound

CVP should be increased
-with intravenous fluid administration

Bilateral jugular vein compression
-additional maneuver to help identify the sound of embolus
-increases central venous pressure
-may slow entrainment
-may cause back pressure and increase back bleeding
-back bleeding may help assist the surgeon in identifying the source of embolus

Hypotension should be treated
-vasopressors