An audible alarm for monitoring air embolism during neurosurgery

The sensitivity of various monitors during experimental pulmonary air embolization was measured in five dogs. Changes in vital signs, electrocardiogram, Doppler sound, and end tidal CO2 were observed. An audible alarm was incorporated in the end tidal CO2 monitor. The results showed that a 0.01 cc to 0.02cc/kg dose of air triggered the Doppler consistently, but the changes in heart sounds were heard only transiently. The fall in end tical CO2 occurred at 0.5 to 1.0 cc/kg of intravenously injected air and was confirmed by pen write-out. The audible alarm device was constructed to sound after four consecutive breaths with preset percentage fall in end tidal CO2. The changes in vital signs occurred at doses of 4 to 8 cc/kg of air and death occurred at 8 to 10 cc/kg. The sound alarms was used in 30 patients during sitting craniotomies. Eight patients developed air emboli initially detected by transient change in Doppler sound. The alarm called attention to the fall in end tidal CO2 and confirmed the occurence of air emboli.