Negative pressure
A negative value of a pressure variable.
Air embolism is a potentially fatal complication of any operation when an opening to air occurs in a non-collapsible vein (e.g. diploic vein or a dural sinus) when there is a negative pressure in the vein (e.g. when the head is elevated above the heart) 1).
Air is entrained in the vein and can become trapped in the right atrium of the heart, which may impair venous return, causing hypotension. May also produce cardiac arrhythmias. Paradoxical air embolism can occur in the presence of a patent foramen ovale 2) or pulmonary AV fistula, and may produce ischemic cerebral infarction.
Greater negative pressures occur in the sitting position due to the extreme elevation of the head, but AE can occur in any operation with the head elevated higher than the heart. Incidence: a wide range has been quoted in the literature, and depends on the monitoring method used: ≈ 7–25% incidence with the sitting position using Doppler monitoring is an estimate 3).