The mainstay of detection is vigilance and a high index of suspicion. The main period of risk is when the posterior cervical muscles are cut and the craniectomy is being performed. Bone is usually removed as a craniectomy in the posterior fossa rather than by raising a bone flap. Although the incidence of major air embolism is vastly lower than when the sitting position was used, small amounts of air still enter the circulation quite frequently. The severity of the problem depends upon the volume of air entrained and the fact that air bubbles expand in the presence of nitrous oxide.
The main practical method of detection is by end-tidal carbon dioxide monitoring, because the airlock produced in the pulmonary circulation results in a rapid reduction in CO2 excretion (usually together with a fall in oxygen saturation). Arterial pressure decreases and cardiac arrhythmias are frequently seen. The use of an oesophageal stethoscope permits auscultation of the classic 'mill-wheel' murmur with large quantities of air, but requires continuous listening. Doppler ultrasonography is probably the most accurate method of early detection, before the embolus leaves the heart, but frequently suffers from interference.
In practice, provided that the sitting position is not used, large air emboli are uncommon. Treatment consists of preventing further entry of air by telling the surgeon, who immediately floods the operative field with saline, lowering the level of the head and increasing the venous pressure by jugular compression to raise intrathoracic pressure. Ideally, the air should be trapped in the right atrium by placing the patient in the left lateral position; it is then occasionally possible to aspirate air through a central venous catheter, which is commonly inserted in posterior fossa explorations. Vasopressors are sometimes required until the circulation is restored; occasionally full cardiopulmonary resuscitation is necessary.
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