Surgery in the posterior cranial fossa involves lesions of the cerebellum and fourth ventricle. In addition, this position facilitates operations on the foramen magnum and upper cervical spine.
In the past, some surgeons favoured the sitting position because this produced superb venous drainage, relative hypotension and excellent operating conditions. The patients were frequently allowed to breathe a volatile anaesthetic agent (usually trichlo-roethylene) spontaneously so that changes in their respiratory pattern could be used to monitor the progress of fourth ventricular surgery in the region of the respiratory centre. This posed several major anaesthetic problems. Patients in the sitting position are prone to hypotension, which results inevitably in poor cerebral perfusion. Air embolism is also a severe potential problem because, when the skull is opened, many of the veins within the bone are held open and, if the venous pressure at this point is sub-atmospheric, air may enter the veins, leading to systemic air embolism.
For these reasons, the sitting position is no longer used other than in exceptional circumstances and posterior fossa surgery is carried out in the 'park bench' position; operations on the cervical spine are performed with the patient prone and supported on blocks (see above). Although this change has diminished the risks of cerebral hypoperfusion and consequent hypoxia, air embolism is still a potential problem. The operative site, particularly with a moderate head-up tilt, is still above the level of the heart and the veins are still held open by the surrounding structures.
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