Preoperative evaluation and preparation

Patient physical status, particularly in reference to cardiovascular and pulmonary stability and airway manageability, is a determinant of the choice of patient position for posterior fossa surgery. The efforts to obtain optimal operating conditions and maintain a stable perioperative course may sometimes be at cross-purposes. For example, patients with previous cerebrospinal fluid shunting procedures may be at greater risk for subdural pneumocephalus with surgery in the head-up position. Thus a thorough evaluation of previous operations and cardiopulmonary problems, current cardiac and respiratory status, evidence of cerebrovascular compromise, and suitability of vascular access for right atrial catheter placement are of particular importance in the patient undergoing posterior fossa surgery.

In patients with altered limits of cerebral autoregulation, impaired cerebral perfusion, or abnormal baroreceptor function resulting from hypertension, cardiovascular disease, cerebrovascular insufficiency, or prior carotid endarterectomy, the occurrence of hypotension during anesthesia in the head-up position may be especially detrimental.

Assessment of vascular access for right atrial catheter placement helps determine the most promising route. Patients who are obese, have poor vasculature due to disease or chronic intravenous cannulation, or have short, thick necks should be identified early so that necessary time may be allotted for catheter placement. Some authorities have advocated echocardiography to detect patent foramen ovale (PFO) in patients scheduled for surgery in the head-up position; the use of an alternative position for those who have PFO might reduce the occurrence of paradoxical air embolism (PAE). ^, A detection rate of 10% to 30% with use of echocardiography is comparable with the 20% to 30% incidence reported in autopsy findings. The noninvasive nature of echocardiography makes it attractive for screening purposes; its specificity is reported to be 64% to 100%. However, preoperative screening echocardiography lacks sensitivity (ie, nondetection of PFO does not guarantee its absence). ^, Transesophageal echocardiography (TEE) is used after induction of anesthesia in some institutions, but it is not 100% sensitive for detection of PFO. More recently, Feigl and associates described their experience in 200 patients scheduled for posterior fossa surgery in the sitting position. After induction of anesthesia, transesophageal echocardiography was performed to check for PFO. Fifty-two patients (26%) had a detectable PFO with a venous air embolism (VAE) rate of 54%. Only one patient had significant clinical manifestations but was without neurologic sequelae.