OBJECTIVE The semi-sitting position has numerous advantages in various neurosurgical procedures. Venous air embolism is one of the associated complications of this position. Currently, there are no prospective studies of the relationship between the degree of head elevation and the rate and severity of venous air embolism for patients undergoing procedures in this position. This study compared changes in the severity of venous air embolism according to the degree of head elevation (30 or 45 degrees) in patients undergoing elective cranial neurosurgical procedures in the semi-sitting position. 
METHODS  One hundred patients undergoing an elective infratentorial craniotomy in the semi-sitting position were included, and patients were divided into two groups. In group 1, each patient’s head was elevated 30 degrees; in group 2, head elevation was 45 degrees. Patients were assigned to groups according to the location of their lesion. During surgery, the standard anesthetic protocol was used with total intravenous anesthesia, and transesophageal echocardiography was used to detect air in the blood circulation. Any air embolism seen on the echocardiography screen was classified as grade 0 to 4. If multiple events occurred, the worst graded attack was used for statistical analysis. During hemodynamic changes caused by emboli, fluid and vasopressor requirements were recorded. Surgical and anesthetic complications were also recorded. All results were compared statistically and a p value < 0.05 was considered statistically significant.
RESULTS  There was a statistically significant difference between groups for the total rate of venous air emboli detected on transesophageal echocardiography: 22.0% (n=11) in group 1 and 62.5% (n=30) in group 2 (p < 0.0001). The rate and severity of air embolism were significantly lower in group 1 than in group 2 (p < 0.001). The rate of clinically important venous air embolism (grades 2, 3, and 4: venous air embolism with decreased end-tidal carbon dioxide levels and/or hemodynamic changes) was 8.0% (n=4) in group 1, and 50.0% (n=24) in group 2 (p < 0.0001). There was no association between the rate and severity of venous air embolism with patients’ demographics (p>0.05). There was association, however between the rate of venous air embolism and the type of surgical pathology (p< 0.001). The venous embolism was more frequent in patients with menengiomas. There were no major surgical or anesthetic complications related to the patients’ position during the postoperative period.
CONCLUSION  For patients in the semi-sitting position, an increase in the degree of head elevation is directly related to a higher rate of venous air embolism. With 30 degrees of head elevation and our standardized technique of positioning the semi-sitting position can be utilized safely in neurosurgical practice.

Effect of the degree of head elevation on the incidence and severity of venous air embolism in cranial neurosurgical procedures in the semi-sitting position