Sitting Position as a Venous air embolism risk factor
Venous air embolism (VAE) is a known neurosurgical complication classically and most frequently occurring in patients undergoing posterior cranial fossa or cervical spine surgery in a sitting position or semi-sitting position.
Greater negative pressures occur in the sitting position due to the extreme elevation of the head, but venous air embolism can occur in any operation with the head elevated higher than the heart.
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 1).
The risk of venous air embolism (VAE) is the major deterrent for surgeons and anesthesiologists, despite the fact that sitting position and semisitting positions are commonly used in these operations.
To demonstrate a reduction in the risk of VAE and tension pneumocephalus throughout the operation period while taking advantage of the semisitting position.
In a study, 11 patients with various diagnoses were operated on the Department of Neurosurgery, Ondokuz Mayis University, School of Medicine, Samsun, Turkey using the supracerebellar approach in the dynamic lateral semisitting position. They used end-tidal carbon dioxide and arterial blood pressure monitoring to detect venous air embolism.
None of the patients had clinically significant VAE in this study. No tension pneumocephalus or major complications were observed. All the patients were extubated safely after surgery.
The ideal position, with which to apply the supracerebellar approach, is still a challenge. In the study, Durmuş et al. presented an alternative position that has the advantages of sitting and semisitting positions with a lower risk of venous air embolism 2).
Awad et al., present a gravity-dependent supine (GDS) positioning to avoid certain drawbacks of sitting, lateral, and prone positions in 2 illustrative cases.
The first patient underwent surgical resection of a right cerebellar arteriovenous malformation that drained superiorly with the draining vein adjacent to the tentorium after a ventricular/subarachnoid hemorrhage. The second patient underwent surgical resection of a brainstem cavernous malformation in the left pontomesencephalic region with the GDS supracerebellar approach.
Postoperative imaging demonstrated complete resection in both patients. There were no perioperative complications related to positioning or the surgical resections postoperatively, with an uneventful hospital course in both cases.
The GDS lateral SCIT approach allows natural cerebellar relaxation via gravity without the need for lumbar drainage and is a novel, straightforward operative technique with inherent advantages over the prone, lateral decubitus, and sitting positions 3).
The pure endoscopic SCIT approach could enable safe and effective resection of pineal region tumors, even for relatively large lesions. The endoscope could provide a panoramic view and illumination of the deep-seated structures. Compared with the sitting position, this modified ergonomic position could be implemented easily 4).
There is an ongoing debate about the sitting position (SP) in neurosurgical patients. The SP provides a number of advantages as well as severe complications such as commonly concerning venous air embolism (VAE). The best monitoring system for the detection of VAE is still controversial.
In a retrospective analysis Günther et al compared 208 patients. Transesophageal echocardiography (TEE) or transthoracic Doppler (TTD) were used as monitoring devices to detect VAE; 101 cases were monitored with TEE and 107 with TTD.
The overall incidence of VAE was 23% (TTD: 10%; TEE: 37%), but the incidence of clinically relevant VAE (drop in end-tidal carbon dioxide above 3 mmHg) was higher in the TTD group (9 out of 17 VAE, 53%) compared to the TEE group (19 out of 62 VAE, 31%). None of the patients with recorded VAE had clinically significant sequelae.
In this small sample they found more VAE events in the TEE group, but the incidence of clinically relevant VAE was rare and comparable to other data. There is no consensus in the definition of clinically relevant VAE. 5).