Clinical studyVenous air embolism and the sitting position: A case series
Introduction
The sitting position is sometimes used for patients undergoing procedures on the cervical spine and within the posterior cranial fossa. Advantages of the position include excellent surgical access, improved ventilation, and improved drainage of blood and cerebrospinal fluid from the head and neck, leading to decreased intracranial pressure. However, there are a number of potential complications that can be associated with the sitting position, including haemodynamic instability due to venous pooling in the lower limbs, decreased cerebral perfusion pressure secondary to gravitational effects, pneumocephalus, subdural haematoma, quadriplegia, macroglossia, damage to pressure areas such as the brachial plexus and buttocks, and venous air embolism.
Venous air embolism can lead to serious mortality and morbidity including hypotension and cardiovascular collapse (secondary to right heart outflow obstruction and pulmonary hypertension) and systemic paradoxical embolism via intracardiac or intrapulmonary shunts causing stroke and arterial ischaemia. Venous air embolism requires a breach of the venous system and a pressure gradient for air entry. The sitting position increases the risk of venous air embolism because of decreased venous pressure at the surgical site due to gravitational effects. The most common sources of entry are the major cerebral venous sinuses, which are non collapsible due to their dural attachments, but air can also enter through any vein that is open during the operation. This leads to negative venous pressure, which facilitates the entry of atmospheric air.
The reported incidence of venous air embolism during neurosurgery ranges between 7 and 76% in previously published series (Table 1).[1], [2], [3], [4], [5], [6], [7], [8], [9] This wide variation can be explained in part by the different sensitivities and specificities of the various detection techniques. Transoesophageal echocardiography (TOE) has detected venous air embolism in 76% of cases3 and precordial Doppler in up to 50% of cases,4 whereas pulmonary artery pressure, central venous pressure, end tidal carbon dioxide partial pressure (ETCO2), end tidal nitrogen, and arterial blood pressure are less sensitive.5 In the present study, a prospective audit was conducted in 100 consecutive patients whose neurosurgery was performed in the sitting position, under the care of one anaesthetist, and using ETCO2 to detect venous air embolism.
Section snippets
Methods
One hundred consecutive patients who underwent neurosurgery in the sitting position were studied. All patients were anaesthetized by the same specialist anaesthetist between April 1997 and August 2004. The study was approved by the Human Research Ethics Committee of the Royal Melbourne Hospital, but patient consent for the study was not obtained, because the ethics committee considered that this activity was an audit.
Anaesthesia was administered according to the needs of the patient and the
Results
Patient characteristics are summarized in Table 2. The incidence of venous air embolism in this case series was 9% (95% confidence interval: 3.3–14.7%). Venous air embolism was more common in patients undergoing posterior fossa surgery than in those undergoing cervical surgery (5 [15%] vs. 4 [6%]; P = 0.27), although this difference was not statistically significant. There were no statistically significant differences between patients who suffered venous air embolism and those who did not (Table 3
Discussion
The incidence of venous air embolism in this audit was 9% (6% in cervical cases and 15% in cranial cases). This figure is similar to those reported for other series.[1], [2], [3], [4], [5], [6], [7], [8], [9] Venous air embolism was not associated with any other disturbance in physiological variables, including arterial blood pressure and oxygen saturation. Patients suffering venous air embolism in this audit did not have a significantly higher incidence of hypotension during the surgery than
Acknowledgements
We would like to thank the neurosurgeons at the Royal Melbourne Hospital and Melbourne Private Hospital for their cooperation.
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