Perioperative anesthetic management for neurosurgical operations in a lounging, sitting or semi-sitting position

In neurosurgery, positioning the patient in a lounging or (semi-)sitting position is used for surgical treatment of processes in the area of the posterior cranial fossa, as this can offer a number of advantages compared to other forms of positioning, e.g., good drainage of blood and cerebrospinal fluid as well as potentially better preservation of cranial nerve function. At the same time, this positioning requires vigilant perioperative anesthesiological monitoring. In addition to ensuring adequate cerebral blood flow, it is crucially important to recognize and treat a venous air embolism (VAE). The underlying mechanism of VAE is due to the elevated surgical area in relation to the heart and the resulting hydrostatic pressure difference between an open vein and the heart. If the incoming air enters the pulmonary arterial vascular bed, the effects are primarily equivalent to a pulmonary artery embolism and can lead to right heart failure and the need for resuscitation. It should be emphasized that the effects of a VAE are not primarily depending on the total volume of air entering the vasculature, but rather on the volume entering per time. Especially patients presenting with a persistent foramen ovale (PFO) are at high risk during operations in a (semi-)sitting position. In the case of VAE, this can lead to a direct passage of air bubbles from the right heart to the left heart, leading to cerebral and coronary vascular embolism with subsequent stroke or myocardial infarction. Therefore, there is a need for anesthesiologists to recognize and assess both a PFO before the start of positioning and an intraoperative VAE, as well as to treat this in a targeted manner in communication with the surgeon. Using transesophageal echocardiography (TEE), VAE can be directly visualized and objectively graded according e.g., to the “Tuebingen Venous Air Embolism Grading Scale”. Depending on the severity of the VAE, various measures must be taken: information of the surgeon, avoidance of further air entry, treatment of the hemodynamic depression, evaluation of the grade of VAE and, if necessary, aspiration of the entered air or the so-called “air lock”. Here, the authors discuss advantages and disadvantages, implementation, as well as special risks of this neurosurgical positioning, physiological changes caused by the sitting position itself, hemodynamic monitoring of the patient and intraoperative ventilation. A special focus is on pathophysiology, incidence, and TEE diagnosis of VAE in lounging or (semi-)sitting position, including discussion of the question of a PFO.