<s>[INST] context: Bleeding is less when patient is placed in this position. This position is most commonly utilized for posterior fossa surgery and cervical laminectomy. Benefits: This position provides optimal surgical exposure for posterior fossa surgery because tissue retraction and risks of cranial nerve damage are reduced, cerebral venous drainage is improved and bleeding is less. The patient's airway is accessible to the anesthesiologist. Risks: Venous air embolism (VAE), paradoxical air embolism, bradycardia or cardiac arrest due to brain stem manipulations can occur. Macroglossia, upper airway obstruction, pneumocephalus, subdural hematoma, and quadriplegia have also been repoed. Despite well described risks involved with the sitting position, there is no evidence of increased moality rate. Relative contraindications to the sitting position include: open ventriculoatrial shunt, signs of cerebral ischemia when upright and awake, right-to-left shunt as with patent foramen ovale, and cardiac instability. venous air embolism in sitting craniotomy incidence highest 20-40% Hemodynamics and Ventilation. The classic sitting position causes postural hypotension in about 1/3 of patients, and 2-5% of patients suffer severe hypotension (decrease in blood pressure more than by half from baseline). The major hemodynamic consequence is decrease in venous return, leading to decrease in cardiac output and hypotension. Therefore, hemodynamic instability and cardiac disease are relative contraindications for prone positioning. Wrapping of the legs with elastic bandages (e.g. ACE bandage) prevents pooling of blood in the lower extremities and should be applied in every case. Ventilation in sitting position is improved compared to the supine position due to downward shift of the diaphragm, which decreases intraabdominal pressure, improves ventilation of the dependent zones, and decreases ventilation-perfusion mismatch. However, low perfusion pressure secondary to decreased venous return may affect oxygenation. Therefore, preventing hypovolemia and maintaining normal pulmonary perfusion pressure are crucial for maintaining an adequate oxygen delivery in sitting position. Venous Air Embolism (VAE) The mechanisms of VAE include negative venous pressure and exposure of veins and bony venous sinuses to air. When the site of surgery is exposed to air and located above the level of the hea, air may be entrained in the veins and bony venous sinuses, and air may enter the pulmonary circulation. A large VAE may decrease cardiac output by creating an airlock and decreasing left ventricular output. The incidence of VAE in sitting position may approximate 20-50% when precordial Doppler monitoring is used for detection and 76% with transesophageal echocardiography (TEE) is used for detection. Patent foramen ovale should be excluded before every case as it is a source of paradoxical air embolism. Therefore, preoperative "bubble test" in awake patients using TEE or transthoracic echocardiography is advocated by some authors if the sitting position is considered. In addition to standard monitoring, precordial transthoracic Doppler is recommended for early detection of VAE. Although TEE is more sensitive in detecting VAE, precordial Doppler is inexpensive, readily available, easy to use, and noninvasive. Optimal placement of the precordial probe should be guided by the recognizing the highest pitch over the right upper sternal border with the intravenous injection of agitated saline. When precordial Doppler or TEE are unavailable, VAE should be considered when end tidal CO2 suddenly decreases in the presence of hypotension, not explained by other causes. An atrial catheter (multiorifice or single orifice) placed at the high level of the right atrium may be helpful for air aspiration. Correct positioning may be verified using intravenous electrocardiography, chest radiography, or TEE. However, the therapeutic value of the right atrial catheter may be limited. The most impoant treatment for VAE include irrigation of the surgical site with saline, rescue head-down tilt or left lateral positioning, and cardiovascular suppo with administration of inotropes. Other Complications The incidence of postoperative pneumocephalus in sitting position may reach 100% , and may be due to negative cerebral spinal fluid pressure and/or residual air during closure of the dura. Therefore, nitrous oxide should be discontinued 20-30 minutes before completion of the procedure. However, pneumocephalus can develop even without the use of nitrous oxide and may persist for weeks after surgery. Life-threatening tension pneumocephalus is rare (3%). Quadriplegia is a rare but devastating complication and results from cervical spine ischemia with neck and head hyperflexion. Elderly patients with cervical spine deformities and vascular pathologies have higher risk. During positioning, sufficient distance between chin and neck (at least 2 finger-breadth) is recommended to avoid neck hyperflexion. [/INST] question: Which is not a complication in surgery in sitting position? | options: A: Venous air embolism B: Hemorrhage C: Pneumocephalus D: Tongue edema | answer: B </s>