Introduction

Sitting craniotomies are performed for surgical access to the posterior fossa. This position is advantageous for several reasons. Surgically, the positioning allows for improved access to deep structures within the posterior fossa. The positioning of the head relative to the heart in the sitting position allows for decreased intracranial pressure (ICP) and improved drainage of blood throughout the procedure. However, there is an additional increased risk of venous air embolism (VAE) due to the combination of the operating site being above the level of the heart, surgery near non-collapsable venous sinuses, and decreased venous sinus pressure. There are several other important risks to consider when using the sitting position during a sitting craniotomy including pneumocephalus, macroglossia, quadriplegia, and peripheral neuropathy.^1,2

 

Case Description

A 57-year-old female with a history of a carotid body tumor status post resection and a newly diagnosed pineal gland cyst presented to the hospital for a suboccipital craniotomy for tumor resection. Preoperative magnetic resonance imaging (MRI) showed a cystic structure in the region of the pineal gland with mass effect and flattening of the tectal plate which was increased in size from prior. The patient had headaches and right eye visual disturbances but was otherwise neurologically intact.

Preoperatively, the patient had a transthoracic echocardiogram that showed normal left and right ventricular function with no patent foramen ovale (PFO) detected. All other preoperative laboratory data was within normal limits.

The patient was brought to the operating room and connected to standard monitors. General anesthesia was induced uneventfully and maintained with propofol and remifentanil, and mean arterial pressures was maintained greater than 65mmHg with phenylephrine. Invasive lines and monitors were then inserted, including a radial arterial catheter, a right internal jugular multi-orifice central venous sheath with a 7 French balloon catheter, precordial doppler, and transesophageal echocardiography (TEE) probe. The balloon catheter was confirmed to be in the right atrium with TEE. Prior to beginning the procedure, the absence of a PFO was once again confirmed using TEE. The patient was moved to the sitting position and all pressure points were properly padded. Careful attention was paid to neck flexion to ensure that there were at least two fingerbreadths of space between the chin and the chest wall to avoid obstruction of venous drainage. The arterial line was zeroed at the level of the tragus. The TEE probe was left in place.

Approximately one and a half hours into the operation, air was detected on precordial doppler. This was confirmed by visualizing a significant amount of air in the right atrium, a large air pocket in right ventricle, and air in the main pulmonary artery (mPA) on TEE. This was immediately followed by hemodynamic collapse (decreased end-tidal carbon dioxide value, decreasing mean arterial pressures, decreasing oxygen saturation).  The surgeon was immediately notified and additional help was called for. The patient was placed into Trendelenburg position and the surgical field copiously irrigated. Dilute epinephrine was administered and air was aspirated from RA catheter. Additionally, the catheter was floated into the mPA and additional air was aspirated. Ongoing resuscitation included placing the patient on 100% FiO2, administration of a fluid bolus to increase CVP, initiation of an epinephrine infusion, and administration of phenylephrine and dilute epinephrine boluses as needed to maintain a mean arterial pressure greater than 65. On further TEE assessment, air was also visualized on the left side of the heart, concerning for possible right to left shunting through an initially functionally closed PFO in the setting of a large air/pressure burden versus transpulmonary passage of air. The amount of intracardiac air improved after 10 minutes and the surgery was resumed, however, the patient remained on significant pressor support for the remainder of the case. The patient was extubated with a nasal airway in place. At time of emergence, she was awake and following some simple commands though lethargic.  She was continued on an epinephrine infusion at 2 mcg/min and transferred directly to the CT scanner then to the neurocritical care unit.

Post-operatively, the patient’s head CT scan was notable for significant, diffuse pneumocephalus, left greater than right, with 5 mm left to right midline shift. This was improved on interval CT 6 hours post-op. MRI on post-op day 1 was notable for post-surgical changes, scattered infarcts in the bilateral frontal and parietal lobes and the right occipital lobe that were noted to be possibly related to air emboli, and redemonstrated pneumocephalus. The patient’s exam at that time was notable for dysarthria and 2/5 left upper extremity strength that improved over the next several days of her recovery. She also developed pulmonary edema requiring placement on non-rebreather, improved with IV Lasix. She was discharged home on post-op day 5. At her 1 month follow-up appointment, she noted equilibrium and gait issues (“foot scuffing when walking”), dizziness, head throbbing, left upper extremity numbness and intermittent tingling in her right foot.

Discussion

1. What specific studies are important for the preoperative evaluation of a sitting craniotomy?
2. Which monitors and invasive lines are commonly used during a sitting craniotomy?
3. What are specific challenges to the case?
4. What are the signs and symptoms of venous air embolism in a sitting craniotomy?
5. What are important steps to take in the treatment of an acute venous air embolism?

Preoperative evaluation

The preoperative evaluation is particularly important in patients undergoing a sitting craniotomy. A thorough history and physical should be performed to gain an understanding of the patient’s baseline neurologic status and any significant comorbidities. Preoperative labs should include a standard complete blood count, chemistry panel, liver function tests, coagulation studies and type and screen. Additional preoperative testing should include a transesophageal echocardiography to evaluate for a PFO, which is a relative contraindication given the significant risk of venous air embolism.³ Patients with a PFO are at increased risk of stroke and other end organ infarction from paradoxical VAE. Other relative contraindications to the sitting position are pre-existing pulmonary hypertension and right heart failure as small VAE in these patients could be catastrophic.

Monitors

Monitoring for a sitting craniotomy should include standard American Society of Anesthesiologists recommended monitors in addition to an arterial line for close hemodynamic monitoring and frequent blood draws. The arterial pressure should be leveled at the tragus to more accurately estimate cerebral perfusion pressure (CPP). End-tidal carbon dioxide monitoring is important for early detection of VAE by an acute decrease in value. Intraoperative transesophageal echocardiography is the most sensitive monitor for detecting venous air entrainment which can be directly visualized in the right atrium, right ventricle, and pulmonary artery, but comes with the risk of esophageal injury.⁴  A precordial doppler is a non-invasive monitor that is also very sensitive for detection of air embolus when placed over the right parasternal region.⁵ Esophageal stethoscopes can also be used; a ‘mill-wheel’ murmur is the classic description and pathognomonic for a venous air embolism. A vascular sheath with multi-orifices placed in the right atrium can allow for aspiration of air in the event of an embolus and can also be used for central venous pressure (CVP) monitoring. Maintaining increased CVP can decrease the risk of VAE.⁶

Challenges

Careful positioning of a sitting craniotomy is very important to minimize injury, including the risk of peripheral nerve injury and obstruction of venous and lymphatic drainage that can occur secondary to neck hyperflexion. All pressure points should be properly padded and the degree of neck flexion should be monitored to ensure there is at least two fingerbreadths between the chin and the chest.⁷  Care should be taken to ensure there is no overt compression of the tongue and airway from the endotracheal tube, bite blocks, and/or the transesophageal echo probe.⁸  In the sitting position, mean arterial pressure and cerebral perfusion pressure are likely to be lower and therefore it is more likely that initiation of a vasopressor infusion will be required to avoid ischemic injury.

Detection and treatment of venous air embolism

Use of a pre-cordial doppler allows for early detection of intracardiac air. Change in sound of the pre-cordial doppler or auscultation of a ‘mill-wheel’ murmur with an esophageal stethoscope is concerning for VAE. Even more sensitive is the use of TEE, allowing for detection of even small amounts of air in the right atrium. Additional signs include decreased end tidal carbon dioxide, decreased oxygen saturation and decreasing blood pressure, concerning for impending cardiovascular collapse. Treatment includes immediately flooding the surgical field with saline and sealing off any potential points of air entry. If possible, lowering the operating field below the level of the heart can help to prevent further entrainment of air. A right atrial catheter can be used to therapeutically aspirate air from the right side of the heart; optimal positioning of the catheter can be confirmed on TEE or under ECG-guidance. Increasing CVP with fluid administration may also be helpful in preventing further air entrainment by decreasing the pressure gradient. Supportive therapy should be immediately implemented, including increasing the fraction of inspired oxygen to 100% and initiating vasopressor infusions (epinephrine) to support blood pressure. In the event of circulatory collapse, chest compressions can help to break up air lock and force air out of the pulmonary outflow tract.⁹ Standard advanced cardiac life support algorithms should be followed in the event of cardiac arrest. Extracorporeal membrane oxygenation can be used if standard resuscitation is unsuccessful.¹⁰

Conclusion

Patients undergoing sitting craniotomies are at risk of several complications, one of which is venous air embolism.  A thorough preoperative evaluation is critical in determining suitability for the sitting position, and an anesthetic plan should be devised with use of appropriate monitors and invasive lines to aid in rapid detection and treatment of venous air embolism.

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