Anesthesiology APPLIED Exam › Neuroanesthesia and neuromuscular

Venous air embolism and the sitting position

Neuroanesthesia and neuromuscular·~5 min read· Physician-reviewed✓ Sourced · 2

Stem

A 31-year-old woman, 58 kg, is having a pineal region tumor resected through a supracerebellar approach in the semi-sitting position, under propofol and remifentanil for motor evoked potentials. A transthoracic bubble study showed no shunt. A precordial Doppler and a multi-orifice central venous catheter are in place. As the surgeon opens the dura near the torcula, the Doppler tone turns harsh and roaring, end-tidal CO2 falls from 36 to 23 mmHg, and blood pressure drops from 112/68 to 86/52.

Question

What is happening, and what do you do?

Ideal answer

HeadlineA sudden fall in end-tidal CO2 with hypotension, the site above the heart, and the Doppler confirms air: venous air embolism. I tell the surgeon at once.

Flood the field and cover the wound, because only the surgeon can stop air entry.
No nitrous oxide, 100% oxygen,¹ and jugular compression to raise venous pressure at the site so the open vein can be found.
Aspirate the multi-orifice central catheter, without letting it delay resuscitation; fluids, vasopressors and epinephrine for the pressure.¹ Head down if safe; no PEEP, which can push air across a foramen ovale.
If unstable: flat or left lateral head-down, compressions, because chest compressions break up the air lock in the right ventricle.
Once stable, I watch for recurrence on end-tidal CO2 and the Doppler, send a blood gas, and examine her for paradoxical embolism after the case.

Breaking down the answer

The decision before the case — would you agree to the sitting position, monitor for the risk you accepted, and act when the capnograph drops: judgment and adaptability.
Who sits — the surgeon gains exposure and less bleeding; I accept it only with no right-to-left shunt, so a murmur or any history prompts an echo with bubble study.
Shunts and PEEP — a patent foramen ovale is a relative contraindication, because venous air becomes arterial air to the brain and heart. PEEP raises right atrial above left atrial pressure and promotes exactly that, and the evidence that it prevents air entry is weak.
Monitors ranked — TEE is most sensitive but needs an operator; precordial Doppler at the left parasternal border is the most sensitive continuous noninvasive monitor, about 0.25 mL of air;¹ end-tidal CO2 says it is physiologically significant. End-tidal nitrogen rises early if measured.
Multi-orifice catheter — tip at the junction of the superior vena cava and right atrium, confirmed by intravascular ECG or chest radiograph; a pulmonary artery catheter aspirates poorly. Evidence that aspiration improves outcome is weak.¹
Why no nitrous oxide — nitrous enters air 34-fold faster than nitrogen leaves, so a small embolus becomes dangerous.
Jugular compression and position — compression is temporary and raises intracranial pressure. The Durant position, left lateral head-down, moves air away from the right ventricular outflow tract but is rarely practical with pins; flat is achievable.

Follow-up question

Despite these measures, the pressure falls to 58/30 and end-tidal CO2 is 11 mmHg. Then the arterial trace loses its pulsatility.

What now?

Follow-up answer

HeadlineA massive embolus with right ventricular failure or an air lock; I call it out loud, with the steps in order.

Field flooded, nitrous off, 100% oxygen, jugular compression, catheter aspirated, table flat, epinephrine in escalating doses, and chest compressions now, because the arterial trace shows no pulse. Resuscitation sitting is difficult, so we close the wound temporarily and reposition.¹ I ask for TEE to look for air in the right heart and air crossing the septum. Once she recovers, we decide whether to continue. Afterward I look for paradoxical embolism: a new neurologic deficit prompts a stroke alert and CT,² or ischemic ECG changes; I consider hyperbaric oxygen for neurologic or end-organ injury, ideally within 6 hours.¹

Pitfalls that cost points

Saying "I would never allow the sitting position"; the examiner wants the risk-benefit discussion.
Waiting for hypotension: a 2 mmHg end-tidal fall with a changed Doppler tone is already the trigger.
Missing that air enters in any procedure with the wound above the heart: shoulder arthroscopy, laparoscopy, cesarean section, central line placement.

Rapid-fire

Q: Why does end-tidal CO2 fall? A: Air obstructs pulmonary arterioles; ventilated alveoli lose perfusion, dead space rises and the exhaled CO2 falls while PaCO2 rises.

Q: What volume of air is lethal? A: Estimates span 50 to 500 mL; rapid entry kills at smaller volumes than slow entrainment;¹ across a patent foramen ovale or in a child, a few milliliters reaching coronary or cerebral vessels can be catastrophic.

Q: Why is the sitting position risky even without air? A: Venous pooling drops cardiac output and cerebral perfusion pressure; I zero the arterial pressure at the external auditory meatus and keep the patient full.

Q: What study do you want before a sitting case? A: An echo with a bubble study, because a right-to-left shunt such as a patent foramen ovale turns venous air into arterial air.

Numbers to know

Item	Value
Precordial Doppler sensitivity	~0.25 mL air
Lethal volume, adult	~50–500 mL, depends on rate¹
Catheter tip position	SVC–right atrial junction
Transducer zero in the sitting position	External auditory meatus
Nitrous oxide	Avoid in any at-risk case
VAE incidence, sitting craniotomy	15–76% by TEE¹

Sources

1.Venous Air Embolism · OpenAnesthesia (IARS) · openanesthesia.org
2.Delayed Emergence · OpenAnesthesia (IARS) · openanesthesia.orgCited for postoperative neurologic workup: stroke alert, CT, neurology consult.

Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.