Anesthesiology APPLIED Exam › Thoracic and respiratory

One-lung ventilation and hypoxemia

Thoracic and respiratory·~5 min read· Physician-reviewed✓ Sourced · 2

Stem

A 67-year-old man, 82 kg, is having a video-assisted thoracoscopic right lower lobectomy in the left lateral position through a left double-lumen tube placed and confirmed with a bronchoscope before turning. Twenty-five minutes into one-lung ventilation on an FiO2 of 0.5, his SpO2 drifts from 97% to 87%. Peak airway pressure has risen from 24 to 31 cmH2O, and blood pressure is 112/68.

Question

His saturation is falling. What are you going to do?

Ideal answer

HeadlineTube malposition until proven otherwise, and the rising peak pressure fits. I go to 100% oxygen, tell the surgeon, and look with the bronchoscope.

Bronchoscope down the lumens, suction, reposition under vision, while I check the circuit, the capnograph and the airway pressures.¹
Tube right: recruit, 5 cmH2O PEEP to ventilated lung, because what remains is shunt.
Then CPAP with oxygen to the non-ventilated lung, about 3–8 cmH2O if the surgical field allows, warning the surgeon that it will partly inflate the lung.²
Below about 85–88% or still falling: intermittent reinflation, with the surgeon pausing; if unstable, two-lung ventilation, or early ligation of the lobar pulmonary artery branch, which removes that lobe's share of the shunt.
Cardiac output up, tidal volumes small, throughout, because low mixed venous saturation worsens any shunt, and the ventilated lung needs protecting.

Breaking down the answer

Tube position first — the double-lumen tube migrates with positioning and surgical retraction; an unrecognized herniated bronchial cuff or a tube slipped out of the bronchus is the commonest reversible cause; no amount of PEEP fixes it. Most dangerous first is graded (organization, adaptability).
Why 100% oxygen — it cannot reach shunted blood, but it maximizes the content of the blood that is ventilated and buys time; the small absorption-atelectasis cost is irrelevant in an emergency.
PEEP, cautiously — it recruits the dependent lung, but too much raises its vascular resistance and diverts blood to the non-ventilated lung, worsening shunt. I start at 5 and reassess.
Why CPAP works — highly effective because it oxygenates blood flowing through that lung without ventilation; it impairs VATS exposure, so it comes after the dependent-lung measures have failed.¹^,2
Intermittent two-lung ventilation — the fastest way to restore oxygenation;² the cost is surgical interruption, so it is for a saturation that will not respond, a hemodynamically unstable patient, or an SpO2 in the low 80s.
Pulmonary artery clamp — for pneumonectomy, surgically reducing pulmonary artery flow eliminates the shunt and is the surgeon's definitive contribution;² in a lobectomy the equivalent is early lobar artery branch ligation, not a main pulmonary artery clamp.
Protect the ventilated lung — tidal volume 4–6 mL/kg ideal body weight, PEEP 5–10 cmH2O, plateau pressure under 30 cmH2O and lower if possible, brief permissive hypercapnia;¹ high volumes cause post-pneumonectomy lung injury.

Follow-up question

The tube looks correctly placed, CPAP has not helped, and the saturation is now 80%. The surgeon says the pulmonary vein is about to be ligated and asks you to hold on. What do you do?

Follow-up answer

HeadlineI tell the surgeon plainly that I need the lung back: let me reinflate, or finish the vessel in seconds.

I hand-ventilate and confirm with the bronchoscope that nothing has moved; if there is any doubt about the tube, I pull it back into the trachea and ventilate both lungs through it. Then a second problem: bronchospasm, a kinked lumen with secretions or blood, a pneumothorax of the dependent lung, or a drop in cardiac output from bleeding or compression of the heart. I reassess saturation, capnograph and blood pressure after each step. If two-lung ventilation does not restore the saturation, the problem is not the lung isolation, and I move to the circulation.

Pitfalls that cost points

Listing eight maneuvers without an order; the examiner wants the first thing you would do and why.
Saying "I would call for help" as the only action.
Explaining hypoxic pulmonary vasoconstriction correctly but then giving large volumes to the dependent lung.

Rapid-fire

Q: Why is a right-sided double-lumen tube harder to place? A: The right upper lobe bronchus leaves about 1.5–2 cm below the carina,¹ so the tube's side port must align with it; a left-sided tube is the default for most cases.

Q: How does volatile anesthetic affect hypoxic pulmonary vasoconstriction? A: Modern agents at 1 MAC blunt it modestly; the effect is small compared with surgical and positional factors,¹ so I do not switch to TIVA for this reason alone.

Q: Why does lateral decubitus with an open chest worsen V/Q mismatch under anesthesia? A: The dependent lung is better perfused but compressed by the mediastinum and abdomen and ventilated less; the non-dependent lung is ventilated more, perfused less, and with the chest open not ventilated at all.

Q: What is the expected shunt fraction on one-lung ventilation? A: Hypoxic pulmonary vasoconstriction cuts the non-ventilated lung's share of cardiac output from about 50% to about 25%, so PaO2 on 100% oxygen falls from over 400 to about 100–150 mmHg.²

Numbers to know

Item	Value
Tidal volume during OLV	4–6 mL/kg ideal body weight¹
Plateau pressure	<30 cmH2O, lower if possible¹
PEEP, ventilated lung	5–10 cmH2O, titrated¹
CPAP, non-ventilated lung	~3–8 cmH2O with oxygen²
Right upper lobe bronchus take-off	~1.5–2 cm below carina¹
Significant hypoxemia during OLV	<5% of cases²

Sources

1.Lung Isolation Anesthesia · StatPearls · statpearls.com
2.One-Lung Ventilation (CME article) · Anästhesiologie & Intensivmedizin (DGAI), 2020 · ai-online.infoLoop T. Hypoxaemia incidence <5%; CPAP 3–8 cmH2O; non-ventilated lung perfusion ~50% → ~25% with HPV.

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