Anesthesiology APPLIED Exam › Thoracic and respiratory

One-lung ventilation and hypoxemia

T1Thoracic and respiratory·950 words · ~5 min read·v0.1✓ Sourced · 2DRAFT

Why examiners ask this

Desaturation during one-lung ventilation is the classic intraoperative curveball in a thoracic stem: it tests whether you can run a differential in order, act on the most dangerous cause first, and explain the physiology of shunt and hypoxic pulmonary vasoconstriction while you do it. Examiners commonly ask exactly this, then push the saturation lower. It stresses adaptability and organization more than knowledge.

The 60-second answer

A falling saturation on one-lung ventilation is tube malposition until proven otherwise. I increase the inspired oxygen to 100%, tell the surgeon, and look at the tube with the fiberoptic bronchoscope and suction the lumens while checking the circuit, the capnograph and the airway pressures.¹ If the tube is correctly placed, I treat shunt: a recruitment maneuver and 5 cm H2O of PEEP to the ventilated lung, then low-level CPAP of about 3–8 cm H2O with oxygen to the non-ventilated lung if the surgical field allows it.² If the saturation keeps falling or is below about 85–88%, I ask the surgeon to pause and reinflate the operative lung intermittently, and if the patient remains unstable I ask them to clamp the pulmonary artery or go to two-lung ventilation. Throughout I keep cardiac output up, because low mixed venous saturation makes any shunt worse.

Decision points and the "why"

Tube position first — the double-lumen tube migrates with positioning and surgical retraction; an unrecognized herniated bronchial cuff or a tube that has slipped out of the bronchus is the commonest reversible cause, and no amount of PEEP fixes it.
FiO2 to 100% — oxygen 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 to the ventilated lung, 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.
CPAP to the non-ventilated lung — highly effective because it oxygenates blood flowing through that lung without ventilation; it partially inflates the lung and impairs VATS exposure, so I warn the surgeon and use it 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
Protect the ventilated lung — tidal volume 4–6 mL/kg ideal body weight, PEEP 5–10 cm H2O, plateau pressure under 30 cm H2O and lower if possible, brief permissive hypercapnia;¹ I avoid high volumes that cause post-pneumonectomy lung injury.

The complication they will inject

The saturation is now 80% and the surgeon says the pulmonary vein is about to be ligated. I tell the surgeon plainly that I need the lung back and ask them to either reinflate or finish the vessel in seconds. I hand-ventilate the ventilated lung, confirm with the bronchoscope that nothing has moved, and if there is any doubt about the tube I pull it back into the trachea and ventilate both lungs through it. I look for 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. If two-lung ventilation does not restore the saturation, the problem is not the lung isolation and I move to circulation.

Pitfalls that cost points

Listing eight maneuvers without an order; the examiner wants the first thing you would do and why.
Adding PEEP before checking the tube.
Applying CPAP without mentioning that it partially inflates the lung in the surgeon's field.
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-why

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 is compressed by the mediastinum and abdomen and is ventilated less; the non-dependent lung is ventilated more but perfused less, and with the chest open it is 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 the PaO2 on 100% oxygen falls from over 400 to about 100–150 mmHg, and significant hypoxemia now occurs in under 5% of cases.²

Numbers to know

Item	Value
Tidal volume during OLV	4–6 mL/kg ideal body weight¹
Plateau pressure	<30 cm H2O, lower if possible¹
PEEP, ventilated lung	5–10 cm H2O, titrated¹
CPAP, non-ventilated lung	~3–8 cm H2O with oxygen²
Right upper lobe bronchus take-off	~1.5–2 cm below carina¹

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.