Hypoxaemia (low SpO₂)
Oedema, PE or shunt — the saline bubble study and its timing separate intracardiac from intrapulmonary shunts.
Echo approach, step by step
Low oxygen saturation comes from V/Q mismatch, shunt, hypoventilation or diffusion limitation. Echo identifies the cardiac and pulmonary vascular causes, and the agitated saline study localises right-to-left shunts [176].
- LV and filling pressurePulmonary oedema (with B-lines) [2,202].
- RV and PA pressurePE, PH [8,124].
- Atrial septumASD, PFO, atrial septal aneurysm [175].
- Agitated saline at rest and with ValsalvaBubbles in the LA within 3 cardiac cycles → intracardiac shunt. Bubbles appearing after 3 cycles, via the pulmonary veins → intrapulmonary shunt (hepatopulmonary syndrome, pulmonary AVM) [176,204].
- Upright studyIn platypnoea–orthodeoxia, repeat the saline study sitting up [175].
- EisenmengerLarge VSD, ASD or PDA with bidirectional or right-to-left flow and systemic PA pressure [25].
- LV and filling pressure
- RV, PASP
- IAS morphology
- Saline study: timing of bubbles
- Upright study (if positional)
- Eisenmenger physiology
Causes & echo clues
| Cause | Echo clue | Next |
|---|---|---|
| Cardiogenic pulmonary oedema | Low EF or high filling pressure; B-lines | Treat HF [12] |
| Pulmonary embolism | RV dilatation, McConnell | CTPA [124] |
| PFO with raised RA pressure (PE, RV infarction, high PEEP) | Early bubbles across the IAS | Treat the cause; closure selected [175,176] |
| Hepatopulmonary syndrome | Late bubbles (after ≥3 cycles) with liver disease | A–a gradient, liver team [204] |
| Pulmonary AVM | Late bubbles; hereditary telangiectasia | CT chest [176] |
| Eisenmenger syndrome | Large shunt, reversed flow, systemic PA pressure | ACHD centre [25] |
| Lung disease | Normal heart, or RV strain | CT, spirometry |
Clinical pathway to the diagnosis
- Arterial blood gasMeasure PaO₂, the A–a gradient and PaCO₂. A normal A–a gradient means hypoventilation.
- Response to 100% oxygenPoor correction suggests true shunt.
- Chest imagingX-ray, lung ultrasound, CT.
- Echo with salineLocalise any shunt [176].
- TOE or CTDefine septal anatomy, sinus venosus defects and pulmonary vessels [175].
Clinical pearls & pitfalls
Practical tipVentilated patient with refractory hypoxaemia and a PFO: raising PEEP increases RA pressure and the right-to-left shunt. Echo at the bedside shows it [152,176].
- Inadequate Valsalva gives a false-negative bubble study. The septum must bow into the LA on release [176].
- Pulse oximetry is inaccurate with poor perfusion, dark skin pigmentation and carboxyhaemoglobin. Confirm with a blood gas.
Red flags
- Hypoxaemia refractory to oxygen → shunt [176]
- Hypoxaemia worse when upright (platypnoea–orthodeoxia) → PFO or hepatopulmonary syndrome [175,204]
- Hypoxaemia with a dilated RV → PE [124]
- Clubbing and cyanosis → Eisenmenger physiology or pulmonary AVM [25]