Pneumothorax on the Supine Radiograph: Where the Air Actually Goes
Supine pleural air collects anteriorly, not at the apex. The deep sulcus sign, the etched mediastinal edge, and where pneumothorax hides in the trauma bay.
Last reviewed 2026-05
A supine AP chest goes up during the primary survey, and the question comes from the foot of the bed before the patient is anywhere near a scanner: any pneumothorax? You look, and both lungs seem expanded, with no pleural line at either apex. That apical check is the one erect films taught you, and on a supine patient it’s looking in the wrong place.
Why the Apical Check Fails#
Pleural air rises to the least dependent part of the chest, wherever that happens to be. Stand the patient up and the least dependent point is the apex, so air collects there and shows you a visceral pleural line in profile, which is the finding every textbook drew for you. Lay the patient flat, and every trauma activation is flat, and the geometry changes. The highest point of the pleural space is now anterior and inferior, along the front of the chest and in the anteromedial and subpulmonic recesses.
So the air slides forward and down, and instead of a cap at the apex it spreads out as a sheet in front of the lung. A sheet of air viewed face-on casts almost no edge, the beam no longer catches the pleural line in tangent, and the direct sign disappears. What’s left are the indirect signs, the places where that anterior air outlines structures that used to blend into the lung around them. On the supine film you’re rarely looking for a line. Mostly you’re looking for what the air outlines.
Because the film hides the sheet so well, a third to a half of pneumothoraces later proven on CT are invisible on the supine trauma radiograph, and the ones that do show up do it mostly through the signs below.
The Signs, and What the Air Is Doing#
| Sign | What the air is doing | Where to look |
|---|---|---|
| Deep sulcus | Pooling in the lateral costophrenic recess, deepening it | Compare the two costophrenic angles |
| Etched cardiac or mediastinal border | Replacing lung against the heart, sharpening the silhouette | Heart borders, especially the apex |
| Hyperlucent upper quadrant | Layering in the subpulmonic recess over liver or spleen | Compare upper abdominal density, side to side |
| Double diaphragm contour | Outlining both the dome and the anterior insertion | Each hemidiaphragm, front to back |
| Sharp pericardial fat tags | Lifting fat pads free of the cardiac border | Cardiophrenic angles |
Two of these do most of the work and are worth spelling out.
The deep sulcus sign. Air pooling in the lateral costophrenic recess pushes it deeper and makes it more lucent (Fig. 1). The affected sulcus plunges caudally, sometimes far down along the flank. The technique is the side-to-side comparison: one sulcus looks normal, and the other is deeper and darker with a sharp edge.
The etched border. Aerated lung sitting against the heart makes a soft, slightly blurred silhouette, because two similar densities meet there. Put air in that gap instead and the interface becomes unusually crisp. So when a heart border or a hemidiaphragm looks better defined than it has any business being, take it seriously, because air may be what’s outlining it.
The mimics
Skin folds are the classic false positive: a fold casts an edge that fades at one end, often runs beyond the ribcage, and has lung markings lateral to it rather than a truly lucent pleural space. The medial scapular border and large bullae can mimic the erect pleural line the same way. If you can follow the edge outside the thorax, it’s a skin fold.
What to Do With a Maybe#
The supine film usually leaves you with a maybe, and the tiebreakers are close at hand.
The definitive answers are nearby
In the bay, the extended FAST already includes anterior pleural views, and absent lung sliding answers the question in seconds. The trauma CT that follows settles it completely, down to pneumothoraces of a few millimeters. While you’re in that scan, the solid organs deserve the same systematic read: see the approach to splenic trauma.
An occult pneumothorax, one visible only on CT, is usually observed rather than drained. The read on the radiograph still matters because decisions sometimes have to be made before the patient gets to the scanner.
Before positive pressure
A small or occult pneumothorax under positive pressure ventilation can convert to a tension pneumothorax, because every delivered breath pumps a little more air through the same one-way defect. If the patient is headed for intubation, the OR, or a helicopter, a suspected pneumothorax changes what the team does next. Say it out loud before the tube goes in.
The Supine Sweep#
- Compare the costophrenic angles. A sulcus that plunges is air until proven otherwise.
- Trace both heart borders and both hemidiaphragms. Unusual clarity along any of them is a finding.
- Compare the upper abdominal quadrants for a lucent side.
- Check the apices anyway. Large pneumothoraces still show up there, and you’ll feel silly explaining the one you skipped.
Then report what you actually know. “No pneumothorax” is a claim the supine film can’t fully back. “No visible pneumothorax on a supine radiograph, CT to follow” is what the film supports, and it tells the anesthesiologist exactly how much weight to put on it.
Notes
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CT-confirmed blunt trauma series put supine radiograph sensitivity for pneumothorax in the 30 to 50 percent range. The exact figure moves with detector and reader, and the series agree on the direction. ↩︎
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Observation has held up in trials even for ventilated patients with small occult pneumothoraces, provided the team knows the air is there and can act if it grows. ↩︎
CT-confirmed blunt trauma series put supine radiograph sensitivity for pneumothorax in the 30 to 50 percent range. The exact figure moves with detector and reader, and the series agree on the direction.
Observation has held up in trials even for ventilated patients with small occult pneumothoraces, provided the team knows the air is there and can act if it grows.