Ultrasound Signs of Pneumothorax
The four key ultrasound signs for diagnosing pneumothorax are: (1) absence of lung sliding, (2) absence of B-lines, (3) absence of lung pulse, and (4) presence of a lung point 1.
Diagnostic Algorithm
The international evidence-based guidelines provide a clear sequential approach 1:
Primary Signs to Rule OUT Pneumothorax:
- Lung sliding present = NO pneumothorax (100% negative predictive value) 1
- B-lines present = NO pneumothorax (proves visceral pleura is opposing parietal pleura) 1
- Lung pulse present = NO pneumothorax (cardiac oscillations transmitted through apposed pleural layers) 1
Signs to Rule IN Pneumothorax:
- Absence of lung sliding (sensitivity 87.2%, specificity 99.4%) 2
- Absence of B-lines
- Absence of lung pulse
- Lung point present = pathognomonic for pneumothorax (100% specificity, but only 60-82% sensitivity) 2, 3
What Each Sign Means
Lung sliding: The rhythmic horizontal movement of visceral pleura against parietal pleura synchronized with respiration. When air separates these layers in pneumothorax, this movement disappears 1.
B-lines: Vertical artifacts originating from the visceral pleura. Their presence definitively excludes pneumothorax at that scanning location because they can only form when the visceral pleura contacts the parietal pleura 1.
Lung pulse: Subtle rhythmic movement of visceral pleura transmitted from cardiac oscillations. Absence indicates pleural separation 1.
Lung point: The specific location on the chest wall where the pneumothorax pattern (absent sliding/B-lines) transitions to normal lung (present sliding). This represents the physical border of the pneumothorax and is 100% specific when found 1, 2.
Emergency Diagnostic Approach
In extreme emergencies, you can diagnose pneumothorax immediately without searching for the lung point if you find: absence of pleural line movement (both horizontal sliding AND vertical pulse) coupled with absence of B-lines 1.
Clinical Performance
Lung ultrasound demonstrates overall sensitivity of 98.6% and specificity of 85.1% for pneumothorax diagnosis 2. This significantly outperforms supine chest radiography for both ruling in (Level B evidence) and ruling out (Level A evidence) pneumothorax 1.
Common Pitfalls
False positives can occur with:
- Lung bullae
- Pulmonary contusions
- Pleural adhesions
- Severe COPD with reduced lung sliding 1
Unusual presentations to recognize:
- Septate pneumothorax: B-lines and lung pulse may still be visible despite pneumothorax due to adhesions 4
- Double lung point: Alternating patterns of sliding/non-sliding appearing at opposite sides of the scan 4
- Hydropneumothorax: Air/fluid border appears as transition between anechoic space and non-sliding A-pattern 4
The technique requires nuanced interpretation when these confounding factors are present, which explains why specificity can be slightly lower than sensitivity in some studies 1.