During normal respirations, the parietal pleura slides over the visceral pleura. This movement can be visualized with ultrasound. The lack of sliding can be a concerning sign for pneumothorax, although data suggests that such a finding may have a poor positive predictive value in the critically ill population.
So how do we improve our sensitivity and specificity for pneumothorax using ultrasound?
One method is using motion mode (“M mode“) to discern the difference in motion between the visceral and parietal pleural layers. The left side of the following image shows the pleural interface about halfway down the image (white stripe). When I shoot M-mode through (right side), the top half of the image consists of horizontal lines and the bottom half of the image appears grainy – the so called “seashore sign” – which we expect in normal lungs.
Keep in mind that this exam can only tell you what is happening to the lung at that point on the lung field. In other words, we have to scan multiple parts of each lung to feel comfortable there is no overt sign of a pneumothorax.
Drop me a comment below with questions! 🙂
