The Pleural Line

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Hôpital Ambroise Paré Service de Réanimation Médicale, Boulogne (Paris-West University), France

The previous chapters detailed the first three of the seven main principles of lung ultrasound, just evoking the four last. Now it is time to take the probe. Chapter 1 showed how we hold it and how we don’t. The probe is perpendicular to the anterior chest wall and tries to stay perpendicular at the PLAPS-point.

Principle N°4 tells that in LUCI, all signs come from the pleural line. This is an apparently easy statement, but the pleural line must be carefully defined, in all circumstances, especially in agitated, dyspneic, bariatric patients, subcutaneous emphysema, and shaky environments. In bariatric patients who are agitated because of a severe pneumothorax associated with subcutaneous emphysema, all this in an airborne mission, the rules of LUCI should minimize the difficulties.

Any BLUE-protocol must begin by a correct recognition of the pleural line. We do not use transversal scans. This would make lung ultrasound more difficult, since slight movements (of physician or patient) would deeply change the image acquisition (see Fig. 1.2).

Our 5 MHz microconvex probe is perfect for this part of lung investigation.

The Pleural Line: The Basis

General Remarks

The thorax is built by the ribs and lungs. A longitudinal scan in adults makes an alternance of the rib surface on roughly 2 cm, the lung surface on roughly 2 cm, the rib on 2 cm, etc.

The rib is recognized easily: arciform hyperechoic structure and then acoustic shadow.

Between the top of 2 ribs, one can draw a “rib line.”

The lung surface, i.e., the visceral pleural, is normally against the parietal pleural, and both make the pleural line in normal subjects. This is the line visible less than a cm below the rib line in standard adults. This distance is roughly 1/2 cm anteriorly, a little more posteriorly. At any age including neonates, the pleural line is located at roughly 1/4–1/3 of the distance between the two rib borders.

The pleural line appears as a hyperechoic, roughly horizontal line (when the probe is correctly applied, tangential), in actual fact slightly bended because of intrinsic distorsion of the image (visible as well with sectorial as linear probes). The pleural line should be visible in any circumstance, apart from huge surgical emphysema (Fig. 8.1).

Fig. 8.1

The bat sign. The right vertical scale is centimetric. The ribs (cm 1) are recognized by their arciform shape with frank posterior acoustic shadow. A horizontal line below the rib line (1/2 cm in the adult) is highlighted (1.75 cm). This is the pleural line, which basically indicates the parietal pleura (and usually the visceral pleura). The upper rib, pleural line, and lower rib shape a kind of bat flying facing us, hence the bat sign, a basic landmark in lung ultrasonography. We made this figure without arrow, for keeping it preserved (see Figs. 9.1 and 10.1, for more details)

The pleural line indicates the interface between the soft tissues (fluid-rich) of the wall and the lung tissue (gas-rich), i.e., the lung-wall interface. It shows the parietal pleura in all cases and the visceral pleura, i.e., the lung surface, only when there is no pneumothorax (nor pulmonectomy). The pleural cavity is normally virtual. The pleural line makes the parietal and visceral pleuras one line. With our 5 MHz probe, we do not distinguish the two layers, which is not a problem.

Pleural Line and the Bat Sign

The pattern created by the upper rib (left wing), pleural line (belly), and lower rib (right wing) has been labeled the bat sign, the basic first step in any lung ultrasound. It allows to precisely locate the lung surface using a stable landmark. Using longitudinal scans, the pleural line is always under control, even in hard conditions.

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