Assessment of the pleural effusion
Sonographic assessment of the thorax is one of the most common applications of ultasound in the critically ill patient. It is rapid, simple, and highly accurate in determining the presence of an effusion (vs consolidation) and its characteristics (contents, size, etc). Before this skill can be wielded effectively however, there are some important caveats. In this section, we will review sonographic assessment of the pleural space.
First, the examiner must acknowledge the anatomical planes to best understand assessment of the pleural space. The most common anatomical plane for imaging the pleural space is the mid-axillary line at the level of the diaphragm. With the probe marker pointed cephalad, the imaging plane corresponds to the coronal plane.
In the the patient, the space above the diaphragm may be difficult to appreciate as the lung will "veil" the diaphgram itself. This is often referred to as the "curtain sign"---seen here as the patient inhales the lung expands--as you see the lung coming down. Other typical artifacts seen in this view include a "mirror image" artifact, seen in the far field. This occurs as the diaphragm is a strong reflector. Also notice the screen marker is left, standard acquisition format for non-cardiac images.
In this patient, the probe is applied to the right hemi-thorax in the midaxillary line with the probe marker cephalad. In the ultrasound clip we can see a rather large black (anechoic space) and the typical anatomic boundaries including:
1. Diaphragm and sub-diaphragm viscera
2. The spinal column in the far field
3. Lung (with likely atelectasis/ consolidation)
4. Chest wall (in near field)
An oft-quoted finding is the "spine sign". In general, the spinal column is not normally seen above the level of the diaphragm, as alveolar air causes plenty of scatter and reflection. This is because the acoustic impedance of bodily tissue and air-filled alveoli is dramatically different. In the case of a pleural effusion or dense consolidation, the acoustic impedance of the lung tissue changes now fundamentally changing the propagation of ultrasound waves. This change in acoustic impedance now permits the examiner to see tissue as deep as the spinal column.
Additional anatomic planes
With the probe now rotated 90 degrees with the probe marker pointed up we can see an axial or transverse plane. The appearance of the chest here is more similar to an axial plane on a CT chest in terms of its appearance.
Quantification of a pleural effusion
This brief discussion will cover different techniques in quantification of the pleural effusion in patients with critical illness.
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