What am I looking at?

In order to recognize pleural effusions, it is first important to note the following normal anatomic structures and their sonographic appearance at the costophrenic interface.

Lung:

• Cephalad to the diaphragm
• Normally air filled, causing gas scatter: grey homogenous haze obscuring deep structures

Diaphragm:

• Smooth, layered, rounded hyperechoic line, ~5mm thick
• Moves with respiration

Solid Organ (liver/spleen):

• Homogenous echogenic structure
• Caudad to the diaphragm
• Smooth, well demarcated borders

Video 1: Normal anatomy at posterior axillary line

Indications

• All trauma patients in whom a chest x-ray is indicated or a FAST exam is performed
• Clinical suspicion of pleural effusion
• During systematic assessment of lungs with PoCUS in dyspneic patient

Equipment

• Ultrasound machine
• Curvilinear or phased array probe
• Gel

Technique

1. Select the appropriate probe for the patient size
2. Position the patient: sitting preferred over supine
3. Place the probe longitudinally in the posterior axillary line at the level of the xiphoid and fan as posteriorly as possible to identify the vertebral bodies and dependent fluid in the supine patient
4. Center the diaphragm in the screen
5. Identify the diaphragm, liver (or spleen on the left), lung and vertebrae
6. Look for normal artifacts and abnormal findings
7. Repeat on opposite side

Introduction

The most common initial test for respiratory distress is chest x-ray. Recently, growing evidence has shown that PoCUS can reliably detect thoracic pathology with equal if not better sensitivity than chest x-ray (CXR). In the case of pleural effusions, ultrasound is both more sensitive and specific than CXR. It has the advantage of differentiating causes of “white out” on CXR given its ability to characterize effusion and other pathology. It is also useful in guiding diagnostic and therapeutic procedures. Computed tomography, which offers little advantage over US is impractical and comes with the cost of significant radiation.

Why Ultrasound?

Traditionally it was thought that ultrasound was not a useful modality for investigating lung pathology because air scatters ultrasound waves. There has been a growing body of evidence in recent years contradicting this practice. Given that the fluid accumulated in the pleural space transmits ultrasound waves, pleural effusions can easily be detected by ultrasound.

A recent meta-analysis looking at the test characteristics of PoCUS for pleural effusion in trained users found a sensitivity of 93% and specificity of 96% when compared to either CT or fluid on aspiration [1]. This is significantly superior to CXR that has a sensitivity of 20-75% and a specificity of 50-90% [2]. Therefore, ultrasound is therefore the first-line imaging choice in investigating pleural collections [3,4,5,6].

Implementation of lung ultrasound has been shown to decrease the number of chest x-rays and CT scans performed in the ICU. In the emergency department, it has been shown to decrease the length of stay compared to patients who underwent chest radiography [7,8].

PoCUS is safe, timely and effective for the diagnosis of pleural effusion.

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