56: Ultrasound of the Lung

INTRODUCTION

Ultrasound was first used to detect a pneumothorax in a horse in 1986, and then described in humans shortly afterward.¹ Lung ultrasonography was pioneered by Dr. Daniel A. Lichtenstein, a French intensivist, in 1993.² In a series of innovative articles, he defined the scope, application, and terminology of lung ultrasonography in current use.³

The need for lung ultrasonography was born from the inherent limitations of the chest radiograph, which has been the standard initial diagnostic imaging test for the past several decades. As a static imaging modality, chest radiographic findings are often delayed compared to a patient's clinical picture (sometimes as long as 24 hours). Also, it is often nondiagnostic when performed on the supine patient, as it often shows nonspecific patterns. Furthermore, it is time- and labor-intensive.

In contrast, ultrasonography of the lung can be rapidly performed at the bedside, can be repeated serially to gauge treatment without exposing the patient to unnecessary radiation, and is cost-effective with many rapid goal-directed applications.

Lung ultrasonography has been shown to be superior to supine portable chest radiographs and similar in yield to chest computed tomography (CT) for the detection of a normal aeration pattern, pneumothorax, pleural effusion, interstitial syndrome, and alveolar syndrome.⁴

BASICS

A 3.5- to 5-MHz curvilinear transducer is often preferred when performing most ultrasonography of the lung, such as evaluating for interstitial disease and pleural effusions. However, a higher-frequency 5- to 10-Mhz linear transducer can be helpful for focusing on superficial structures such as the pleural line, as in the evaluation for a pneumothorax.

In the intensive care unit, the patient is typically positioned supine at 45 degrees, with the upper extremities abducted, or in lateral decubitus if undergoing a complete examination. The transducer orientation marker is placed on the left of the ultrasound screen. The probe is held in a longitudinal position, with the indicator facing cephalad, and oriented such that rib shadows lay at either edge of the screen. The transducer is moved freely over the thorax following sequential scan lines, named stages. Stage 1 is defined by the anterior chest wall; Stage 2 includes the lateral wall, from anterior to posterior axillary line; Stage 3 involves the external part of the posterior wall; and Stage 4 adds the internal part of the posterior wall and the apex.³

Ribs are calcified and project an anechoic shadow, as sound waves cannot penetrate through bone. The only exception to this finding is found at the costochondral joints, where ribs become cartilaginous and allow sound to penetrate through the rib.

Because of the opposition of the parietal and visceral pleura, the pleural line typically appears as a single, bright white line comprised between rib shadows (Figure 56-1). However, the parietal pleura can be separated from the visceral pleura ...