Ultrasound Estimation of Pleural Effusion in the Sitting Patients

The pleural fluid is the thin film of serous fluid between the visceral and parietal pleurae, whose physiological value is around 20 mL in healthy adults. It plays crucial role in the respiratory mechanics, as allows the pleurae to slide effortlessly against each other during ventilation, and its surface tension leads to close apposition of the lung surfaces with the chest wall. The abnormal collection of fluid in the pleural cavity is defined pleural effusion (PE). The most common causes of PE in adults are congestive heart failure, liver cirrhosis, pneumonia, malignant pleural disease, pulmonary embolism, and gastrointestinal disease. PE represents the 10% of admissions in pulmonary units, and affects about 10 million people each year in industrialized countries. The high incidence demands methods for PE accurate estimation, in order to guide the clinician in the choice of the adequate therapy and its follow up .

Strategies for the estimation of PE can be divided into: i) qualitative methods, which are non-invasive; and ii) quantitative methods, which are invasive. Qualitative methods provide coarse estimation of PE (minimal, small, moderate and massive PE), and usually ultrasonography (US) is devoted for this aim; quantitative approaches give accurate information about PE volume, at the expenses of the invasiveness, because of the need of X-ray, CT imaging and thoracentesis.

The interest in the use of US for the evaluation of chest diseases, especially for the study of bedridden, critically ill patients increased. In fact, US-based methods present various advantages: i) absence of ionizing radiation; ii) noninvasiveness; iii) they can be performed at the bedside; iv) being inexpensive, can be repeated if necessary; v) short examination time when compared with CT-based methods. Moreover, US methods are particularly sensitive in imaging the chest wall, pleura, and pleural space thanks to their superficial locations, and are often used to detect PE and guide thoracentesis and drainage, especially in minimal effusions.

Some authors proposed approaches for the estimation of PE volume by means of US images. The PE is identified like an anechoic area on the US image. The basic idea is to measure characteristic lengths or surfaces, and to correlate them to the volume. Such approaches can be distinguished into linear and planar. In the linear ones, usually one length (e.g., the high of the PE column, or intrapleural distance) measured on US image is correlated to the PE volume. These methods are assessed by comparing their output with the values obtained by the gold standard (CT, or thoracentesis). The pro of these techniques related to the quick evaluation is restrained by the low accuracy of the estimation.

In the planar approaches, volumes are directly calculated by multiplying a specific length (e.g., the height of the PE column) with the effusion areas measured in correspondence of defined anatomical landmarks (e.g., the half height of PE column). For instance, the model proposed by Remérand et al. requires three measurements: 2 for the detection of the PE column extremities, and 1 for the area. The US exam is performed on supine patients, at the end-expiration. Such approach is more time-consuming than the linear one, but allows achieving more accurate results.