Using Critical Care Ultrasonography to Diagnose the Etiology of Acute Respiratory Failure

By Eric C. Walter, MD, MSc

Pulmonary and Critical Care Medicine, Northwest Permanente and Kaiser Sunnyside Medical Center, Portland, OR

Dr. Walter reports no financial relationships relevant to this field of study.

SYNOPSIS: This single-center study suggests that thoracic ultrasound and limited echocardiography may be useful to help differentiate the causes of acute hypoxic respiratory failure in the ICU.

SOURCE: Sekiguchi H, et al. Critical care ultrasonography differentiates ARDS, pulmonary edema, and other causes in the early course of acute hypoxic respiratory failure. Chest 2015 May 21[Epub ahead of print].

Acute hypoxic respiratory failure is common in the ICU, yet determining the etiology can be challenging. Thoracic ultrasound (US) and limited echocardiography (echo) are increasingly being used in the ICU. This prospective study evaluated the effectiveness of thoracic US and limited echo in determining the etiology of acute hypoxic respiratory failure.

This was a single-center study performed at an academic teaching hospital. Adults with acute hypoxic respiratory failure (defined as arterial partial pressure of oxygen to fraction of inspired oxygen ratio [PaO₂/FiO₂] < 300) were eligible (241 screened and 134 enrolled). Thoracic US and limited echo were performed within 6 hours of the diagnosis of acute respiratory failure. The exam was limited to 10 minutes. Thoracic US included five lung zones bilaterally (second and fourth intercostal spaces at the midclavicular and midaxillary lines and at the diaphragmatic arch). Limited echo included the subcostal 4-chamber, parasternal long- and short-axis, and apical 4-chamber views. Images were saved for later interpretation by board-certified radiologists and cardiologists blinded to the clinical data. Medical records were reviewed by two investigators blinded to the US and echo findings. Acute hypoxic respiratory failure was classified by these reviewers into one of three groups: 1) miscellaneous causes (unilateral pneumonia, pulmonary embolism, chronic obstructive pulmonary disease [COPD], mucous plugging), 2) cardiogenic pulmonary edema (CPE), or 3) acute respiratory distress syndrome (ARDS). After chart review, 25% were diagnosed with miscellaneous causes, 44% with CPE, and 31% with ARDS. The authors then used complex statistical modeling to associate imaging findings and cut-points that could predict these diagnoses. Based on these findings, an algorithm was suggested.

The first step is to evaluate for B-lines with thoracic ultrasound. B-lines, or lung comets, are hyperechoic or isoechoic vertical lines arising from the pleural line and spreading down to the edge of the screen. B-lines have previously been found to be associated with extravascular lung water.¹ B-lines in less than three lung zones suggested a miscellaneous cause, whereas B-lines in three or more lung zones suggested either ARDS or CPE. The presence of a left pleural effusion > 20 mm moderately or severely decreased left ventricular function, and a large minimal inferior vena cava diameter (> 23 mm) suggested CPE over ARDS.

COMMENTARY

Ultrasound is a test with distinct advantages. It uses no radiation, results are immediately available at the bedside, and testing can be repeated as the clinical situation changes. Sekiguchi et al add to the growing literature supporting US in the ICU, but this study should be considered more as a proof of concept study than definitive evidence. The authors recorded many different US measurements in an attempt to identify markers that could predict certain diagnoses. The use of so many measures increases the probability of type 1 error (finding an association when it truly does not exist). Furthermore, other questions need to be answered before US can be recommended as a useful tool in the evaluation of acute hypoxic respiratory failure. Is US better than other tests commonly used, such as chest x-ray or basic natriuretic peptide? For clinicians already using these studies, as well as the history and physical exam, will US add appreciable information beyond what is already used? Will US potentially be better than clinical gestalt?

Perhaps the biggest disadvantage to critical care US is the technical skills required to both obtain and interpret images. In this study, it was not clear if the images were obtained by intensivists or certified ultrasonographers. Images were interpreted by board-certified cardiologists and radiologists. In this proof of concept study, it made sense to use experts to obtain and review images. However, this limits the generalizability of the study. Future studies should see if intensivists at the bedside can demonstrate the same results. Critical care US is in its infancy but promises to be a valuable new tool. Too frequently, new tools are adopted in the ICU before being critically evaluated. Studies such as this are welcome evaluations. Over time, they will help us understand how and when US can best be used.

REFERENCE

1. Enghard P, et al. Simplified lung ultrasound protocol shows excellent prediction of extravascular lung water in ventilated intensive care patients. Crit Care 2015;19:36.