What is the role of thoracic ultrasound in Hypervolemia?
Formerly, ultrasound of the lung was thought to be valueless apart from the easy identification of pleural effusions. The lung is composed of numerous air-filled alveoli that represent multiple air-fluid interfaces. These interfaces reflect sound waves and generate reverberation artifacts that impair accurate tomographic visualization of normal lung parenchyma. These artifacts are termed A-lines and can be visualized as serial, equally spaced reflections of the pleural line.
In patients with kidney disease or heart failure, fluid accumulates in the interlobular septa separating alveoli and the A-line pattern gives way to a B-line pattern ( Fig. 71.2 ) with more B-lines corresponding with increasing fluid overload. The B-line pattern is visualized as radially oriented hyperechoic (bright) lines emanating perpendicularly from the pleural line running to the edge of the field. Counting B-lines serially over a predefined pattern of 28 intercostal spaces yields a B-line score.
B-line score has been validated using gravimetry in a post-mortem pig model and using invasive techniques available in the intensive care unit, such as transpulmonary thermodilution. B-line score correlates well with volume overload, outperforming the chest radiograph in detection of pulmonary edema, with higher score correlating with increased lung water. In patients with ESKD on HD, B-lines were demonstrated to disappear dynamically on dialysis correlating with ultrafiltration volume. In these patients, B-line score correlates with cardiovascular outcomes, death, and readmissions in retrospective data. A large, prospective, multi-center randomized controlled trial (Lung Water by Ultrasound Guided Treatment in Hemodialysis Patients or LUST Study) is ongoing to determine whether measurement of B-line score enhances estimation of dry weight and improves cardiovascular outcomes in patients with ESKD on HD and comorbid cardiac disease. Preliminary data from this trial has shown that the prevalence of asymptomatic pulmonary congestion is high and often goes undetected by the physical exam.
Lung ultrasound is easy to learn. Inter-observer reliability has been demonstrated using web-based tutorials and image review by expert trainers. There are limitations to lung ultrasound. A full 28-point exam takes about 5 minutes to perform and requires the patient to disrobe. Importantly, other disease processes can present as a diffuse B-line pattern, including the acute respiratory distress syndrome, diffuse interstitial lung diseases, and multifocal pneumonias.