Ultrasound Offers New Approach to Measure Lung Fluid

A new study suggests that ultrasound may offer a noninvasive way to track and monitor pulmonary edema, which often affects patients with congestive heart failure (CHF).

Developed by researchers at North Carolina State University and the University of North Carolina, the new technique uses ultrasound multiple scattering and exploits the complexity of ultrasound propagation in the lung structure in order to characterize the micro-architecture of the lung parenchyma. The experimental setup consisted of a linear transducer array with an 8-MHz central frequency, which was placed in contact with the lung surface.

When conventional ultrasound waves hit air, the energy is scattered; this occurs multiple times in alveoli, resulting in an amorphous grey image with little useful information. This reflection process means that it takes an ultrasound’s echo much longer to return back to the ultrasound machine. But since no two ultrasound waves take the same path, their echoes take different amounts of time to return to the scanner. By examining all of the returning echoes and how they change over time, the extent to which the space between the alveoli is filled with fluid can be measured.

To test their approach, the researchers conducted two sets of experiments using rats. In the first set, the researchers used lung tissue that had been injected with saline solution to mimic fluid-filled lungs. The new approach allowed the researchers to quantify the amount of lung fluid to within one milliliter. In the second set, the researchers measured the mean distance between two ultrasound scattering events. They found that in fluid-filled lungs, the mean distance was 1,040 micrometers, whereas the mean distance in healthy lungs was only 332 micrometers. The study was published on March 18, 2017, in Ultrasound in Medicine and Biology.

“Historically, it has been difficult to use ultrasound to collect quantitative information on the lung, because ultrasound waves don’t travel through air– and the lung is full of air,” said senior author mechanical engineer Marie Muller, PhD, of NCSU. “However, we’ve been able to use the reflective nature of air pockets in the lung to calculate the amount of fluid in the lung. This is important, because one could potentially track this mean distance value as a way of determining how well pulmonary edema treatment is working.”

Pulmonary edema is fluid accumulation in the tissue and air spaces of the lungs, and is a cardinal feature of CHF. It leads to impaired gas exchange and may cause respiratory failure. It is due to either failure of the left ventricle of the heart to remove blood adequately from the pulmonary circulation (cardiogenic pulmonary edema), or an injury to the lung parenchyma or vasculature of the lung (noncardiogenic pulmonary edema). Treatment is focused on three aspects: improving respiratory function, treating the underlying cause, and avoiding further damage to the lung.