At Sydney HEMS we are believers in the benefit of ultrasound in the prehospital & retrieval medicine environment. Lung ultrasound is of particular use in diagnosing pneumothoraces at noisy trauma scenes or in helicopters.
Yash Wimalasena, one of our Retrieval Fellows from the UK who has a particular passion for mountaineering has recently published an article on another potential role for prehospital lung ultrasound – identifying wet lungs. He focused specifically on the role of ultrasound in identifying patients with high altitude pulmonary oedema (HAPE). [Abstract available here]
According to Yash, studies conducted in remote areas have demonstrated that ultrasound can be used as a measure of subacute pulmonary oedema and HAPE in climbers ascending to altitude. The specific ultrasound findings are “B-lines”, also known as ultrasound lung comets.
What to look for
Just to refresh your memories here is a quick clip of normal lung (from Neurocritical Care Ultrasound 2013)
By contrast, wet lung has multiple vertical lines arising from the pleural line. They move with respiration and go to the edge of the screen. B-lines also rule out pneumothorax. These are B-lines pictured in the video below.
Here’s a clip by Dr Justin Bowra showing us where to put the probe.
Essentially, the ultrasound diagnosis of pulmonary oedema is based on identifying bilateral B-lines in the middle or upper zones. The higher and more abundant the B-lines are the wetter the lungs.
Summary of ultrasound use in high altitude pulmonary oedema (HAPE)
According to Yash, B-lines were first identified at high altitude in a study of HAPE victims conducted at the Himalayan Rescue Association clinic in Pheriche, Nepal (4240 m). Lung ultrasound performed on 11 patients diagnosed with HAPE and 7 healthy control subjects revealed that patients with HAPE had a significantly more B-lines and lower oxygen saturations. In patients with HAPE B-lines cleared and oxygen saturations increased after treatment.
In 2010, Pratali et al conducted a study in Nepal on 18 healthy Italian trekkers ascending to an altitude of 5130 m. Lung US was conducted at sea level and at various points during the ascent. Their results showed that B-lines appeared in 15 of 18 subjects (83%) at 3440 m and in 18 of 18 subjects (100%) at 4790 m. B-lines increased during the ascent, from 1.06 at 1350 m to 16.5 at 5130 m and decreased on descending. These 2 studies conducted in remote high altitude areas demonstrate that ultrasound can aid in diagnosing respiratory pathology even in the most extreme of environments.
Ultrasound and other causes of wet lungs
But you don’t need to be on the side of a mountain to find wet lungs on ultrasound. During interhospital transfers, “B-lines” are just as useful in diagnosing cardiogenic pulmonary oedema and may help differentiate it from other causes of wet lungs such as ARDS. Here is a quick video from the SMACC conference which goes through the differences.
Scott Weingart of EMcrit fame also talks about using the abscence of “B-lines” as a marker of “fluid tolerence”. He suggests that if you don’t see “B-lines” on chest ultrasound you are unlikely to push a patient into pulmonary oedema with a fluid bolus. By contrast if you see B-lines be carful about pushing fluids. Matt and Mike explain this well on the Ultrasound Podcast
If you want to know more about lung ultrasound in general, check out Mike Stone’s talks on the Ultrasound Podcast part 1 & part 2
Lung Ultrasound for High Altitude Pulmonary Oedema
Wilderness Environ Med. 2013 Jun;24(2):159-64