Wheezing is a common occurrence that most of us will have experienced, whether temporarily through something like hayfever or a cold, or more long-term through a condition such as asthma or chronic obstructive pulmonary disease (COPD). But scientists don't understand much about why it happens.
New research has used a combination of modelling and high-resolution video to try and shed some light on the mechanisms of wheezing, finding that there's a "violent" process that can cause our lung pipes to make these raspy sounds.
With this new information available, the team is hoping that wheezing might be better understood and diagnosed in the future.
"Because wheezing makes it harder to breathe, it puts an enormous amount of pressure on the lungs," says engineer Alastair Gregory, from the University of Cambridge in the UK.
"The sounds associated with wheezing have been used to make diagnoses for centuries, but the physical mechanisms responsible for the onset of wheezing are poorly understood, and there is no model for predicting when wheezing will occur."
To get to the bottom of wheezing, scientists had to get to the end of the flexible bronchiole tubes that make up the branching network in the lungs. They built their own lung substitute by adapting a device called a Starling resistor, made up of thin elastic tubes of varying lengths and thicknesses.
Air was forced through the tubes at different degrees of tension and then filmed with a multi-camera stereoscopy setup. The scientists were then able to observe how wheezing might begin and be sustained, through a series of oscillations in the tubes (or in the lungs).
"It surprised us just how violent the mechanism of wheezing is," says Gregory.
"We found that there are two conditions for wheezing to occur: the first is that the pressure on the tubes is such that one or more of the bronchioles nearly collapses, and the second is that air is forced through the collapsed airway with enough force to drive oscillations."
In either case, the oscillations are sustained through a fluttering mechanism, where the travelling waves of air have the same frequency as the opening and closing of the tube. The same sort of resonance scenario can collapse bridges and cause aircraft wings to fail, which shows how damaging it could be to the lungs.
The scientists went on to develop a 'tube law', which factors in the material properties and the geometry of the tubes, as well as the amount of tension they're under, to calculate when oscillations might occur. If the law can be adapted to the human lungs, we might have a new way of analysing wheezes and identifying both the type and the location of serious bronchial problems.
More work is going to be needed to fine-tune the system and better pick up on wheezing sounds, but the researchers are hopeful that a simple audio recording setup could in some circumstances replace X-rays and MRI scans, which are expensive and time-consuming to operate.
"Since wheezing is associated with so many conditions, it is difficult to be sure of what is wrong with a patient just based on the wheeze, so we're working on understanding how wheezing sounds are produced so that diagnoses can be more specific," says engineer Anurag Agarwal, from the University of Cambridge.
The research has been published in Royal Society Open Science.