Prior to the plethysmograph—an enclosure that a person enters to help ascertain the state of their lungs—the only way to determine lung capacity was using a spirometry device where a patient takes a deep breath and blows into a tube. The information available from such a test is limited.
Terry Kapsen, senior vice president of marketing at Angeion Corporation, a company based in St. Paul that makes medical imaging devices, says his company’s Medical Graphics Platinum Elite Series plethysmograph provides data and three-dimensional graphs to aid in lung function assessment.
“In a very short amount of time, [we can] get a complete analysis of the capacity of the lung. We can look at the mechanical properties of the lung—in other words, how well air is moving in and out of the lungs,” Kapsen says. This includes measuring the diffusion of gas into the lungs and air-wave resistance within the lungs.
The plethysmograph is used to help diagnose chronic obstructive pulmonary disease, asthma, emphysema, and chronic bronchitis, and can be found in hospitals and clinics. A patient steps into the unit and sits down and a wrap-around door shuts and seals the space. This last point is important because, as Kapsen points out, within the closed cylinder there is a known volume of air, which makes for more precise lung volume calculations. As the patient breathes into a mouthpiece, the device monitors the pressure of the air coming out of lung and the pressure inside the device. Using a simple equation, the device’s software solves the unknown quantity—the volume of air that’s trapped in the lungs.
An advanced model also performs diffusion tests to determine how well gas is transferred from the lungs to the blood stream, and back from the blood stream and into the lungs. The most comprehensive model can also calculate how much gas is trapped inside the lungs and is not aspirated, which is helpful in determining the lung capacity of people with emphysema.
At about six feet tall and about three feet across, Kapsen says the device has a space-saving footprint but can still accommodate large patients.