TORONTO -- One-third of studied individuals who were hospitalized due to COVID-19 still experienced the aftermath of the virus on their lungs a year after they were discharged, new research shows.

A study published in The Lancet Respiratory Medicine journal found that while most participants showed improvement one year after being hospitalized, 33 per cent showed that the virus had hindered their ability to diffuse carbon monoxide into oxygen for their blood cells.

This was determined through DLCO, a diagnostic test used to measure the ability of the lungs to transfer oxygen into the blood vessels.

The study, which was released on Wednesday, was based on 83 patients who were hospitalized due to COVID-19 and did not require medical ventilation or have any previous underlying health conditions that would affect their lungs. This included, but was not limited to, cardiovascular diseases, chronic lung diseases, or any history of smoking.

Dr. Rebecca Amer, respiratory and critical care doctor at St. Joseph’s Healthcare in Ontario and director of the adult respirology training program at McMaster University, said that COVID-19 can cause changes in the lungs such as scarring, cysts, damage to the blood vessels, blood clotting, and damage to the heart muscle.

“If a patient has developed scarring, it can actually decrease the size of the lung volumes. If they develop cystic lung disease, they can develop what you would say [are] twitchy airways, so they might become short of breath or wheezing. If they develop a problem with the blood vessels, it can reduce the ability of oxygen to be transferred from the airways in the lungs to the blood vessels in the lungs,” Amer told “We’re seeing patients coming through our post-COVID-19 clinic with similar findings in ongoing shortness of breath, and this is multifactorial in nature and it comes from residual disease in the lung.”

When doing imaging and breathing assessments such as the DLCO test, Amer says that the average predicted percentage of gas transfer differs between each individual and can be adjusted based on age, sex, and ethnicity, but anything above 75 per cent would be considered normal and anything below that would be more considered abnormal.

In the study, a third of the patients were showing gas transfer rates that were less than 80 per cent.

“If you have a reduction in your gas transfer, then obviously you’re not getting enough oxygen from the airways into the blood vessels, and the blood vessels in the lungs go back into the heart and then it goes to the rest of your body,” Amer said. “If you're not getting enough oxygen into those blood vessels, then the rest of your body, all of your organs and muscles and everything, vital tissue, your brain, it won’t get enough oxygen so that can affect the person in many different ways.”

Amer says that while scarring in the lungs cannot be fixed, treatments can be put in place to prevent it from getting worse. She adds that the complete recovery of the lungs from patients who have contracted COVID-19 is dependent on factors such as pre-existing medical conditions and how the virus affected their lungs and their heart health, but it could be possible with pulmonary rehabilitation.

“We’ve had a few patients come through our clinic and initially, their exercise tolerance had been greatly impacted, but then with time and a little bit of rehabilitation, they’ve been able to get back to normal,” she said. “I think the older patient population with premorbid conditions are having a more difficult time returning back to baseline, but it's not impossible.”