Why spirometry is not enough: Long COVID (#2 in a series)

Sep 20, 2021 Why spirometry is not enough: Long COVID (#2 in a series)

A great deal has been written about post-COVID 19 syndrome, also known as Long COVID. Numerous papers and articles describe the various lingering symptoms that may characterize this phenomenon, analyze the incidence and prevalence among different populations, and speculate about the possible causes as well as the long-term effects.

The goal of this post is to give a brief overview of the pulmonary injury shown to be associated with post-COVID and to highlight the importance of complete pulmonary function testing (PFT) in the evaluation and follow-up of these patients.

Pulmonary injury in Long COVID as reflected by PFTs

Several studies have evaluated residual pulmonary damage in patients recovering from COVID-19.

In a prospective study conducted in Canada, 60 patients were followed three months after onset of COVID-19. At least one PFT variable was abnormal in 58% of patients. 52% had abnormal DLCO, with 45% of these also having abnormal total lung capacity (TLC), indicating a concurrent restrictive ventilatory deficit. There was strong association between days on oxygen supplementation during the acute phase of COVID-19 and DLCO [1]. 

Additional prospective studies were conducted in Austria, Switzerland, and Spain with cohorts of 113-215 patients, followed between 3 to 4 months post COVID-19 infection. In all three studies, significant functional and radiological abnormalities were found, with 21-25% of patients recovering from severe/critical COVID-19 demonstrating reduced diffusing capacity (DLCO), potentially due to small airway and lung parenchymal disease [2,3,4]. CT scans in these studies showed that 55-63% had persistent pulmonary compromise, mainly ground-glass opacities and/or reticulation in the lower lung lobes. 

A number of studies have been conducted in China. In one, evaluation of 100 COVID patients upon discharge from the hospital showed that impaired diffusion capacity was the most common abnormality of lung function, occurring in 47% of the cohort, followed by restrictive ventilatory defects, which were both associated with the severity of the disease [5]. In another study with 55 COVID patients, 25% still had reduced diffusion capacity three months after discharge from the hospital [6]. In the largest study with 1733 COVID-19 patients, follow-up at six months post infection showed compromised diffusion capacity in 22%- 56% of the patients, depending on the severity of their disease [7]. These findings are similar to those of follow-up studies of the SARS virus, which showed reduced DLCO in 15.5% to 43.6% of patients 0.5-2 years post-infection [5].

Follow-up of Long COVID patients: DLCO for the full picture

It is becoming increasingly clear that Long COVID is here to stay for the foreseeable future. Based on the literature, more than half of hospitalized COVID-19 patients have lung function and chest imaging abnormalities 12 weeks after symptom onset.

Reduced diffusion capacity, followed by restrictive disease, are the most common types of residual pulmonary compromise in post-COVID hospitalized patients and may be associated with duration of oxygen supplementation. 

Based on data from the SARS epidemic, a substantial percentage of these patients may continue to have chronic abnormalities [1]. Experts conclude that complete PFTs including DLCO may need to become the new standard for out-patient follow-up and evaluation of Long COVID patients, especially in severe cases [5]. 

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