Oscillometry—A reasonable option to monitor lung functions in the era of COVID‐19 pandemic

Abstract

The current documented global prevalence of asthma is over 300 million and is continuously rising due to rapid changes in gene-environment interactions.1 Documentation of expiratory airflow obstruction at diagnosis and resolution in follow-up is necessary for its appropriate management. Pulmonary function monitoring is also desired in restrictive lung diseases, including COVID-19 pneumonia, at the start and during subsequent visits. Spirometry, the widely used technique for measuring lung functions, has been questioned in recent times due to the potential risk of outspreading COVID-19 apart from its restricted diagnostic utility for smaller airways and restrictive lung diseases. Another important drawback is a requirement of a patient's understanding and optimal performance during the procedure which makes this procedure a nightmare for the pediatric population. With the pandemic progressing at the current pace, we are far away from the trivialization of the situation. With the limited utility of aerosol-generating and strenuous procedures like spirometry, there is an immediate need for safer alternatives to monitor lung functions in the indigent. Procedures involving vigorous expiratory efforts like coughing, sneezing, and spirometry can impose an enhanced transmission risk of respiratory infections among the healthcare workers and fellow pulmonology patients, due to increased aerosol generation.2 Several studies have reported that forceful deep exhalation increases the particle concentrations as well as the duration of suspension in the surrounding environment (Table 1). The phenomenon of increased aerosol generation during forceful breathing maneuvers can be explained by the “airway reopening hypothesis.”3 During physiological conditions, the apical portion of the lungs is more ventilated than basal segments, the condition is further aggravated and mismatched during disease. A tidal breath, at low volume, will not disturb the dependent airways configuration whereas a forceful breathing effort will reopen the collapsed segments causing more turbulence and hence increased production of aerosols.3 Any procedure involving forceful inspiration and/or expiration is highly likely to cause more airflow disruption and further aerosol generation. As the procedure of spirometry requires cooperation and forceful respiratory efforts, it is cumbersome in children, the elderly, patients with neuromuscular weakness, and in those with learning difficulties. Moreover, its use has been restricted to a minimum during the current pandemic situation.2 Using negative pressure rooms, HEPA filters, adequate ventilation, hand hygiene, complete personal protective equipment, and social distancing will only reduce the spread and contamination but will not eliminate the aerosol generation.2 The current COVID-19 pandemic has reemphasized the unmet need of more child-friendly breathing operations with lesser aerosol production for monitoring pulmonary functions. Forced oscillation technique (FOT), being a tidal breath-based maneuver, satisfies most of the shortcomings of spirometry. FOT is a simple, reliable, and rapid method where the requirement of patient cooperation is minimal. It detects respiratory impedance (resistance and reactance) by superimposing small-amplitude oscillation waves over normal tidal breath.4 Lower frequencies (2–4 Hz) are transmitted to the peripheral pulmonary areas whereas higher frequencies (>20 Hz) reflect characteristics of proximal conducting airways. Pressure and flow, measured at individual midrange frequencies (2–20 Hz), define respiratory impedance in common practice, by using the fast Fourier transform technique.4 Its potential, in detecting lung functions in children, ventilated patients and during sleep, has been tested previously. It requires minimal patient cooperation and can be used in the preschool age group, overcoming a major limitation of spirometry. Gupta et al.5 have recently demonstrated its utility in monitoring bronchodilator reversibility of asthmatic airways in children as young as 2 years of age. Though the concept of FOT was first introduced in the late 1950s and has undergone several modifications over the last six decades, it is still not widely used due to concerns over standardization. Oscillation signals could be of a single frequency, pseudorandom noise (PRN; simultaneous application of several frequency components), or recurrent impulses type as available currently with different manufacturers.6 Mono-frequency sinusoidal pressure signals are useful in monitoring patients with sleep apnea and those on mechanical ventilation or using continuous positive airway pressure. PRN FOT signals describe respiratory characteristics in patients with asthma, bronchitis, emphysema, interstitial lung disease, pulmonary fibrosis, and thoracic wall deformities. Impulse oscillometry uses periodic impulses to better define within breath changes in impedance. Dandurand et al.7 have highlighted the same by comparing various types of commercially available machines. They found greater variation in measured airway parameters with certain devices on subjecting higher mechanical load under experimental conditions. Devices (like Wave Tube, Tremo Flo C-100, and Resmon Pro) working with pseudorandom, relative prime signals fared better than others for measurement of peripheral lung characteristics. FOT is more sensitive to detect peripheral airway obstruction and lung parenchymal diseases with its unique sonic detection technique. As no forceful breathing effort is involved in FOT, the expected aerosol generation is much less than spirometry (Table 1), when used during active viral infections like corona and influenza viruses. This distinctive feature may be more useful in serial monitoring of patients suffering from COVID-19 pneumonia. Pulmonary function measurements (either by spirometry or FOT) can vary with ethnicity and regional reference values should be referred for comparison. In the absence of available benchmark values for a specific height, age, or ethnicity, respiratory mechanical characteristics, and bronchodilator responsiveness can be compared with the baseline measurements from the same machine as used previously.5, 6 Monitoring of periodic lung functions in both obstructive and restrictive disease is necessary for optimal disease control and improved quality of life. FOT seems to have a good potential in the current pandemic and afterward to reliably monitor pulmonary functions with reduced risk of disease transmission, as compared to spirometry. Apart from the universal inhaler (with mask and holding chamber) and individual filter use, when combined with other protective strategies like adequate room ventilation, air exchanges, equipment disinfection, self-isolation, and staff protection, this technique can prove to be a real boon to pulmonologists and their patients. This, the potential use of FOT also highlights an urgent requirement to make global efforts for standardization of this technique and pass on the potential benefits from bench to bedside. The authors declare that there are no conflict of interests.

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