We demonstrate a standard operating protocol to conduct respiratory oscillometry, highlighting key quality control and assurance procedures.
Respiratory oscillometry is a different modality of pulmonary function testing that is increasingly used in a clinical and research setting to provide information regarding lung mechanics. Respiratory oscillometry is conducted through three acceptable measurements of tidal breathing and can be performed with minimal contraindications. Young children and patients who cannot perform spirometry due to cognitive or physical impairment can usually complete oscillometry. The main advantages of respiratory oscillometry are that it requires minimal patient cooperation and is more sensitive in detecting changes in small airways than conventional pulmonary function tests. Commercial devices are now available. Updated technical guidelines, standard operating protocols, and quality control/assurance guidelines have recently been published. Reference values are also available.
We conducted oscillometry test audits before and after implementing a formal respiratory oscillometry training program and standard operating protocol. We observed improvement in the quality of tests completed, with a significant increase in the number of acceptable and reproducible measurements.
The current paper outlines and demonstrates a standard operating protocol to conduct respiratory oscillometry in an outpatient setting. We highlight the key steps to ensuring acceptable and reproducible quality measurements according to the recommended European Respiratory Society (ERS) guidelines, as quality control is critical to measurement accuracies. Potential problems and pitfalls are also discussed with suggestions to resolve technical errors.
Respiratory oscillometry measures the impedance of the lung and is exquisitely sensitive to changes in respiratory mechanics1, particularly to the peripheral lung and small airways, regions of the lung that are not well assessed by traditional pulmonary function tests.
Over the last several years, the availability of commercial devices and updated technical and quality control/assurance standards2,3 have led to increasing use of oscillometry for clinical and research purposes. However, to date it is not a routine test in the repertoire of pulmonary function modalities, but the technique is anticipated to become more widely used with increasing recognition of its clinical utility. The overall goal of respiratory oscillometry is to provide measurement of respiratory mechanics during normal breathing and assessment of lung function, that is not discernible by current methods of spirometry and plethysmography. Oscillometry offers other advantages over traditional pulmonary function tests as it can be performed in the very young, the elderly, or in patients with cognitive impairment where forced expiratory maneuvers needed for spirometry are impossible. Furthermore, oscillometry can be conducted in anyone who can breathe spontaneously while wearing a nose clip. Unlike standard pulmonary function tests, it is not contraindicated following cataract, intra-abdominal or cardiothoracic surgery, nor following acute myocardial infarction and heart failure. Lastly, several of the oscillometry devices currently available are portable, and can be used in settings outside of a diagnostic laboratory, including clinic and office settings, bedside, or in workplaces.
Oscillometry measures the total respiratory impedance (Zrs) to multi-frequency oscillatory pressure waves1,2,4,5,6. Impedance is composed of the complex sum of respiratory resistance (Rrs) and reactance (Xrs). Rrs reflects the resistance of airways and is largely frequency-independent in health4,7,8. In small airway diseases, Rrs becomes frequency-dependent and increases more in the lower frequencies5,9,10, so that a difference in Rrs at frequencies between 5 and 19 Hz (R5–19) or 5 and 20 Hz (R5–20) indicates small airway obstruction and heterogeneity of ventilation in different regions of the lung 10,11,12. Xrs measures the balance of elastic and inertial impedances of the respiratory system. At lower frequencies (e.g., 5 to 11 Hz), Xrs reflects the stiffness or elastance of the pulmonary and chest wall tissues13,14. At higher frequencies, Xrs is dominated by the inertia of the air column in the conducting airways. The resonance frequency (Fres) is the point at which the magnitudes of elastic and inertive reactance are equal. AX is an integrative index of Xrs and is calculated as the area under the Xrs versus frequency graph between 5 Hz and Fres. AX has the units of elastance and is inversely related to the volume of the lung in communication with ventilation. AX increases with restrictive processes and peripheral inhomogeneity. X5 becomes increasingly negative while AX and Fres are increased in both obstructive and restrictive lung diseases4,5. See Figure 1 for depiction of these metrics.
While initially focused on measurement of lung function in children, emerging data show that oscillometry provides useful clinical information in adults as well. It is increasingly used in the clinical setting15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45. Oscillometry has been most extensively studied in obstructive lung diseases where it has been found to offer better diagnostic information than spirometry with respect to asthma control31,32,33,34,35, better correlation with symptoms23,34, and earlier detection36,37,38 of chronic obstructive lung disease (COPD). Our group has shown oscillometry to be more sensitive than spirometry for tracking graft injury following lung transplant46. Several studies have shown that Xrs, specifically the difference in the mean inspiratory and expiratory reactance at 5 Hz, can distinguish restrictive defects in interstitial lung disease (ILD) from asthma and COPD47, and can differentiate combined pulmonary fibrosis and emphysema from ILD-only48,49. Figure 2 demonstrates the typical oscillometry patterns for normal, restrictive, and obstructive lung diseases. There has been increasing interest to implement oscillometry as another routine modality of pulmonary function testing to supplement and potentially replace some of the current testing modalities for lung function monitoring50,51.
We suggest that oscillometry is useful for screening of lung diseases, in follow-up of patients with known obstructive and restrictive lung diseases, and following lung transplant. The commercial devices are suitable for use in children as young as 2 years old. There is ongoing research with even younger populations52, and as the field grows it may be possible to evaluate infants and newborns.
The goal of the current manuscript is to provide a training manual for clinicians, technologists, and research personnel on the appropriate conduct of oscillometry, following international standard operating protocols and quality control guidelines. Due to the small footprint of most commercial oscillometers, oscillometry can be implemented in multiple settings. The protocol outlined is appropriate for pulmonary function laboratories, physician offices, clinic settings, and other outpatient settings such as workplace occupational health units.
The critical steps in a high quality oscillometry measurement can be categorized into the domains of patient, equipment, and operator. Ensuring that the patient is relaxed and comfortable so that the measurements collected are at resting functional residual volume is key. The patient posture is very important; ensure that the patient is sitting upright with both feet on ground with no crossing of legs. The enforcement of cheek and jaw support, good placement of the nose clip, and ensuring lips are sealed around the mouthpiece will eliminate shunting and air leaks1,2,3. The equipment must be calibrated and verified prior to use. The operator must be able to recognize acceptable and unacceptable recordings and able to troubleshoot the underlying cause of unacceptable readings or artefacts to ensure reported measurements have CoV ≤10%1,2,3. Quality control and assurance must be maintained to not only ensure the oscillometry device is validated, but also the quality of tests.
Training of the operator to recognize the patterns produced by common artefacts such as swallowing, leaks, and shunting will allow for timely repeated measurements to obtain quality tests. There are instances when oscillometry is performed at different lung volumes, (e.g., in supine position). Under these circumstances, all the steps described in the protocol can still be applied.
While oscillometry is an easier and more rapid modality of pulmonary function testing, errors in measurements, and thus interpretation, will occur if deviations from the standardized protocol and quality control steps occur. Our protocol is based on the device used at our center. The conduct of oscillometry will be the same across devices. However, there will be differences in the technical aspect of calibration and software applications. Readers are advised to follow the manual for the different instruments.
Oscillometry is faster and easier to perform than spirometry. Moreover, young children and adults with language, physical, and/or cognitive impairment that impede the ability to perform the forced expiratory maneuvers needed for spirometry can still perform oscillometry as it is conducted during normal breathing. In some centers, oscillometry has supplanted spirometry as the initial screening tool for lung disease. Enhancing training in the conduct of oscillometry will facilitate its wider application as a diagnostic tool and ensure quality control of the tests conducted.
Although oscillometry is a fast and easy technique, quality controls are needed to ensure accurate and reproducible measurements. By following international guidelines, research and clinical oscillometry data can be interpreted appropriately so that findings can be applied across different patient populations.
The authors have nothing to disclose.
The study was funded by CIHR-NSERC Collaborative Health Research Projects (CWC), Pettit Block Term grant (CWC), The Lung Health Foundation, and Canadian Lung Association – Breathing as One: Allied Health Grant (JW). We thank the many participants of our oscillometry research studies who have allowed us to develop expertise in conduct of oscillometry.
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