Pulmonary Physiology, Function, and Rehabilitation
Using impulse oscillometry in clinical practice
Impulse oscillometry (iOS) is an effort-independent test that requires minimal cooperation from the patient. It provides measures of respiratory mechanics during normal tidal breathing, including resistance (R), reactance (X), and impedance (Z) (Oostveen E, et al.).
Airway R is largely, but not entirely, determined by cross-sectional area (Poiseuille’s Law). X is a surrogate for lung elastance, which is the inverse of compliance. Z is the combination of R and X and isn’t used clinically.
There are several benefits to using iOS, as opposed to or in conjunction with standard spirometry. First, iOS yields respiratory function measurements for patients, like the elderly and young children, who cannot provide acceptable and reproducible spirometry (Pezzoli L, et al.). Second, it provides a real-world assessment of lung function because R and X values are obtained during tidal breathing. Humans don’t use the forced maneuvers needed for spirometry during normal daily activities, which weakens the correlation of FEV1 with respiratory symptoms. Forced maneuvers also create artifacts from gas compression and cause small airway closure, which limits inferences made from standard spirometry (Brusasco V, et al. ). Lastly, R and X provide information not available from spirometry, and iOS is particularly sensitive for detecting small airway dysfunction (Berger K, et al. ).
Clinical and disease-specific indications for iOS are still being established. As discussed above, iOS is appropriate for any patient unable to perform spirometry. As new inhalers designed to deliver medication to the distal airways become available, subtle abnormalities detected via iOS will provide a target for specific therapies (Lipworth B.). iOS shows significant promise as a noninvasive assessment for supraglottic diseases, like vocal cord dysfunction, and can quantify changes over time following invasive intervention to relieve upper airway obstruction (Bikov A, et al. ; Horan T, et al. ). As their comfort level with interpretation improves, pulmonologists will find iOS is an important tool for disease diagnosis and treatment.
Aaron Holley, MD, FCCP
Steering Committee Member
Pulmonary Vascular Disease
Hemodynamic definition of pulmonary hypertension changed
Many patients worldwide went to bed February 26, 2018, with normal pulmonary pressures and woke up the next morning with pulmonary hypertension (PH). That day, experts met at the World Symposium on PH in Nice, France, and changed the definition of resting PH from a mean pulmonary artery pressure (mPAP) of greater than or equal to 25 mm Hg to a mPAP greater 20 mm Hg (Simmoneau, et al.). The First World Health Organization symposium on PH in 1973 established the 25 mm Hg cutoff to distinguish primary PH from what was then considered less severe forms of PH. This definition, acknowledged as arbitrary and conservative at the time, has persisted due to a paucity of data establishing a definitively abnormal mPAP threshold.
Two contemporary findings provide justification for the definition change: (1)Normal mPAP is 14 ± 3.3 mm Hg in healthy subjects (Kovacs, et al.). (2) Patients with mPAP greater than 20 mm Hg suffer worse outcomes compared with control subjects (Maron, et al. ).
Preserving the other hemodynamic criteria for group 1 PH, pulmonary artery wedge pressure less than or equal to 15 mm Hg and pulmonary vascular resistance greater than or equal to 3 Wood units, experts also recommend applying the new definition to all pre-capillary PH, including groups 3, 4, and applicable group 5 diagnoses.
Importantly, new guidelines do not recommend treating PH patients with mPAP 21-24 mm Hg: “A change in the hemodynamic definition of PH due to [pulmonary vascular diseases] does not imply treating these additional patients, but highlights the importance of close monitoring in this population.”
John Kingrey, MD
Steering Committee Member