Background: Pulmonary function tests utilize flow-volume loops (FVLs) to help detect, diagnose, and monitor the long-term progression of lung disorders such as COPD and asthma. Spirometry is the gold standard for generating FVLs; however, it requires patient cooperation, which is not always feasible. Respiratory inductance plethysmography bands have also been used to analyze tidal breathing FVLs. Monitoring tidal breathing FVLs has been proposed as a means of monitoring disease progression, responsiveness to therapy, reaction to broncho-constrictive agents, and changes in breathing during exercise, but has not been widely adopted due to technology limitations. The objective here was to demonstrate the utility of a non-invasive respiratory volume monitor (RVM) in measuring continuous tidal FVLs.

Methods: Volume traces were collected using an RVM (ExSpiron 1Xi, Respiratory Motion, Inc., Waltham, MA) for 6 breathing trials. In trials 1 and 6, subjects were instructed to breathe normally. In trials 2-5, subjects alternated between fast (25 bpm) and slow (5 bpm) breathing. Flow traces were generated by taking the first derivative of the volume. We assessed the characteristics of the shape of FVLs for different trials.

Results: 48 subjects (33 males, age: 46.1±14.3 yrs; BMI: 27.6±6.2 kg/m2, mean ± SD) completed the study. Respiratory rates for the normal, fast, and slow breathing trials were 12.6±0.6, 24.6±0.1, and 6.9±0.3 bpm (mean ± SEM), respectively. Fig 1 depicts representative volume (top row), flow (middle row), and FVLs (bottom row) for normal, fast, and slow breathing trials. During normal breathing the FVL has a convex shape with a steady flow during the second half of the expiratory limb. The FVLs during fast breathing appear elliptical. During slow breathing a concave expiratory limb is observed near the end of expiration indicating an expiratory flow limitation.

Conclusion: We demonstrated the capability of the non-invasive RVM to generate continuous tidal FVLs in healthy volunteers. We observed distinctive shapes of the FVLs when the subjects varied their respiratory rate. The RVM vastly expands the potential applications in which FVLs can be measured.

10 / Quantification of flow-volume loops using a non-invasive respiratory volume monitor