Changes in lung function and tidal airflow patterns after increasing extrathoracic airway resistance

On the relation between tidal and forced spirometry

Spirometry is a lung function test involving deep inhalation and forceful deep exhalation. It is widely used to obtain objective information about airflow limitation and to diagnose lung diseases. In contrast, tidal spirometry is based on normal breathing and therefore much more convenient, but it is hardly used in medical care and its relation with conventional (forced) spirometry is largely unknown. Therefore, the objective of this work is to reveal the relation between tidal and forced spirometry. Employing the strong correspondence between the forced flow-volume curves and the Tiffeneau-Pinelli (TP) index, we present a method to obtain (a) the expected tidal flow-volume curve for a given TP-index, and (b) the expected TP-index for a given tidal curve. For patients with similar values of the TP-index, the tidal curves show a larger spread than the forced curves, but their average shape varies in a characteristic way with varying index. Therefore, just as with forced curves, the TP-index provides a useful objective ranking of the average of tidal curves: upon decreasing TP-index the expiratory flow rate changes in that its peak shifts towards smaller expiratory volumes, and its post-peak part becomes dented.

Tidal spirometric curves obtained from a nasal cannula

Spirometry is a gold standard to assess lung function, and to identify respiratory impairments seen in obstructive lung diseases. The method is used for periodic monitoring, but it only provides snapshot information, and it requires forceful exhalation which is associated with limited reliability and repeatability. Several studies indicate that tidal flow-volume curves measured by pneumotachography or plethysmography can also be used to assess lung function. These methods avoid the forced manoeuvre, but are complex to set up or sensitive to movement. In the present work we address the long-standing problem of the unavailability of an easy-to-use and accurate method for monitoring tidal breathing frequently or even continuously. We show that pressure recordings from a nasal cannula can be accurately converted into scaled flow-volume curves by means of an algorithm that continuously calibrates itself. The method has been validated by feeding realistic healthy and unhealthy breathing patterns to anatomically correct 3D-printed upper airways of an infant and an adult, and by comparing the imposed flow-volume curves to the pressure-derived flow-volume curves. The observed very high level of accuracy opens the route towards remotely monitoring patients with chronic lung diseases.

Tidal breathing patterns derived from structured light plethysmography in COPD patients compared with healthy subjects

PURPOSE

Differences in tidal breathing patterns have been reported between patients with chronic obstructive pulmonary disease (COPD) and healthy individuals using traditional measurement techniques. This feasibility study examined whether structured light plethysmography (SLP) - a noncontact, light-based technique - could also detect differences in tidal breathing patterns between patients with COPD and healthy subjects.

PATIENTS AND METHODS

A 5 min period of tidal (quiet) breathing was recorded in each patient with COPD (n=31) and each healthy subject (n=31), matched for age, body mass index, and sex. For every participant, the median and interquartile range (IQR; denoting within-subject variability) of 12 tidal breathing parameters were calculated. Individual data were then combined by cohort and summarized by its median and IQR.

RESULTS

After correction for multiple comparisons, inspiratory time (median tI) and its variability (IQR of tI) were lower in patients with COPD (p<0.001 and p<0.01, respectively) as were ratios derived from tI (tI/tE and tI/tTot, both p<0.01) and their variability (p<0.01 and p<0.05, respectively). IE50SLP (the ratio of inspiratory to expiratory flow at 50% tidal volume calculated from the SLP signal) was higher (p<0.001) in COPD while SLP-derived time to reach peak tidal expiratory flow over expiratory time (median tPTEFSLP/tE) was shorter (p<0.01) and considerably less variable (p<0.001). Thoraco-abdominal asynchrony was increased (p<0.05) in COPD.

CONCLUSION

These early observations suggest that, like traditional techniques, SLP is able to detect different breathing patterns in COPD patients compared with subjects with no respiratory disease. This provides support for further investigation into the potential uses of SLP in assessing clinical conditions and interventions.