Abstract
<b>Background</b> Respiratory pattern analysis is conventionally reduced to simplified metrics, such as respiratory rate (RR), tidal volume (Vt) or respiratory rate variability (RRV). Respiratory waveforms are nonetheless complex and generally sampled with high fidelity. A novel mathematical method, Symmetric Projection Attractor Reconstruction (SPAR), analyses waveforms without discarding any of this high-fidelity data, focusing on subtle changes in morphology and variability. SPAR creates a simpler visual representation of waveforms (attractor) which can be qualitatively and quantitatively analysed.
<b>Aim</b> To investigate whether SPAR attractors can provide additional physiological biomarkers of pathophysiological respiratory changes in COPD patients.
<b>Methods</b> Pneumotachograph airflow waveform data, recorded during resting tidal breathing and maximum voluntary ventilation (MVV), were analysed conventionally and using SPAR. Data from 56 healthy subjects (43.5 years median age, 7% female) and 10 COPD patients (68.5 years median age, 10% female, 39% median%predicted FEV1) were compared. Receiver operating characteristics area under the curve (ROC AUC) was used to ascertain sensitivity and specificity of classifying changes between healthy and COPD subjects (arbitrary threshold > 0.7).
<b>Results</b> During resting tidal breathing, RR, Vt and attractor morphology metrics were not notably different between the two groups (ROC AUC = 0.70, 0.58, 0.50). RRV was higher in COPD patients (ROC AUC = 0.74) but more sensitively measured using attractor variability metrics which map wave-to-wave variability (ROC AUC = 0.86). During MVV, RR did not change (ROC AUC = 0.58). Vt was lower in COPD patients (ROC AUC = 0.95). Clear morphological differences were seen between the groups where COPD attractors were ‘pinched’ corresponding to a flattening of the expiratory trace (ROC AUC = 0.95). RRV was not different (ROC AUC = 0.54), but attractor variability was decreased in the COPD group (ROC AUC = 0.72) (figure 1).
<b>Conclusions</b> SPAR provides a new ‘at-a glance’ representation of waveform morphology and variability. SPAR differentiated COPD patients from healthy subjects by highlighting changes in the non-spirometric, resting and MVV airflow waveforms. The morphological change of MVV attractors most likely reflects expiratory airflow limitation. Further work is needed to explore the use of SPAR to facilitate respiratory diagnostics and disease monitoring.