MacLeod, Dominic Paul; (1997) Use of a within-breath forced oscillation technique - Development and clinical applications. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

Introduction: The forced oscillation technique can track respiratory resistance non-invasively during breathing. Here, the clinical potential of within-breath oscillatory resistance measurements was evaluated. Methods using a 10 Hz input impedance set-up, oscillatory resistance (R10) was gated to respiration using the trans-thoracic electrical impedance. Absolute R10 values and their repeatability and sensitivity to changing airway calibre were studied in; healthy subjects (n=65), patients with airflow obstruction (n=21), subjects undergoing bronchochallenge (n=38) and, in the upper airway, in healthy (n=8) and rhinitic subjects (n=12). Results: In healthy subjects, gating to end-inspiration and end-expiration reduced absolute oscillatory resistance values compared to ensemble-averaged values (P < 0.05). Inspiratory values (mean [sd]: males; 0.22[0.05] kPa.l-1s, females; 0.26[0.05] kPa.l-1.s) were lower than expiratory values (mean [sd]: males; 0.23[0.06] kPa.l-1s, females; 0.29[0.09] kPa.l-1s) (n=62, P < 0.05). Within-session repeatability (intra-subject coefficient of variation, CV%) remained slightly better using ensemble-averaging (mean CV%=5.5-5.8) (P < 0.05). R10 behaviour mainly reflected changing respiratory flow and glottic cross-sectional behaviour had little influence on repeatability (P < 0.05). Studying patients with lower airway obstruction, inspiratory and expiratory R10 differed markedly. Late expiratory rises in R10 occurred, maximally in laryngectomized subjects studied per tracheostoma. Significant correlations to airway resistance and spirometry were found using gating, repeatability falling with rising airway resistance at end-inspiration (P < 0.05). Sensitivity to bronchodilatation was greatest at end-expiration (P < 0.05). These findings were felt to reflect collapsible airway segments with expiratory flow- limitation. A recently described 'bronchomotility' index was elevated in patients with bronchial hyper-reactivity compared to normals at baseline and after bronchochallenge (P < 0.05). Studying nasal resistance, gating had less effect on absolute values. Repeatability and sensitivity to changing nasal patency in rhinitis were unaffected by gating. Conclusions: Gating enhances the information yield of forced oscillation measurements, especially in the lower airway, improving their clinical and investigative potential.

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