Low frequency forced oscillation technique in infants

Abstract

The respiratory system in infants undergoes profound changes in the first few years of life. We applied two adaptions of the FOT to characterise 1 ) changes in airway and parenchymal mechanics with growth in the first two years of life; 2) contribution of the nose to total respiratory impedance (Zrs); and 3) the influence of the chest wall to Zrs. Methods: For studies 1 (n=34, 1 -24 months) & 2 (n=20, 3-21 months) a pseudo-random forcing signal (0.5-21 Hz) was applied to sedated infants via a face mask & Zrs was determined at a transrespiratory pressure of 20cm HzO. A model containing an airway compartment [airway resistance (R) & inertance (I)] & a frequency dependant constant-phase tissue compartment [tissue damping (G) & tissue elastance (H)] was fitted to Zrs. In the second study, Zrs was partitioned into nasal impedance (Zn) & lower respiratory system impedance (Zlrs). In the third study, Zrs & chest wall impedance (Zw) were determined at FRC in patients undergoing cardiac surgery (n=5, 3-7.5 years). Lung impedance (Zl) was calculated as Zl = Zrs - Zw. Results: In infants with no history of lung disease the growth of the airways was found to lag that of the pulmonary tissues. Zn contributed 42.5±4.4(SEM)% and 73.4±6.7% of the total R & I respectively, however it's contribution to the total G and H was negligible. The chest wall contributed substantially to the parenchymal parameters of total respiratory G (46.5±6.7%) & H (39.9±7.4%) while having negligible influence on R & I of the total respiratory system. Conclusions: The different growth patterns of the airways and parenchyma support the concept of dysanaptic growth in this population. The relative contribution of Zn to Zrs for all parameters was constant with growth, however the relative resistive and elastic properties of the chest wall decreased with age. In summary the FOT allows changes in airway and parenchymal mechanics to be examined in infants and young children