Respiratory Mechanics During High Frequency Oscillatory Ventilation : 1 A Physical Model and Preterm Infant Study

39 Accurate mechanics measurements during high frequency oscillatory ventilation (HFOV) 40 facilitate optimizing ventilator support settings. Yet, these are substantially influenced by 41 endotracheal tube (ETT) contributions which may dominate when leaks around uncuffed ETT 42 are present. We hypothesized that 1) the effective removal of ETT leaks may be confirmed via 43 direct comparison of measured versus model-predicted mean intra-tracheal pressure (mPtr); and 44 2) reproducible respiratory system resistance (Rrs) and compliance (Crs) may be derived from 45 no-leak, oscillatory Ptr and proximal flow. Using ETT test lung models, proximal (Pao) and 46 distal (Ptr) pressures and flows were measured during slow cuff inflations until leaks are 47 removed. These were repeated for combinations of HFOV settings [Frequency, mean-airway48 pressure (Paw), oscillation amplitudes (ΔP) and inspiratory time (%tI)] and varying Test-Lung 49 compliance. Results showed that leaks around the ETT will 1) systematically reduce the effective 50 distending pressures and lung-delivered oscillatory volumes, and 2) derived mechanical 51 properties are increasingly non-physiologic as leaks worsen. Mean pressures were systematically 52 reduced along the ventilator circuit and ETT (Paw> Pao>Ptr) even for no leak conditions. ETT 53 size-specific regression models were then derived for predicting mPtr based on: mean Pao, ΔPao, 54 %tI and Frequency. Next, in 10 of 11 studied preterm infants (0.77±0.24 kg), no-to-minimal leak 55 was confirmed based on excellent agreement between measured and model-predicted mPtr and, 56 consequently, their oscillatory respiratory mechanics were evaluated. Infant resistance at the 57 proximal ETT (Rao=RETT+Rrs; p<0.001) and ETT inertance (IETT; p=0.014) increased 58 significantly with increasing oscillation amplitude (ΔP = 50%, 100% and 150% baseline) 59 whereas Rrs showed a modest non-significant increase (p=0.14) and Crs was essentially 60 unchanged (p=0.39). We conclude that verifying no-leak conditions is feasible by comparison of 61 by 10.0.33.4 on A ril 6, 2017 http://jaysiology.org/ D ow nladed fom Singh et al. (JAPPL-01120-2011-R2) 3 model-derived versus measured mean distending Ptr. This facilitated the reliable and accurate 62 assessment of physiologic respiratory mechanical properties that can objectively guide 63 ventilatory management of HFOV-treated preterm infants. 64 65