Determinants of Dynamic Hyperinflation in a Bench Model

BACKGROUND: Previous in vivo data suggest that high airway resistance (R_aw) promotes dynamic hyperinflation, especially when coupled to high minute ventilation (V̇_E). However, no studies have systematically examined the relative effects of various mechanical parameters on dynamic hyperinflation.

METHODS: Intrinsic positive end-expiratory pressure (PEEPi) was measured with a ventilator-lung model, over a range and various permutations of R_aw, V̇_E, respiratory system compliance (C_RS), and duty cycles/flow regimes.

RESULTS: Substantial dynamic hyperinflation (PEEPi > 5 cm H₂O) occurred at various V̇_E, even when R_aw was low (4 cm H₂O/L/s) or just above normal (18 cm H₂O/L/s). A V̇_E ≥ 15 L/min was associated with increasing PEEPi in this model, across a broad range of mechanical permutations. PEEPi was significantly higher in all models during descending ramp flow than during constant flow, at equivalent peak flows (wherein duty cycle during descending ramp flow was twice that of constant flow). PEEPi was equivalent when duty cycles (and all other mechanical parameters) were equal. PEEPi was significantly greater, irrespective of duty cycle, R_aw, and C_RS, when delivered with lower tidal volume (0.6 L vs 1.0 L). The change in peak airway pressure associated with development of dynamic hyperinflation was consistently greater than the observed PEEPi. Higher V̇_E, resistance, compliance, and duty cycles were all independently associated with dynamic hyperinflation.

CONCLUSIONS: In this bench model, dynamic hyperinflation occurred with high V̇_E, even at low R_aw. Since moderate R_aw and V̇_E frequently occur in vivo, even without obstructive lung disease, occult dynamic hyperinflation is likely to occur commonly. PEEPi was greater with high frequency and small tidal volume (0.6 L) than with equal V̇_E of lower frequency and larger tidal volume (1.0 L).