Abstract
BACKGROUND:
Previous in vivo data suggest that high airway resistance (Raw) promotes dynamic hyperinflation, especially when coupled to high minute ventilation (V̇E). 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 Raw, V̇E, respiratory system compliance (CRS), and duty cycles/flow regimes.
RESULTS:
Substantial dynamic hyperinflation (PEEPi > 5 cm H2O) occurred at various V̇E, even when Raw was low (4 cm H2O/L/s) or just above normal (18 cm H2O/L/s). A V̇E ≥ 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, Raw, and CRS, 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 V̇E, resistance, compliance, and duty cycles were all independently associated with dynamic hyperinflation.
CONCLUSIONS:
In this bench model, dynamic hyperinflation occurred with high V̇E, even at low Raw. Since moderate Raw and V̇E 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 V̇E of lower frequency and larger tidal volume (1.0 L).
Keywords
Get full access to this article
View all access options for this article.