Heliox administration in anesthetized rabbits with spontaneous inspiratory flow limitation

We investigated the effects of heliox administration (80% helium in O₂) on tidal inspiratory flow limitation (tIFL) occurring in supine anesthetized spontaneously breathing rabbits, regarded as an animal model of obstructive apnea-hypopnea syndrome. 22 rabbits were instrumented to record oro-nasal mask flow, airway opening, tracheal and esophageal pressures, and diaphragm and genioglossus electromyographic activities while breathing either room air or heliox, and, in 12 rabbits, also during the application of continuous positive airway pressure (CPAP; 6 cmH₂O). For the group, heliox increased peak inspiratory flow, ventilation (18 ± 11%), peak inspiratory tracheal and dynamic transpulmonary pressures, but in no animal eliminated tIFL, as instead CPAP did in all. Muscle activities were unaffected by heliox. In the presence of IFL the increase in flow with heliox (ΔV̇ifl) varied markedly among rabbits (2 to 49%), allowing the distinction between responders and non-responders. None of the baseline variables discriminated responders and non-responders. However, fitting the Rohrer equation (R = K₁ + K₂V̇) to the tracheal pressure-flow relationship over the first 0.1 s of inspiration while breathing air allowed such discrimination on the basis of larger K₂ in responders (0.005 ± 0.002 versus 0.002 ± 0.001 cmH₂O·s²·ml^-2; P < 0.001), suggesting a corresponding difference in the relative contribution of laminar and turbulent flow. The differences in ΔV̇ifl between responders and non-responders were simulated by modeling the collapsible segment of the upper airways as a non-linear resistor and varying its pressure-volume curve, length, and diameter, thus showing the importance of mechanical and geometrical factors in determining the response to heliox in the presence of tIFL.NEW & NOTEWORTHY In an obstructive sleep apnea rabbit model, heliox never abolishes tidal inspiratory flow limitation (IFL), but increases inspiratory flow and tidal volume, substantially in some and nearly nil in other animals. Positive response to heliox cannot be predicted on the basis of breathing pattern characteristics or upper airway resistance that preceded IFL onset, but is related to the mechanical and geometrical features of upper airway collapsible segment, as indicated by model simulation.