Effect of heliox breathing on tidal expiratory flow limitation and dynamic hyperinflation in chronic heart failure patients

Background: Patients with chronic heart failure (CHF) exhibit orthopnea and tidal expiratory flow limitation in the supine position. At present it is not known if the flow limiting segment occurs in the peripheral or in the central part of the tracheobronchial tree. Objectives: The location of the flow limiting segment can be inferred from the mechanical response of the respiratory system to heliox (80% He, 20% O2) breathing. If the choke point were located in the central airways, where flow is limited by the wave-speed mechanism, the use of heliox should increase the maximum available expiratory flow and reduce the associated dynamic hyperinflation. If the choke point were located in the peripheral airways, where maximal flow is limited by a viscous, density independent mechanism, heliox breathing should have no effect on flow limitation and dynamic hyperinflation. Methods: 14 patients with CHF were studied sitting and supine, during air and heliox breathing. Tidal expiratory flow limitation was assessed with the negative expiratory pressure (NEP) technique and dynamic hyperinflation from changes of inspiratory capacity. Results: No patient exhibited tidal expiratory flow limitation in the sitting position. In contrast in the supine posture 8 patients exhibited tidal expiratory flow limitation. Heliox breathing had no effect on flow limitation and dynamic hyperinflation, and only minor effects on breathing pattern. Conclusions: These findings indicate that during spontaneous breathing at rest the choke point in CHF patients is located in the peripheral airways, where flow is limited by a viscous mechanism.