1. Inspiratory flow limitation is involved in the pathophysiology of sleep-related breathing disorders. Since the definition of flow-limited cycle is based on a dissociation between flow and respiratory efforts, identification of inspiratory flow limitation requires upper airway or intrathoracic pressure measurements. We examined the accuracy of the analysis of the flow—volume loop of a tidal breath in identifying inspiratory flow limitation during sleep in ten patients with a sleep apnoea—hypopnoea syndrome.
2. Measurements were taken during continuous positive airway pressure trials. After data acquisition, the presence of inspiratory flow limitation was identified by the presence of an inspiratory plateau or decrease in inspiratory flow independently of the increase in inspiratory efforts. The flow—volume loop was reconstructed for each breathing cycle by plotting the instantaneous flow and the tidal volume. The instantaneous inspiratory and expiratory flows were measured at 50% of the respective portion of the tidal volume, and a breath-by-breath analysis of the midtidal volume—flow ratio (inspiratory/expiratory ratio) was obtained. The analysis of the flow—volume loop was compared with standard inspiratory flow limitation criteria using different values of the inspiratory/expiratory ratio threshold, below which breathing cycles were classified as flow-limited. With a lower limit of the normal inspiratory/expiratory ratio threshold of 0.97, the sensitivity and specificity of the method were both 76%. In each subject, the proportion of breathing cycles identified as flow-limited according to the inspiratory/expiratory ratio progressively decreased with an increasing positive pressure level.
3. We conclude that analysis of the flow—volume curve is accurate in identifying most of the inspiratory flow limitation breathings in sleep apnoea—hypopnoea syndrome.