Inspiratory flow limitation (IFL) 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 IFL requires upper airway or intrathoracic pressure measurements. We examined the feasibility and accuracy of the analysis of the flow-volume loop of a tidal breath in identifying IFL. The tidal volume was obtained by integration of the instantaneous airflow signal, and 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 a 50% of the respective (inspiratory or expiratory) portion of the tidal volume, and a breath-by-breath analysis of the mid tidal volume flow ratio (I/E ratio) was obtained. There was a positive significant relationship between I/E ratio and V.sub.Imax (maximal inspiratory volume) for flow- limited breathing (correlation coefficient range: 0.25-0.54). With a lower limit of the normal I/E ratio threshold of 0.97, the sensitivity and specificity of the method were both 76%. Patients having a I/E ratio lower than 0.97 to 1 are classified as suffering IFL. Therefore, the prevent invention relates to the above-described non- invasive method which is applicable in the evaluation IFL and to an apparatus measuring I/E ratio and correcting IFL.
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