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Immediate Physiological Responses to Inspiratory Flow Limited Events in Mild Obstructive Sleep Apnea.

RATIONALE
Inspiratory flow limitation (IFL), characterized by flattening of individual breaths on the airflow/time tracing, is a non-invasive indicator of elevated upper airway resistance. An IFL "event" in isolation has not been defined, nor has the ability to reproducibly identify event occurrence been tested. IFL events and their association with immediate physiological responses, as well as the impact of characteristics such as age, sex, sleep stage, sleepiness and event duration on their association with such outcomes also has not been studied. Symptomatic patients with a normal to mildly abnormal apnea hypopnea index (AHI) who have predominant IFL on their polysomnography may benefit from treatment.


OBJECTIVE
To test the reproducibility of identifying IFL events and their termination and to determine the frequency of the immediate physiological response to their occurrence, including desaturation, EEG arousal and increased heart rate.


METHODS
Fifty-eight patients with obstructive sleep apnea underwent full diagnostic polysomnography. IFL events and their termination were identified manually using predefined rules from the unscored nasal cannula flow channel alone and evaluated for responses such as EEG arousal, oxygen desaturation of ≥ 3% and heart rate increase.


RESULTS
Interscorer reliability was acceptable with an average percent agreement for occurrence of 82 ± 3%. 24% of all IFL events regardless of the definition were not associated with an EEG arousal, increase in HR or O2 desaturation. Of all IFL events scored, 25% caused O2 desaturation, 40% were associated with an EEG arousal, and 55% with an increase in HR. 67% caused either an EEG arousal and/or increase in HR. Responses were observed to occur either in isolation or in combination. IFL events that terminated with at least two non-IFL breaths, one of which had a 200% increase in amplitude, were significantly associated with O2 desaturation, EEG arousal and increase in HR compared to events that ended in one non-IFL breath. IFL events that had a greater than 50% reduction in flow amplitude compared to baseline were significantly associated with O2 desaturation compared to events that had a 30% reduction or less.


CONCLUSION
Most IFL events resulted in immediate physiological responses and no single consequence reliably occurred after every event. We propose a framework that can incorporate scoring of IFL events into assessing the diagnosis and severity of OSA and suggest that no single consequence be used to define IFL as a respiratory event. The relationship of IFL events to OSA outcomes remains to be tested.