Comparison of respiratory event detection by a polyvinylidene fluoride film airflow sensor and a pneumotachograph in sleep apnea patients

Study objectives: Compare the ability of a polyvinylidene fluoride (PVDF) thermal sensor and a pneumotachograph to detect respiratory events in patients with obstructive sleep apnea. Design: Single night of monitoring, single blinded scorer. Setting: Veterans Affairs medical center. Patients: Ten male subjects with obstructive sleep apnea. Interventions: Nasal-oral airflow was simultaneously detected by a PVDF thermal sensor attached to the upper lip and a pneumotachograph in a mask over the nose and mouth. Measurements: Events were scored from display, views showing only the airflow tracings of the sensor in question and the events scored from that sensor. The apnea-hypopnea index was computed using two definitions for hypopnea. Hypopnea-1 was defined as a 50% reduction in flow for >= 10 s in duration. Hypopnea-2 was defined as any reduction in airflow for >= 10 s associated with a 3% drop in the arterial oxygen saturation or followed by an arousal. The level of agreement (K) for the sensors was determined by comparing whether or not they identified candidate events determined by a second blinded scorer. Results: For the apnea-hypopnea-1 index (mean +/- SD), the event rate for the pneumotachograph (26.0 +/- 27.9 events/h) was slightly greater than that for the PVDF sensor (20.1 +/- 27.1 events/h; p < 0.05). For the apnea-hypopnea-2 index, the event rate for the pneumotachograph (29.4 +/- 26.8 events/h) and for that of the PVDF sensor (26.4 +/- 25.9 events/h) were similar (difference not significant). The mean 2 SD difference was 3.0 +/- 8.5 events/h. The level of agreement between the sensors was in the "good range," whereby kappa = 0.69. For 20 randomly, selected breaths per patient, the maximum deflections of the PVDF sensor varied linearly with pneumotachograph airflow deflections. Conclusion: The PVDF sensor compared favorably with a "gold standard" method of detecting respiratory events during sleep in patients with obstructive sleep apnea.