Montserrat and colleagues showed that the square root of the signal from a standard nasal cannula attached to a sensitive differential pressure transducer was comparable to the tracing from a pneumotachograph, obviating the need for an obtrusive, tight-fitting face mask. Hosselet et al showed similar results. An earlier publication described the increased sensitivity of this device when compared to a nasal thermistor in 11 OSAS and 9 UARS patients; even liberal thermistor criteria, defined as any change in the appearance of the tracing, detected only 75% of the events in the OSAS patients and 38.6% of the events in the UARS patients that had been detected using a nasal cannula. A flow-limited (flattened) pattern was included in the scoring of events by nasal cannula. This particular investigation, however, did not include an assessment of accuracy by comparison with a face mask/pneumotachograph. The authors reported that pressure swings of 0.5 cm h2o were detected by nasal cannula during quiet breathing. The pressure transducer used in this type of system should be able to detect differential pressures in the range of ±5 cm H2O, and a direct current or an alternating current amplifier with a time constant of > 5 s is necessary to correctly identify the flow limitation. asthma inhalers
Hosselet and colleagues also reported the use of the nasal cannula system in a more advanced clinical application. They studied 10 symptomatic and 4 asymptomatic subjects with an esophageal or supra-glottic pressure catheter and a nasal cannula/differential pressure transducer. The symptomatic subjects included one snorer, five OSAS patients, and four UARS patients. A statistically significant correlation existed between the calculated upper airway resistance and the shape of the inspiratory flow tracing; high resistance correlated with the flattening of the inspiratory flow tracing, interpreted as flow limitation. The symptomatic group showed a significantly greater percentage of flow-limited breaths. The investigators did not use the square root of the nasal cannula signal, resulting in what they interpreted as increased sensitivity; therefore, they chose an RDI of > 20 rather than 10 as abnormal.