Internal thermistors in differentiating between oral and nasal breathing during sleep

To select patients with sleep apnoea hypopnoea syndrome (SAHS) who will benefit from surgery, we use information from four different pressure sensors in the upper airways and oesophagus during polysomnography (PSG). These pressure sensors also have the ability to act as internal thermistors and can hence indirectly measure flow as well as pressure. This new method for measuring flow has proven to be very accurate for scoring hypopnoeas as well as apnoeas. The aim of this study was to determine whether the flow and pressure sensors located in the epipharynx, oropharynx and hypopharynx could differentiate between nasal and oral breathing. The design was a prospective cross-over study in 124 patients referred to the hospital for SAHS diagnosis. The awake patients were asked to breathe first through the nose and then through the mouth while the nose was blocked with a clip. A standard nocturnal PSG with pressure and flow measurement in the upper airways was performed in all patients. The procedure was repeated in a lateral position, and again the next morning in 32 of the patients. Reduction in flow signals from the nose was calculated, and a paired t-test was performed for statistical analysis. The difference between nasal and oral breathing was quite distinct in the flow tracings from the internal thermistors. The mean reduction in nasal flow signals when changing from nasal to oral breathing was 83.7% (SD 14.5, p < 0.0001). The same was seen in a lateral position, 82.2% (SD 16.4, p < 0.0001). Testing for changes in properties of the internal thermistors revealed no significant difference between the reduction in flow when the test started and after the patients had slept the whole night (p > 0.1). It is possible to differentiate between nasal and oral breathing using internal thermistors.