Diagnosing respiratory events and tracing air flow by internal thermistors

We have developed a new method to measure flow in patients with sleep-related breathing disorders (SRBD). These flow sensors are modified thermistors located in the same sensors we use for pressure measurement in the upper airways to find the obstructive segments during apnoeas. The aim of this study was to test if using internal thermistors as indicator of air flow has advantages compared with the external thermistor method in detecting respiratory events. A total of 50 consecutive patients with an apnoea-hypopnoea index (AHI) of more than 15 were studied. A standard nocturnal polysomnography (PSG) with both internal and external thermistors was performed in all patients. To estimate the patients' AHI, a detailed analysis viewing all parameters except external and internal thermistors was performed. This was followed by an analysis viewing only internal thermistors signals indicating airflow in the pharynx, and finally an analysis viewing only external thermistor signals indicating air flow at the mouth and nose. Mean AHI measured by the three methods showed 49.8 (SD 23.4) by the PSG, 47.8 (SD 24.9) by internal thermistors alone, and 31.5 (SD 22.2) by external thermistors. There was no statistical difference between AHI detected by PSG and internal thermistors, but highly significant differences between PSG and external thermistors (p < 0.001). The external thermistors missed an average of almost 20 respiratory events per hour.