Diagnosing respiratory events and tracing air flow by internal thermistors

We have developed a new method to measure flow in patients with sleep-relat ed breathing disorders (SRBD). These flow sensors are modified thermistors located in the same sensors we use for pressure measurement in the upper ai l-ways to find the obstructive segments during apnoeas. The aim of this stu dy was to test if using internal thermistors as indicator of air flow has a dvantages compared with the external thermistor method in detecting respira tory events. A total of 50 consecutive patients with an apnoea-hypopnoea in dex (AHI) of more than 15 were studied. A standard nocturnal polysomnograph y (PSG) with both internal and external thermistors was performed in all pa tients. To estimate the patients' AHI, a detailed analysis viewing all para meters except external and internal thermistors was performed. This was fol lowed by an analysis viewing only internal thermistors signals indicating a irflow in the pharynx, and finally an analysis viewing only external thermi stor signals indicating air flow at the mouth and nose. Mean AHI measured b y the three methods showed 49.8 (SD 23.4) by the PSG, 47.8 (SD 24.9) by int ernal thermistors alone, and 31.5 (SD 22.2) by external thermistors. There was no statistical difference between AHI detected by PSG and internal ther mistors, but highly significant differences between PSG and external thermi stors (p < 0.001). The external thermistors missed an average of almost 20 respiratory events per hour.