[Methodologic study for measuring oronasal airflow using thermistors]

In an attempt to simplify the diagnostic work-up of sleep apnea syndrome, a method has long been sought that would permit the recording of apnea on an ambulatory basis. To this end, we have since developed a face mask and special electronics capable of recording expiration and inspiration via thermistors. We made use of a respirator and a ventilation mannikin in order to simulate respiratory activities in the patient. Since the heat given off during respiration is influenced by a variety of factors, we investigated the following parameters: room temperature (12, 17 and 22 degrees) respiratory rate (15, 18, 21, breaths per minute), maximum airflows (0.5 and 0.6 l/sec) and tidal volume (TV 0.3, 0.5, 0.7 l). Erroneous recording of apneic episodes was observed only with changes from complete mouth to nose breathing; otherwise, the apneic episodes were correctly recognized and counted. TV following an apneic phase must exceed at least TV/2 in order to be recorded as a breath. With a set duration of apnea of more than 30 seconds, apnea is considered to be terminated only by a breath of more than 60% of the last amplitude preceding the apneic phase. Changes in the respiratory rate and maximum airflow, or a change between 12 an 22 degrees within the mask has no effect on the recording of apneic phases. We conclude, therefore, that devices for the automatic detection of apnea should be calibrated by simulated respiratory activities, and specified accordingly.