An apnea monitor using a rapid-response hygrometer

Objective. The aim of this study was to detect cyclic changes in the relative humidity (RH) occurring with spontaneous respiration using a rapid-response hygrometer, and to evaluate its potential applicability as an apnea monitor in nonintubated subjects. Methods. Respiratory monitoring using a rapid-response hygrometer was performed in spontaneously breathing, nonintubated subjects. Changes in RH during spontaneous breathing were measured in adult volunteers, breathing room air and nonintubated infants who underwent cardiac catheterization under intravenous anesthesia. The detection of apnea by the hygrometer was assessed in the adult patients during the induction of anesthesia and those undergoing minor gynecologic surgery under epidural anesthesia. The hygrometric sensor was positioned in front of the nostril, a simultaneous recording of CO2 was obtained from nasal cannulas by a sidestream capnograph. Each waveform was collected and acquired by a PC-based computer, and data were analyzed off-line. Results. The hygrometer showed a rapid response to the cyclic changes in RH during spontaneous respiration and could identify respiratory phases of tachypnea as high as 60 breaths per minute in infants. RH rapidly increased to 80% with a plateau on expiration and decreased to 40% on inspiration. These phasic changes, consisting of three distinct phases, expiratory upstroke, plateau and inspiratory downslope, preceded the corresponding capnographic changes by nearly two seconds. Expiratory increase in RH appeared to be influenced by the respiratory flow rate as well as the response time of the equipment, As respiration was depressed due to airway obstruction, the magnitude of RH gradually decreased and then disappeared at the time of apnea. Conclusions. The hygrometer could detect cyclic changes in RH during spontaneous respiration. Apnea was immediately detected by the decreases in the magnitude of RH. The results demonstrate the potential usefulness of a rapid-response hygrometer for monitoring respiratory rate and early detection of apnea.