A NEW RESPIRATORY RATE MONITOR - DEVELOPMENT AND INITIAL CLINICAL-EXPERIENCE

The need for continuous, noninvasive, and reliable respiratory rate mo nitoring during recovery from general anesthesia has long been recogni zed. Alternative principles can be grouped into those detecting the re spiratory effort, and those detecting the actual result, i.e. the resp iratory gas flow. The second category is of greatest interest for pati ent monitoring. In this paper, we report the development and initial c linical experience with a new acoustic air-flow sensor. By differentia l, multipoint detection of the air-flow in the mouth and nose region, the sensor can easily discriminate against different kinds of interfer ence, including motion artefacts. The sensor is nonexpensive, rugged, simple to apply, and inherently safe. An instrument with continuous di splay of respiratory rate, and an audiovisual apnea alarm has been des igned and built. The complete system has been tested on patients durin g recovery after general anesthesia. In 16 patients, the respiratory r ate displayed by the instrument has been correlated against that visua lly observed. A good correlation was obtained. Minor discrepancies can be explained from the fact that visual observation corresponds to the respiratory effort, whereas the sensor detects the actual air flow. I n 12 patients, 24 hour simultaneous recordings were made of respirator y rate with the new sensor, with simultaneous recording of the oxygen saturation and the heart rate with a pulse oximeter. It was found that the new sensor reliably recorded respiratory depression and apnea. Su ch events may in some patients be as frequent as one incident per hour . One case of 'Ondine's curse' provided clear evidence that pulse oxim etry has a low sensitivity to respiratory disorders.