Recently, RRa monitoring has been reported to be more effective than impedance methods or capnometry to measure RR [10,11,12]. Most of these studies, however, do not take into account the differences in settings of the RRa monitor, such as averaging time and respiratory pause time. In the present study, we examined the effects of changes in RR monitor settings on the results of RRa measurement under two averaging time settings and two respiratory pause time settings.

In the apnea test, the length of averaging time of 10 or 30 s had little effect on the detection time of apnea, but the differences of respiratory pause time setting, 20 or 40 s, had a significant effect on it. In the tachypnea test, on the other hand, a shorter averaging time of 10 s resulted in a shorter detection time and in a greater number of detections of 30 breaths/min within 60 s than a longer averaging time. If the RR is maintained at a constant rate, it should be displayed on the monitor within the set averaging time. The finding that the RR did not decrease to 0 breaths/min within the respiratory pause time suggests that the RRa monitor may have mistakenly identified ambient noise, throat movement, coughing, body movement, or airflow in the airway as breathing [13,14,15]. However, this does not explain why the monitor did not display 30 breaths/min within the set averaging time during the tachypnea test. Although the reason for this is not clear, a sudden change of the respiratory pattern from resting respiration to tachypnea of 30 breaths/min in the present study may result in delayed recognition. The RRa monitor has an RR Fresh Time out function for eliminating the effect of sounds such as talking, eating, and drinking, which displays the RR up to the point before picking up these sounds.

There is a limitation to the present study. In the tachypnea test, changes in breathing patterns were very sudden. In real-life clinical practice, however, not all increases and decreases in RR are so rapid and it cannot be ruled out that the suddenness of these changes may have affected RR measurements.