Abstract
The importance of compact-size medical equipment which is capable of wireless monitoring of human patients to evaluate their clinical condition has been growing in recent years. To achieve this goal, we have been striving to develop an earphone-type small and wireless sensor which could enable non-invasive measurement of human vital signs, including heart rates. We developed such a sensor, and compared its precision with a conventional monitor in our study. Using anesthesized rabbits (white Japanese cross-bred female rabbits, weighing approximately 3kg) with the earphone sensors mounted on their external auditory canal, we evaluated their heart rate by measuring the changes in pressure as well as the vibratory dynamics within the canal. The heartbeat and pressure changes could be obtained as heart pressure signals within a range of approximately 2Hz to 20Hz by means of sealing the canal using our earphone sensors. Theoretically, the heart pressure signals are thought to represent the small changes in pressure generated from the ear drum and the ear canal in association with their heartbeat. The rabbits underwent general anesthesia and a conventional monitor or the earphone-type sensor was mounted on the rabbits in our experiments. Subsequently, the rabbits were placed in an environment with a progressively reduced oxygen concentration (reduced by 5% every 10minutes); thereafter, the heart rate results based on both sensors were compared for reference. Our earphone-type sensor was capable of measuring the heart rate by analyzing the heart pressure signals based on a data processing program. The data obtained by the earphone-type sensor could be successfully validated by these animal experiments. When the heart rate values were plotted with the data obtained by conventional monitors on the horizontal axis and the data by the earphone-type sensor on the vertical axis, both data showed statistically significant relationships. When the heart rate was changed by decreasing the oxygen concentration, the root mean squared error of more than 5% oxygen concentration was less than 11. The earphone-type vital sign (heart rate) sensor is definitely useful in terms of its tiny size, wireless and non-invasive nature and its measurement accuracy as well.
