Noninvasive measurement of heartbeat, respiration, snoring and body movements of a subject in bed via a pneumatic method

We have developed a noninvasive pneumatics-based system by which to measure heartbeat, respiration, snoring, and body movements of a subject in bed. A thin, air-sealed cushion is placed under the bed mattress of the subject and the small movements attributable to human automatic vital functions are measured as changes in pressure using a pressure sensor having an almost flat frequency response from 0.1 to 5 kHz and a sensitivity of 56 mV/Pa. Using the newly developed system, heartbeat, respiration, apnea, snoring and body movements are clearly measured. In addition, the optimal signal-to-noise (S/N) ratio by which to evaluate the reliability of the heart rate measurement is presented. Heart rates were measured for four different body postures, 13 different subjects, four different bed mattresses, and three different sensor positions. For these measurements, the S/N ratios ranged from 15.9 to 23.5 dB, and so were determined to be reliable.

I. Introduction

The HOME health care system is an important part of the national welfare system. Noninvasive bio-measurement is central to the home health care system. A variety of measurement devices have been presented in the literature. As examples of such devices, a finger-ring plethsmographic sensor [1] and a watch-type vital-sign sensor [2] have been proposed. These devices are not truly noninvasive because subjects may experience physical and psychological stress. Although bed sensing methods [3]–[10] restricts measurement to the time when subjects are in bed, such methods are truly noninvasive. One bed sensing method employs a static charge sensitive bed [3], which detects the heartbeat, respiration, and body movements of the subject. Another method uses piezo-elements to detect body movements [4]. These methods are not always stable, and when the human body is not in contact with the sensors, these methods fail to detect bio-signal and are easily corrupted by noise. Furthermore, because these methods use electricity, they may be invasive in the sense of exposing subjects to electromagnetic waves. Some of these methods induce high levels of energy, such that polygraph signals are strongly corrupted.