1. Field of the Invention
The present application relates to a technique for extracting respiration. More particularly, the present application relates to a technique for measuring impedance of a living body by using a plurality of electrodes placed on a body of a user, and for extracting information regarding respiration.
2. Description of the Related Art
In recent years, methods for measuring and recording body conditions of a user electrically and mechanically over an extended period of time are coming into widespread adoption. Examples of basic electrical information that represents body conditions of a user include an electroencephalogram (EEG) related to a brain and an electrocardiogram (ECG) related to motion of a heart. Among these information items, the electrocardiogram is acquired, for example, as fundamental biological information (vital sign) at a hospital. In addition, the electrocardiogram can also be acquired by using a portable electrocardiograph called a Holter electrocardiograph, if there is suspicion of a cardiac disease. Use of the Holter electrocardiograph enables recording of an electrocardiogram over a long period of time, for example, 24 hours, at a place other than a hospital, such as at home. In recent years, this Holter electrocardiograph has been downsized, and a user can measure the electrocardiogram more simply.
Recording the electrocardiogram over a long period of time by using the Holter electrocardiograph allows detection of symptoms that cannot be detected during a short examination at a hospital, such as arrhythmia. However, there are examination items (cases) that can be detected by a prolonged examination besides the electrocardiogram. Examples of such a case include sleep apnea syndrome. Sleep apnea syndrome is a respiratory disease closely related to arrhythmia.
Estimation of sleep apnea syndrome cannot be performed only with the electrocardiogram, but information regarding respiration is also required. Estimation of sleep apnea syndrome requires an all-night sleep polygraph examination for measuring the electrocardiogram, respiration, and electroencephalogram simultaneously. This examination needs to be performed by a patient staying at a hospital, and is burdensome to both the patient and the hospital. For this reason, in a stage of suspected disease, it is not realistic to perform such burdensome examinations.
If information regarding a respiratory disease, specifically, information regarding a respiratory rate can be acquired as simply as acquiring the electrocardiogram by using the Holter electrocardiograph, earlier detection of a disease and acceleration of diagnosis are likely to be achieved.
So far, a medical device called a pulse oximeter has mainly been used for simple respiratory measurement. This is a measuring instrument for examining arterial oxygen saturation. Arterial oxygen saturation is measured by wearing a sensor, which is called a probe, on a fingertip. This measuring instrument has a red light source or an LED that emits red light, and measures oxygen content contained in an artery inside a finger in real time by measuring transmitted light of the finger with the sensor. In this way, when both the electrocardiogram and respiratory information are required, it is necessary to place electrodes for the electrocardiograph on a thorax, and to wear the probe of the pulse oximeter on a fingertip.
So far, simultaneous acquisition of the electrocardiogram and the respiratory information with one device, and separation of the respiratory information from data acquired by using the electrocardiogram information have been addressed. One approach is an impedance method. According to the impedance method, an electric current is passed through a body of a user, and the electrocardiogram and impedance change due to respiration are measured with electrodes placed on a thorax. For example, NPL 1 describes a method for extracting respiratory information from thoracic impedance in low electric current (10 nA).
Before description of a concept of the method described in NPL 1, basic electrocardiographic components will be described. FIG. 1 illustrates one cycle of basic electrocardiographic components. The electrocardiogram includes peaks called a P wave, a Q wave, an R wave, an S wave, and a T wave. A portion of the QRS waves represents ventricular activation.
FIG. 2A to FIG. 2C illustrate a concept of the method described in NPL 1. In measurement, four electrodes are placed on a center of a thorax (see FIG. 2A). In FIG. 2A, potential is measured with two inner electrodes among the four aligned electrodes, excluding a ground electrode. A low electric current (10 nA) is passed between two outer electrodes. FIG. 2B illustrates thoracic impedance measured from the potential. NPL 1 defines an envelope curve of the T wave of components derived from the electrocardiogram as respiratory information.
In NPL 1, the four electrodes are attached to the center of the thorax to measure thoracic impedance. In an experiment of NPL 1, a subject is asked to breathe four phases of respiration: normal breath, deep breath, stop breathing, and normal breath. The subject was instructed to take a breath 15 times with a three-second cycle during the normal breath phase. The subject was instructed to take a breath eight times with a five-second cycle during the deep breath phase. The subject was instructed to stop breathing for 30 seconds during the stop-breathing phase.
FIG. 2C illustrates an extraction result of respiration. The cycle in the envelope curve has correlation with an actual breath. In addition, amplitude while breathing is stopped is extremely small, and amplitude during the deep breath is also larger than amplitude during the normal breath, and thus extracted respiratory information represents actual breath.