MEDICAL ELECTRONICS FEATURE
Non-contact heart rate monitor using MEMS technology
Ulf Meriheinä, Senior MEMS Application Specialist at Murata Electronics Oy discusses how the latest non intrusive contactless heart monitoring equipment is offering advanced patient care
EMS (Micro Electro Mechanical System) accelerometers, pressure sensors and gyroscopes are finding an increasing number of applications in healthcare. One new application is in Ballistocardiology (BCG) – monitoring the function of the heart through the recoil effect caused by the blood flowing into the aorta. Compared to the traditional way of
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measuring the functionality of the heart using Electrocardiogram (ECG) with a set of electrodes attached to or in contact with the body, in BCG there are no electrodes and the measurement in addition to timing data also delivers information on the volume of every stroke. Thus this is a completely non-intrusive measurement. BCG offers good perspectives for application in preventive medicine, e.g. in detection of physical or mental stress and sleep quality or in the early detection of pathological conditions or attacks. The mechanical BCG signal follows the
electrical signal with a delay of 30…40ms. The amplitudes of the waves (Fig. 2) are a measure of the heart's stroke volume and from the timing one can see the general functionality of the heart as well as the heart rate and its variability, the latter clearly describes the recovery state or stress of the person measured. Murata has developed BCG sensor
technology (Fig. 1) utilising its very low noise accelerometer components with a noise density less than 20μg/√Hz, a level necessary for measuring the BCG signal. The technology also offers other
important features for the reliability of the measurement, a controlled frequency response with mechanical damping and a 50 000 g shock resistance. Advanced filtering algorithms in the embedded software (SW) in the module separate the BCG signal e.g. from the effects of the person moving, noise or bed resonances. SW detects every beat (Fig. 2: *). It reports the time for Heart Rate (HR) and Heart Rate Variability (HRV), the amplitude (Stroke Volume (SV)) for Stroke Volume Variability (SVV) as well as a status number based on the signal level. The filtered status indicates bed occupancy, i.e. “0” means empty bed, “1” occupied bed and “2” person moving in the bed respectively. This parameter as such can be used for bed occupancy analysis or together with HRV it is very
respiration. Using HRV and SVV one can construct an index describing sleep cycles and the average of which being a measure of recovery. By combining filtered status and sleep cycles you get the total sleep time and the number and length of interruptions during the night. One can then add to this the relative amount of recovery and the result is a potential SQI. The formula for SQI could be something like: SQI = Iduration + Icontinuity + Irecovery = = (tsleep/tref) + (1 – 4 x n x
tawake/tsleep) +
HRVrel.increase (1) Here tref is the average adequate length
useful for creating a Sleep Quality Index (SQI). When the person is moving in the bed, the BCG signal cannot be measured, but this is an important status for SQI. This status does not normally occur very frequently and as a consequence does not really affect overnight statistical analysis for e.g. sleep cycles or recovery. The parameters reported by the BCG
measurement can be filtered for overnight analysis. For sleep cycle and recovery analysis the most important parameters are HRV, HR and SVV, giving information on the sympathetic and parasympathetic nervous systems and the effects of
Figure 1: BCG sensor module and wireless sensor node
of sleep and n the number of interruptions respectively. Murata has analysed the recovery of young sports athletes using the above formula. The BCG measurement also gives the opportunity to have early indication on pathological conditions and potential near future sudden attack, in case they can be seen as an increased stress level in the autonomous nervous system. In addition overnight arrhythmia can be statistically analysed e.g. using a Lorentz plot, i.e. by plotting on the horizontal axis the nth Beat-to-Beat (B2B) time and the following one ((n+1)th) on the vertical axis. BCG is a totally non-intrusive cardiologic
measurement with a large variety of application areas. These include for example, home healthcare, elderly and infant care, sleep quality monitoring, athletes’ recovery monitoring and hospital care.
Figure 2:
Measured beats and status
Murata
www.murata.com 01252 811666
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ELECTRONICS | FEBRUARY 2015
S7
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