Contact-free monitoring of human vital signs via a microwave sensor

Guohua Lu, Fang Yang,Xijing Jing,Xiao Yu, Hua Zhang, Huijun Xue,Jianqi Wang Department of Electronics

Fourth Military Medical University Xian,710032, China

lugh1976@fmmu.edu.cn

AbstractHuman vital signs such as heart rate and breathing rate are widely used to assess the health state of human in clinic. Tranditional method uses eletrodes or sensors touching the body to measure electrocardiography (ECG) and respiratory signals.A vital signs monitor via a micorwave sensor was disscused to contact-free measurement of the heart rate and breathing rate. Comparison of vital signs derived from the microwave sensor and tranditional contact monitor demonstrated that there were no significant differences between each other, which suggested the contact-free vital signs monitor may prove a practical alternative method to measure heart rate and breathing rate.

Keywords-vital signs; microwave sensor;contact-free

I. INTRODUCTION Heart rates and breathing rates are attractive and have

the potential to be useful in various clinical settings.Heart rates can be determined by electrocardiography (ECG) recordings using three Ag/AgCl electrodes attached to specific anatomical positions, in accordance with Einthoven. Breathing rates can be determined by respiration sensor around a humans chest or abdomen. However, none of these monitors are truly noninvasive, as they all require that the sensor be physically attached to human body surface. This may limit their usefulness, particularly in sick premature neonates and seriously burned patients.

Contact-free measurement of vital signs via a microwave sensor is a new technique, which can penetrate some non-metal media like wood, clothes and remotely sense the respiration and heartbeat signals without any electrodes or sensors touching the body of human subjects [1-10].

The work described in the paper was aimed at evaluating the use of a microwave sensor for measuring the vital signs including heart rates and breathing rates in healthy subjects. Here we show under controlled research conditions that measures of heart rates and breathing rates derived from the tranditional contact methods and the microwave sensor are similar by comparing 5-minute data segments.

II. DESCRIPTION OF THE MICROWAVE SENSOR The block scheme of the custom-developed contact-free

microwave sensor is shown in Figure 1. The microwave

wave was generated by the oscillator via a directional coupler. The oscillator, made of a GaAs Gunn diode was chosen to meet the demands of low noise and low cost, can also provide linear continuous waves. The oscillator operated at 24 GHz and the maximum transmission power was about 7 mW. The microwave beams were radiated through a two-way parabolic antenna via a circulator. The gain of the antenna is 24 dB, and the beam width is 12 degree. Another signal from the directional coupler acted as a local oscillatory signal for the receiver. The echo signal was received by the antenna and then passed through the circulator to get into the mixer where it was mixed with the local oscillatory signal. The output of the mixer was conditioned by a signal conditioner, composed of an amplifier with the gain of 60 dB, an analog low-pass filter with cutoff frequency of 0.5 Hz, an analog band- pass filter with lower cutoff frequency of 0.7 Hz and the upper cutoff frequency of 5.0 Hz, and a 50 Hz notch filter. The custom developed rechargeable power supply could provide 5 Volt up to 5 Amp Hours and the power consumption of the microwave sensor was less than 3 Watts. Thus the sensor could continuously work over 8 hours after the lithium batteries were fully recharged. The outputs of the signal conditioner were called heartbeat and brathing signals.

Figure 1. The block scheme of the microwave sensor.

III. SIGNAL RECORDING AND ANALYSING For recording of the electrocardiogram, disposable

Ag/AgCl resting ECG electrodes (Red Dot-2352; 3M Company; MN, USA) were attached to the lower of left

Signal conditioner

Oscillator Directional coupler

Circulator

978-1-4244-5089-3/11/$26.00 2011 IEEE

chest (Ground), upper of right chest (Negative) and the upper of left chest (Positive). Wires from the electrodes (LEAD108A;BIOPAC Systems Inc.; Goleta, CA, USA) were attached to the ECG amplifier (ECG100C; BIOPAC Systems Inc.). For recording of the respiratory signal, the respiration sensor (TSD201; BIOPAC Systems Inc.) around the chest surface connected to the respiration amplifier(RSP100C; BIOPAC Systems Inc.).

Outputs of ECG100C, RSP100C and the microwave sensor were connected to a 16-channel A/D converter (MP150; BIOPAC Systems Inc.), which was in turn directly connected to a desktop computer through a 10M / 100Mbps Ethernet adapter. All the signals were sampled at a frequency of 1000 Hz simultaneously recorded for 20 minutes using the AcqKnowledge software package (v3.8.1; BIOPAC Systems Inc.) and saved to a text file for further processing.

Sixteen volunteers (16 males; 23.0 4.0 years (mean S.D.)) participated in this study. Ethic Committee of the Fourth Military Medical University approved the study. All subjects were healthy and informed consent was obtained prior to their participation. Subjects sat on a chair and remained still throughout the recording period when they were instructed to minimize their movement. The distance between the subject and the microwave sensor was 3.6 m.

A. Calculation of heart rate and breathing rate Three 5-minute segments derived from the microwave

sensor ,ECG and respiration sensor were selected by an experienced researcher. Power spectra of the selected segments were obtained by Welchs method into 50% overlapping sections. A Hamming window of 1024 data points was used to reduce the variance of the resulting spectra estimate. The heart rate and breathing rate were calculated by the frequency occurrence of the peak of the spectra time sixty. The average of the three heart rates and breathing rates was used to analyze the relationship between the microwave sendor and BIOPAC System.

B. Statistical analysis To assess how similar vital signs derived from the

microwave sensor with those d from the BIOPAC System, correlation analysis was performed. Before analyses, raw values of all variables were examined for deviations from normality by the KolmogorovSmirnov test. Statistical analysis was performed in SPSS@ (v13, SPSS Inc.; Chicago, IL, USA) and graphs were plotted using Origin@ (v7.5776, Northampton, MA, USA).The level of significance was set at p < 0.05 (two-tailed).

IV. RESULTS AND DISCUSSION In order to compare the difference between the vital

signs extracted from micorwave sensor and Biopac system recordings, the linear regression analysis is used. The results are showed in Fig.2. and Fig.3.. The horizontal ordinates expresse the breathing rate and heart rate derived from Biopac system,respectively.And the vertical ordinates demonstrate the breathing rate and heart rate derived from the microwave sensor.Both of the figures showed there is

strong correlation between the vital signs from the microwave sensor and the trandional contact recording system,i.e. the vital signs are similar with each other, which suggested the contact-free vital signs monitor via a microwave sensor may prove a practical alternative method to measure heart rate and breathing rate.

12 14 16 18 20 2212

14

16

18

20

22

Bre

athi

ng ra

te (B

eat/m

in)

Breathing rate (Beat/min)

Figure 2. Correlation between the breathing rates derived from microwave sensor and respiration sensor. Solid line=linear regression, Outer dashed line=upper and lower 95% confidence intervals. (2=0.92, p

respiration is overlapped with that of the heartbeat, thus it is difficult to separate the heartbeat signal from the overlapped signals using the simple analog or digital filter. Adaptive filter might be used to reduce the impacts of the interferences above on the heart beat recorded from the microwace sensor .

Here we showed heart rate and breathing rate can be remotely monitored without any electrodes and sensors touching the subject, which will make the subject feel relaxed and comfortable. Vital signs monitor made from this technology can be widely used in clinic, such as seriously burned patients, infectious disorders patients and new-born baby , as well it can be used in the filed of psychophysiology study.

ACKNOWLEDGMENT This work was supported by the General Department of

Logistics of the PLA, China (No. 10KS01) and Shaanxi Technology Committee (No. 2010K01-204).

REFERENCES [1] C. G. Caro and J. A. Bloice, "Contactless apnoea detector based on

radar," Lancet, vol. 2, Oct. 1971,pp. 959-961. [2] C. I. Franks, B. H. Brown, and D. M. Johnston, "Contactless

respiration monitoring of infants," Med. Bio. Eng., vol. 14, May. 1976,pp. 306-312, doi: 10.1007/BF02478126.

[3] Y. Schutz, E. Ravussin, R. Diethelm, and E. Jequier, "Spontaneous physical activity measured by radar in obese and control subject studied in a respiration chamber," Int. J. Obes., vol. 6, Jan. 1982,pp. 23-28.

[4] G. Matthews, B. Sudduth, and M. Burrow, "A non-contact vital signs monitor," Crit. Rev. Biomed. Eng., vol. 28, Jan. 2000,pp. 173-178.

[5] M. Uenoyama, T. Matsui, K. Yamada, S. Suzuki, B. Takase, S. Suzuki, M. Ishihara, and M. Kawakami, "Non-contact respiratory monitoring system using a ceiling-attached microwave antenna," Med. Biol. Eng. Comput., vol. 44, Sep. 2006, pp. 835-840,doi: 10.1007/s11517-006-0091-8.

[6] K. M. Chen, Y. Huang, J. Zhang, and A. Norman, "Microwave cardiopulmonary signs monitoring systems for searching human subjects under earthquake rubble or behind barrier," IEEE Trans. Biomed. Eng., vol. 47, ,Jan. 2000,pp. 105-114,doi: 10.1109/10.817625 .

[7] C. W. Wu and Z. Y. Huang, "Using the phase change of a reflected microwave to detect a human subject behind a barrier," IEEE Trans. Biomed. Eng., vol. 55, Jan. 2008,pp. 267-272,doi: 10.1109/TBME.2007.910680 .

[8] M. Brink, C. H. Muller, and C. Schierz, "Contact-free measurement of heart rate, respiration rate, and body movements during sleep," Behav. Res. Meth., vol. 38, Aug. 2006, pp. 511-521.

[9] H. J. Kim, K. H. Kim, Y. S. Hong, and J. J. Choi, "Measurement of human heartbeat and respiration signals using phase detection radar," Rev. Sci. Instrum., vol. 78, Oct. 2007,pp. 104703,doi: 10.1063/1.2798937.

[10] K. H. Chan and J. C. Lin, "Microprocessor-based cardiopulmonary rate monitor," Med. Biol. Eng. Comput., vol. 25, Jan. 1987,pp. 41-44,doi: 10.1007/BF02442818.

[11] J. Q. Wang, C. X. Zheng, X. J. Jin, G. H. Lu, H. B. Wang, and A. S. Ni, "Study on a non-contact life parameter detection system using millimeter wave," Space Med. Med. Eng., vol. 17,Jun. 2004, pp. 157-161.

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 200 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 2.00333 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 400 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 600 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.00167 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False

/CreateJDFFile false /Description > /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [ > /FormElements false /GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks false /IncludeInteractive false /IncludeLayers false /IncludeProfiles true /MultimediaHandling /UseObjectSettings /Namespace [ (Adobe) (CreativeSuite) (2.0) ] /PDFXOutputIntentProfileSelector /NA /PreserveEditing false /UntaggedCMYKHandling /UseDocumentProfile /UntaggedRGBHandling /UseDocumentProfile /UseDocumentBleed false >> ]>> setdistillerparams> setpagedevice