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T. Yamakawa et al. / Journal of Advanced Research in Physics 3(1), 011205 (2012) 1 A Low-Cost Long-Life R-R Interval Telemeter with Automatic Gain Control for Various ECG Amplitudes Toshitaka Yamakawa1, 2, *, Genki Matsumoto3, and Toru Aoki4 1 Faculty of Engineering, Shizuoka University Fuzzy Logic Systems Institute 3 Department of Bioengineering, University of Toledo 4 Research Institute of Electronics, Shizuoka University 2 Abstract — A low-cost R-R interval telemeter with automatic gain control is the innovative ECG measurement system for both clinical care and homecare. Highly-reliable R-R interval detection was achieved by the automatic gain control for subjects at different ages. The gain is automatically adjusted to extract R waves at the desired voltage amplitude. The high accuracy detection with automatic gain control was proved by the estimation of error occurrence at each subject. The errors of R-R interval detection did occur at less than 2% probabilities. The accuracy detection of long-term measurement is also a crucial issue for the appropriate diagnosis. The results showed 0.752% probabilities of reoccurring errors when 48 hours measurement was executed. Therefore, a low cost R-R interval telemeter with automatic gain control is the handheld device with the high accuracy detection of R waves and the high reliability of measurement at different ages. Keywords — Electrocardiography, R-R interval, biomedical telemetry, automatic gain control I. INTRODUCTION Low cost means of vital signs monitoring are demanded according to that healthcare costs are rapidly increasing. As a mean for early detection of diseases and of acute symptoms, long-term monitoring of electrocardiogram delivers beneficial evidences in both clinical care and homecare. The analysis of R-R intervals on long-term monitoring gives diagnostic evidences of arrhythmia [1], chronic obstructive pulmonary disease [2], Parkinson’s disease [3], and etc. The development of low cost RR interval telemeter has been aimed at the simple uses, high mobility, and high-accuracy detection. This telemeter is regarded as the consumer friendly device of what can be commercially available at lower cost and used by any subjects at different ages. Telemetry system is the supportive monitoring device since the collected data are able to be transferred as the files with lower size into hospital’s monitoring systems to obtain an immediate diagnosis. The measurement of R-R interval at different sex and ages can be achieved by the automatic gain control system that is * Manuscript received September 1, 2011. Corresponding author: ([email protected]) capable of adjusting the gain automatically for different subjects to detect R wave. This innovative program plays an important role on the high accuracy detection and the quick adjustment of gain. ECG measurement performed at home is an ideal financial aid for patients and even for healthy people as a mean of health promotion and early detection of disease. As the device used by patients’ hand at home, the important matter to be considered is lower cost, easier usage, and longer life with modest measurement performance. In order to confirm that the proposed system satisfies the above needs, the experiment was directed for the analysis of the function of automatic gain control and the accuracy of long term measurement. II. SYSTEM DESCRIPTION The proposed telemetry system consists of two devices: the R-wave telemeter and the receiver. The R-waves telemeter measures the electrocardiogram (ECG), detects the R-waves from the ECG, and wirelessly transmits R-wave occurrences with 315MHz amplitude modulation. The receiver receives and demodulates the wireless signal from the telemeter, calculates the R-R interval, and stores into a PC through the USB connection. The details of the devices are explained in following subsections. A. R-waves telemeter Circuit Description Fig. 1(a) and (b) show the block diagram of the proposed R-wave telemeter and the circuit diagram of the analog frontend, respectively. The ECG measured by three electrodes (+, -, GND) is amplified and conditioned in the analog frontend. In order to achieve the single power supply of a 3 V battery CR2032, the common-mode feedback (CMFB) structure is adopted for the differential signal amplification as shown in the left dashed rectangle in Fig. 1(b). By using this structure, the output common-mode voltage of the instrumentation amplifier is set to Vref what is the reference voltage made by a resistor chain. The 1st-order variable-gain low-pass filter was designed to reduce the high-frequency noise (>38Hz). The gain is defined by the resistor ratio R2/RC where RC is the resistor value of the variable resistor. Here, RC is controlled by the automatic gain T. Yamakawa et al. / Journal of Advanced Research in Physics 3(1), 011205 (2012) control program which is embedded in the microcontroller, as described later. Though this LPF reduces the amplitude of the R-waves, the sufficient amplitude is obtained since it is adequately amplified due to the automatic gain control. The output common-mode voltage of the analog frontend is set to Vref, since the positive nodes of OPAs are connected to Vref. Here, Vref of 2.49 V was obtained by a resistor chain. The power dissipation of the analog frontend was kept under 1.2 mW by adopting the following low power ICs: INA122 for the instrumentation amplifier, MCP6042 for OPAs, and MCP4011 for the variable resistor. R1 of 1MΩ, R2 of 1MΩ, C1 of 1µF, and C2 of 4.7nF were used in this prototype fabrication. The output of the analog frontend was connected to an input node of a MSP430F2011 microcontroller as shown in Fig. 1(a). This input node is observed by the built-in comparator, and the comparator outputs the positive pulse during the input signal is lower than 1.5 V. In other words, the amplified ECG signal whose amplitude is larger than 0.99 V (=Vref-1.5) is extracted as an R-wave candidate. The R-wave candidates are treated by the embedded program as described in following subsections. The transmitter module modulates the detected R-wave signal with 315 MHz On-Off-Keying (OOK) modulation, and transmits it through an antenna wire within the output power of 10 mW. Digital Filter and the R-Wave Detection Program Mounted on the Microcontroller A quasi-band-pass filtering program and the R-wave detection program were installed in the microcontroller as shown in Fig. 1(a) by “BPF” and “R or not?” blocks. The BPF program counts the pulse width of the R-wave -candidate signal which is obtained by the comparator, and forwards the signal only where the pulse width is in the range from 3 ms to 50 ms in order to compress the noises (e.g. HF noise originated in the poor hysteresis of the comparator, wider ECG pulses such as P or T waves). The R-wave detection program counts the pulse interval (assuming R-R interval) of the BPF output. Only where the Automatic Gain Control Scheme The amplitude of ECG can be reduced due to the electrodes alignment, the condition between skin and the electrodes, and the subjects’ age. In order to maintain reliable R-wave detection even under the above situations, an automatic gain control (AGC) function is installed. The AGC mode is activated when the button mounted on the circuit board is pushed for more than 2 seconds. Fig. 2 shows an example of the proposed AGC flow scheme. The gain of the LPF in the analog frontend is controlled by RC value which is chosen from 32 steps between 0~50kΩ. In the begging of the AGC mode, the RC value is set to 16*Rstep (the half value of maximum RC) where Rstep=50kΩ/32. If the R wave is detected with this gain, the RC value is changed to the half between maximum RC and previous RC (e.g. RC=24*Rstep) in order to reduce the gain. Then, if the R wave is not detected with the new gain, the RC value is changed to the half between the previous RC and the RC value of two times before (e.g. RC=20*Rstep) in order to raise the gain. After this process is repeated 6 times as shown in Fig. 2, the final gain is decided. This AGC scheme finds out the appropriate gain within 6 steps due to the limitation of available RC value. Since the R-wave detection procedure and the RC modification only take about 1.5 s and 1 ms, [0] [1] [2] [3] [5] [4] [6] 50kΩ (b) Fig. 1. (a)The block diagram of the R-waves telemeter and (b)the circuit diagram of the analog frontend. 1 2 1 2 32 steps (a) interval is in the range from 200 ms to 1500 ms, the R-wave -candidate signal is forwarded to the following blocks as the detected R wave. The above limitation was settled by the basis of clinical cardiology [4] and by the healthy various-aged volunteers’ ECG characteristics measured by the frontend circuitry since this system was premised on the application for the monitoring of healthy volunteers in this study. The values are easily modified through the connector equipped on the circuit board, in the case of the diseased subjects. Though the time lag of the R-wave can be caused by the signal processing in the microcontroller, it is less than few tens of microseconds since the 16 MHz master clock of the microcontroller is fast enough. In addition, the delay hardly fluctuates in each R wave. Therefore the delay does not affect the values of R-R interval practically. 1 2 1 step 1 step R wave : True R wave : False R wave : True R wave : True R wave : True R wave : False Gain : Down Gain : Up Gain : Down Gain : Down Gain : Down Gain : Up 0Ω 2 Fig. 2. An example flow of the proposed AGC scheme. The black cells and the gray cells show the selected RC values and the previous RC values, respectively. T. Yamakawa et al. / Journal of Advanced Research in Physics 3(1), 011205 (2012) 3 respectively, the whole ACG procedure is finished within 10 seconds. Energy Reduction Techniques An energy reduction technique is applied on the microcontroller as well as the low-power circuit design of the analog frontend. The microcontroller operates under a stand-by mode whose power consumption is 0.3 µW when the output of the built-in comparator is low. Additionally, the wireless transmitter which dissipates the largest power in the telemeter is shut down when the R wave is NOT detected. The measured power consumptions when the R wave is detected and when the R wave is not detected are 13 mW and 62 µW, respectively. This allows more than 620 hours of operation time with a CR2032 coin-shaped battery. The timing delay of several tens of microseconds occurs at the beginning of the transmission due to the boot-up time of the transmitter. However, the measured R-R interval is not influenced since the receiver counts the interval of the falling edge of a transmitted R wave as mentioned in following subsections. B. Receiver Circuit Description The wirelessly transmitted signal is received by the on-board antenna printed with the meander pattern. The received signal is demodulated by the 315 MHz OOK receiver module, and the demodulated R wave is forwarded to the built-in comparator of the R5F21294SNSP microcontroller. The comparator output is observed by the microcontroller’s built-in counter which counts the interval of the falling edges with the millisecond-order precision. The hexadecimal number of the counter value is converted to the decimal number with 4 digits as an R-R interval, and it is sent to the PC through the USB connection. The whole electric power is supplied by the USB power source of 5V DC. III. FABRICATION AND EXPERIMENTAL RESULTS A. Low-cost Fabrication of the Proposed System Fig. 3 shows the photographs of the fabricated R-R interval telemetry system. The components of the proposed R-waves telemeter was mounted on a 30 mm×50 mm 4-layer printed circuit board (PCB) as shown in Fig. 3(b). In Fig. 3(a), the red, yellow, and black cables are the connecting wires of the ECG electrodes. The receiver was realized on a 80 mm× 40 mm 2-layer PCB as shown in Fig. 3(c). The antennas were realized by a meander-patterned metal line on the PCBs. By using the low-cost electric components and the PCB manufacturing technology suitable for mass production, the volume cost of the proposed system, excluding the initial costs of the PCB fabrication, was reduced to less than US$100 / a system. B. R-R interval Measurement Accuracy The measured R-R intervals by using the proposed telemetry system were compared with the actual R-R intervals. In order to obtain the correct R-R intervals, the bio-signal amplifier (AB-611J, Nihon Kohden), the digitizer (a) (b) (c) Fig. 3. (a) A general view and (b) a top view of the PCB of the R-waves telemeter, (c) a general view of the receiver. The PCB of the telemeter was put in a plastic case in order to maintain electrical isolation from human body. (PowerLab 4/26, AD Instruments), and the software (LabChart 6, AD Instruments) for medical application were used for measurement and analysis of ECG signal. During the measurement of a 24 year-old male subject for 30 minutes, the errors of the R-R interval measured by the proposed method were maintained less than 3.2 ms and the average error was 1.3 ms. Even though the above errors are caused due to that the proposed telemetry system counts the intervals of the falling edges of the R wave whereas the ECG monitor counts the intervals of the peaks, the millisecond-order errors are ignorable in the medical diagnosis [1]-[4]. C. Automatic Gain Control under Various Ages In order to investigate the automatic gain control of the telemeter, four various-aged subjects were selected to observe how accurate a gain was adjusted to detect R waves at different ages. The subject’s ECG was measured by using the disposable electrodes (Vitrode Bs-150, Nihon Kohden) in accordance with Lead II of Einthoven’s low. The study population comprised four subjects; 5 and 73 year-old females and 24 and 74 year-old males. All of subjects were doing a normal daily activity during the measurement. Fig. 4 shows the R-R interval of all of subjects (the five minutes in the middle of the measurement period were shown in order to highlight the fluctuation of R-R interval). Even though the ECG amplitudes varied by the subjects’ age and sex [5], the R-R interval was successfully measured with millisecond-order precision by the automatic gain control. The probability of reoccurring errors was estimated to prove the accuracy and reliability of R-wave detection. The total numbers of collected data of a 24 year-old male was 2768 for half an hour. In his case, the error region where the R-R interval data is regarded as the inappropriate value should be under 0.3 seconds and over 1.3 seconds. Here, the error region was determined by considering the average value of the measured R-R intervals and the expected range of the heart rate of a healthy subject (based on [4] and [5]). Eight errors were found within the error region of this subject. The major reason of errors would be the motion artifacts. Table 1 describes each subject’s results including the number of the collected data, errors, and the probability of error occurrences. Here, the probabilities or reoccurring errors were simply calculated with dividing the number of errors by the total number of data. As shown in Table 1, almost all cases have less than 2% probabilities. These minimum probabilities are negligible since the effect of the errors can be compressed in the frequency analysis which is frequently used in the diagnosis of long-term ECG monitoring [6], [7]. Therefore, 4 T. Yamakawa et al. / Journal of Advanced Research in Physics 3(1), 011205 (2012) the results prove the high reliability of this system even when subjects’ ages are different. D. Long Term R-R Interval Measurement Capability of long term measurement is also a crucial role on ECG measurement since the patients with acute symptoms are required to be monitored continuously. In this subsection, the accuracy of R-R interval detection was analyzed and quantified when the measurement period was 48 hours. A 24 year-old male was selected and the measurement was executed during his daily routines excluding bedtime. The total number of collected data was 268,312, and the error region was set to be the same region as pervious subsection. To study the accuracy detection of telemeter, the probability of reoccurring errors was calculated by the same method used in previous subsection. Its value turns out to be 0.752%, which can be concluded that the physiological signals in long term measurement were successfully detected with minimum errors. Therefore, it is unnecessary for patients to be restrained at hospital to get a long-term ECG since the recording can be easily performed at home with lower cost. IV. CONCLUSION An R-R interval telemetry system was realized within the mass-production cost of US$100. The automatic gain control scheme has been developed for the easy uses and the high accuracy R-R interval detection toward both clinical care and homecare for various-aged subjects. The desired functions and the validity of the system were proved with the subjects different in age and sex. In addition, the result of long term measurement was the critical evidence that the proposed system has high accuracy and high reliability. TABLE I THE PROBABILITY OF REOCCURRING ERRORS AT DIFFERENT SEX AND AGES. Time Period (min) Error Region Total Errors Total Probabilities 5 year-old female 24 year-old male 73 year-old female 74 year-old male 18 30 30 30 <0.3s, 1.3s> 27 2021 1.336% <0.3s, 1.3s> 8 2768 0.289% <0.3s, 1.3s> 35 2091 1.674% <0.5s, 1.5s> 33 1638 2.014% Fig. 4. R-R interval of 4 subjects at different sex and ages within 5 minutes. ACKNOWLEDGMENT This research was partially supported by Adaptable and Seamless Technology Transfer Program through Target-driven R&D (231Z04347), Japan Science and Technology Agency. REFERENCES [1] [2] [3] [4] [5] [6] [7] J. A. Taylor, D. L. Eckberg, “Fundamental Relations Between Short-term RR Interval and Arterial Pressure Oscillations in Humans,” Circulation, 93, 1996, pp. 1527-1532. M. Pagani, D. Lucini, P. Pizzinelli, M. Sergi, “Effects of aging and of chronic obstructive pulmonary disease on RR interval variability,” Journal of the Autonomic Nervous System, 59, 1996, pp. 125-132. R. Bordet, J. Benhadjali, A. Destee, J. F. Hurtevent, J. L. Bourriez, J. D. Guieu, “Sympathetic skin response and R-R interval variability in multipele system atrophy and idiopathic Parkinson's disease,” Movement Disorders, 11, 1996, pp. 268-272. D. Dubin, Rapid Interpretation of EKG's: 6th Ed., Tampa, FL: Cover Publishing Co., 2000. E. Simonson, “The effect of age on the electrocardiogram,” The American Journal of Cardiology, 29, 1972, pp. 64-73. J. P. Saul, P. Albrecht, R. D. Berger, R. J. Cohen, “Analysis of Long Term Heart Rate Variability: Methods, 1/f Scaling and Implications,” Computers in cardiology, 14, 1988, pp. 419-422. N. Aoyagi, K. Ohashi, Y. Yamamoto, “Frequency characteristics of long-term heart rate variability during constant-routine protocol,” Am. J. Physiol. Regul. Integr. Comp. Physiol., 285, 2003, pp.R171-R176.