Development Of A Miniature Cardiac Function Monitoring Device With Two Electrodes

Thoracic impedance cardiac function detection is to detect the dynamic changes of the thoracic impedance, to evaluate cardiovascular function. This method is a kind of noninvasive cardiac detection method, which has characteristics such as safe, repeatability and convenient. To develop miniaturized cardiac function monitoring device based on the traditional four electrodes method principle that can be used for continuous cardiac function monitoring in communities, families, and emergency department with two electrodes.First, we analyzed the research status of impedance cardiograph instrumentin recent years, expounded the physiological basis, physics model and technical methods. Then, we designed a miniaturized heart function monitoring device.A high frequency current source was excited through a pair of defibrillation/monitoring electrodes. The ECG and transthoracic impedance signals were simultaneously detected with the same electrodes based on a MSP430 microcontroller system. The hardware system consisted of power supply module, constant current source driver, ECG signal detection circuit, thoracic impedance signals detection circuit, MCU interface controller, LCD module and SD storage unit.The system was supplied by 5V rechargeable lithium battery. A high frequency low amplitude sinusoidal constant current source(1MHz/1mA) was used as excitation input. ECG signal detection circuit included preamplifier, band-pass filter, 50 Hz notch filter and main amplifier circuits. The transthoracic impedance signal was filtered, demodulated, differentiated and amplified to obtain the baseline impedance(Z0), the alternate impedance(ΔZ) and the first derivative of ΔZ(dZ/dt). The analog signalswere then digitalized by MCU A/D interface, displayed by LCD, and storedby SD card for further analysis.After completion of the prototype design and performance test, signals of 14 patients with cardiovascular disease and 7 age-matched healthy controlswere collected. The data were then analyzed through a PC application program developed by the authors. Heart rate(HR), baseline impedance, Peak value of the impedancedifferential signal, Peak-trough value of the impedancedifferential signal and Area under the dZ/dt curve was calculated. Characteristics of impedance signals were compared between patients and the age-matched healthy controls. The relationship between characteristics of impedance signal and left ventricular systolic function for patients with cardiac dysfunction was analyzed through correlation analysis.Dimensions of the prototype were 200mm*120mm*110mm and power consumption was 5W. The preliminary clinical test showed that both Peak value of the impedancedifferential signal(p=0.007) and Area under the dZ/dt curve(p=0.012) were significantly lower in patients with impaired cardiac function compared to the healthy controls. Furthermore, Peak value(r=0.811, p<0.001) and Area under the dZ/dt curve(r=0.922, p<0.001) were correlated with left ventricular ejection fraction with Pearson correlation analysis. Peak value(r=0.773, p=0.001) and Area under the dZ/dt curve(r=0.893, p<0.001) were correlated with left ventricular frachional shortening with Pearson correlation analysis. Peak value(r=-0.748, p=0.002) and Area under the d Z/dt curve(r=-0.826, p<0.001) were correlated with left ventricular end diastolic volume with Pearson correlation analysis.The two electrodes based cardiac function monitoring deviceis portable, safe and reliable; the device not onlymeasures cardiac dysfunction qualitatively, but also detects left ventricular systolic function quantitatively.