Respiration detection circuit for multi-vital parameter monitor

    Abstract: This paper introduces a breathing detection circuit used in multi-vital parameter patient monitors. It adopts the principle of respiratory impedance method, uses ECG electrodes to collect respiratory wave signals, and sends the respiratory wave signals to the A/D in the 80c196 microcontroller to convert them into digital signals, calculate the respiratory frequency, and display it on the LCD at the same time.

    Keywords: respiratory impedance method respiratory signal modulation and demodulation

    With the development of sensing technology and electronic technology, patient monitors are being widely used in clinical monitoring. Due to their single monitoring parameters, simple functions, and large size, traditional monitors are only limited to monitoring surgical procedures and ICU wards, which limits their use value and cannot meet the needs of all clinical departments. To this end, we have developed a set of miniaturized, low-power multi-vital parameter patient monitors, which can monitor the patient's electrocardiogram (ECG), respiration (RESP), blood oxygen saturation (SPO2), and blood pressure in real time for a long time. (BP) and body temperature (Temp). Under abnormal circumstances, such as a lead falling off, an alarm can automatically alert the doctor. At the same time, the device can also realize computer communication through the RS232 interface, and gradually realize the networking of multiple patient monitors to meet the application needs of all clinical departments.

    1 Principle of respiration detection circuit for monitor

    The respiration detection circuit for monitors uses the principle of respiratory impedance method. It borrows chest monitoring electrodes for measuring ECG, uses high-frequency excitation pulses to modulate the respiratory wave signal on it, and then demodulates, amplifies, and filters the modulated signal to obtain a clear and stable respiratory curve. The circuit block diagram is as follows Shown in Figure 1.

    LL and RA in Figure 1 respectively represent the left abdominal electrode and the right upper chest electrode among the ECG electrodes. EN is a control signal, controlled by the 80C196C microcontroller. When EN is low level, the high-frequency excitation pulse generating circuit does not generate pulses and this circuit does not work; when EN is high level, the high-frequency excitation pulse generating circuit applies high-frequency excitation voltage to the human body through the ECG electrodes LL and RA. On the device, a safe current is injected, and the electrical signal caused by the impedance change caused by breathing between the two electrodes is modulated on the high-frequency excitation pulse. The modulated signal is demodulated, amplified, and filtered to obtain the respiratory wave signal RESP. Finally, the RESP signal is sent to the CPU, and the CPU calculates the respiratory frequency. In order to ensure the electrical safety of the patient, the circuit is powered by a high-energy battery, so no optoelectronic isolation circuit coupling is required.

    2 Principle of high-frequency excitation voltage generating circuit

    As shown in Figure 2, EN is the respiration measurement enable signal sent by the control system. When there is no need to detect the respiration signal, the control system sets EN to low level, the D flip-flop does not work, and the outputs Q and Q remain high or Low level, due to the DC blocking effect of capacitor C, no excitation voltage is applied to the human body at this time. When it is necessary to detect the respiratory signal, EN is set to high level, and the D flip-flop divides the 125kHz square wave generated by the oscillator by two. , get a 62.5kHz square wave of 5V (or -5V). In each cycle of the square wave, C3 and C4 are charged and discharged through the human body resistance between LL and RA, that is, the respiratory impedance Rb and two fixed resistors R3 and R4. The equivalent circuit diagram is shown in Figure 3.

    In the figure, Rb is the impedance of the human body. According to the dispersion theory of biological impedance, it can be seen that in the frequency band near 62.5kHz, the impedance of the human body shows approximately pure resistance characteristics, with almost no influence of film capacitance, in the order of 10Ω~10kΩ. Take R3=R4=30kΩ, C3=C4=1000pf. In this way, the maximum current flowing through the human body is about 0.08mA, which is within the safe current range, and the time constant τ of the circuit is about 32μs, and the square wave period T is about 16μs. Therefore, each charge and discharge is incomplete, and the two-point potentials of A1 and B1 are shown in Figure 4.

    Since breathing expands the thorax, Rb changes according to the respiratory frequency, and its change range is 0.1~0.3Ω. It is a slowly changing signal relative to Q. In each Rb change period, the equivalent circuit time caused by the change of Rb The weak change of the constant causes the potential of points B1 and B2 to change with the change of Rb, and the absolute value Ub(t) of the potential difference between points B1 and B2 at each instant is proportional to Rb. In this way, the respiratory signal Ub(t) is equivalent to being modulated on a 62.5kHz carrier wave, and the modulation method is amplitude modulation. Thus it is distinguished from the electrical frequency ECG signal. As long as the waveform of Ub(t) can be obtained, the respiratory wave signal can be obtained.

    3 Preamplifier

    As mentioned before, the ECG signal and the high-frequency pulse signal modulated with respiratory information are extracted from the ECG electrodes (LL and RA). Since the ECG signal and respiratory signal are very small, the small signal should be amplified before demodulation and filtering to facilitate demodulation and filtering. This part of the work is done by the preamplifier.

    According to the characteristics of ECG signals and respiratory signals, the preamplifier is required to have low noise, low drift, low power consumption, and high common-mode rejection ratio performance. For this reason, the author chose AD620 as the preamplifier. As shown in Figure 5.

    4 Demodulation processing circuit

    The signal output by the AD620 preamplifier contains the amplitude-modulated signal of the respiratory signal. In order to obtain information about human respiratory impedance, the signal needs to be demodulated. This part of the work is completed by the demodulation processing circuit. The demodulation circuit uses a diode detection circuit.

    Figure 6 shows a full-wave rectifier circuit, also called an absolute value circuit. It consists of a half-wave rectifier circuit and an adder circuit. Its amplitude modulation detection uses the one-way conductivity of the diode.

    When VI>0, D1 is turned on, D2 is turned off, the voltage VA=-VI, at this time the output voltage of the rectifier circuit Vo=VI;

    When VI<0, D1 is turned off, D2 is turned on, and the voltage VA=0. At this time, the rectification process has nothing to do with A1, and the output voltage of the rectifier circuit Vo=-VI>0.

    It can be seen that the output Vo=|Vi| of this circuit can detect the modulated signal unilaterally.

    5 Amplification and filter circuit

    After demodulation, the signal containing human respiratory impedance information contains a large amount of DC components and high-frequency noise, which requires high-pass and low-pass filtering. At the same time, the demodulated signal is only at the millivolt level, so it needs further amplification. To this end, the design block diagram of this part is shown in Figure 7.

    The filter circuit uses passive RC high-pass and BUTTERWEALTH second-order low-pass. A two-stage amplification was used for amplification.

    The respiration detection circuit for multi-parameter monitors introduced above has been cascaded with the ECG detection circuit. Experiments on 4 groups of samples of different ages and genders have proven that this circuit can display clear and stable respiratory wave RESP signals in real time, and Concentric electrical signal parts do not interfere with each other. This circuit has the advantages of low power consumption, portability, and low cost, and fully meets the requirements for multi-parameter life monitors. In this monitor, the RESP signal is sent to the on-chip A/D converter of the 80C196 microcontroller after shaping and converted into a digital signal. The respiratory frequency is calculated and matched with the corresponding software to realize the artificial intelligence of the instrument, thereby achieving real-time Monitor the patient's respiratory signal, display the respiratory frequency in real time on the LCD display, and alarm in real time when breathing is abnormal and other monitoring requirements.