Medical Applications User Guide (pdf) - Freescale Semiconductor

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Defibrillators 11.1 Automated External Defibrillator (AED) An AED is a portable device used to restore normal heart rhythm to patients in cardiac arrest by delivering an electrical shock to a patient through the chest wall. Cardiac arrest is an abrupt loss of heart function. This medical emergency occurs mainly because of ventricular fibrillation. Ventricular fibrillation is a condition where there is an uncoordinated contraction of the ventricles in the heart, making them tremble rather than contract properly. The urgency of ventricular fibrillation requires that the heart must be defibrillated quickly, as a victim’s chance of surviving drops by seven to 10 percent for every minute a normal heartbeat is not restored. An MCU or MPU calculates whether defibrillation is needed and a recorded voice indicates whether to press the shock button on the AED. This shock momentarily stuns the heart and stops all activity, giving the heart an opportunity to resume beating effectively. The charge is generated by high-voltage generation circuits from energy stored in a capacitor bank in the control box. The capacitor bank can hold up to 7 kV of electricity. The shock delivered from this system can be anywhere from 30 to 400 joules. 62 Medical Applications User Guide
11.2 Circuit for Capacitive Discharge Defibrillators In Figure 11-2, a step-up transformer (T2) drives a half-wave rectifier and charges the capacitor (C1). The voltage where C1 is charged is determined by a variable autotransformer (T1) in the primary circuit. A series resistor (R1) limits the charging current to protect the circuit components, and determines the time constant Tao (T = R x C). Five times the time constant for the circuit is required to reach 99 percent of a full charge. The time constant must be less than two seconds to allow a complete charge in less than 10 seconds. 11.3 Circuit for Rectangular- Wave Defibrillators In a rectangular-wave defibrillator, the capacitor is discharged through the patient by turning on a series of silicon-controlled rectifiers (SCR). When sufficient energy has been delivered to the patient, a shunt SCR short circuits the capacitor and terminates the pulse. This eliminates the long discharge tail of the waveform. The output may be controlled by varying either the voltage on the capacitor or the duration of discharge. Figure 11-3 shows a general diagram of circuit implementation. Bipolar defibrillators are more efficient because they need less energy while providing the same results as unipolar defibrillators. A bipolar defibrillator needs just 120 J to discharge. It has the same efficiency as the 200 J of discharge used by a unipolar defibrillator. An ECG unit must be included in the defibrillator’s system to monitor heart activity and to control the moment when the discharge can be applied to the patient. The electrodes perform both functions, capturing the patient’s ECG and delivering a high current. Defibrillator Defibrillator Figure 11-1: Defibrillators General Block Diagram Syncronization Circuit Syncronization Circuit Electrodes Electrodes Discharge Circuit Discharge Circuit ECG Amplifier ECG Amplifier Freescale Technology Freescale Technology Optional MCU/MPU Diagnostic and Therapy Devices Signal Conditioning Signal Conditioning Unipolar Bipolar freescale .com/medical 63 Electrical Isolation Electrical Isolation Display Optional Display MCU/MPU Keypad or Touch Screen Keypad or Touch Screen USB USB Wireless Comm Wireless Comm Power Management Power Management Figure 10-2: Basic Circuit Diagram for a Capacitive Discharge Defibrillator Figure 11-2: Basic Circuit Diagram for a Capacitive Discharge Defibrillator Figure 10-3: 11-3: Block Block Diagram Diagram for for a Rectangular-Wave a Rectangular-Wave Defibrillator Defibrillator Figure 10-4: Unipolar Defibrillator Waveform Volts 2500 2000 1500 1000 500 0 0 1 2 4 6 8 10 Time (ms) 12 14 16 18 Charge Control A Charge Circuit A Charge Circuit B Charge Control B Capacitor Bank A Capacitor Bank B Monitor Circuit Monitor Circuit Figure 11-4: Unipolar and Bipolar Defibrillator Waveforms Figure 10-5: Bipolar Defibrillator Waveform Volts 2500 2000 1500 1000 500 0 0 1 5 10 15 20 25 Time (ms) 30 35 40 45
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Medical Applications User Guide TM
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freescale.com/medical 3 Introductio
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Introduction 1.1 Freescale Offers T
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Monebo Kenetic ECG Algorithms Free
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Home Portable Medical 2.1 Introduct
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Page 15 and 16: MC34704 The MC34704 is a multi-chan
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Page 25 and 26: 4.2 Heartbeat Detection The heartbe
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Page 31 and 32: Heart Rate Monitor 5.1 Introduction
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Page 43 and 44: AN4327 Pulse Oximeter Fundamentals
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Page 103 and 104: Summary Summary Applications Freesc
Page 105 and 106: Appendix Digital Signal Processing
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