Medical Electronics - Hints for Slow Learner
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The document provides a comprehensive overview of defibrillators, detailing their function, types, operation methods, and specific parameters for internal and external use. It explains different defibrillator technologies, including AC and DC versions, their advantages and disadvantages, and their operational characteristics regarding voltage, current, and energy requirements. The document also includes descriptions of advanced defibrillation techniques such as synchronized, square wave, double square pulse, and biphasic defibrillators.
Medical Electronics - Hints for Slow Learner
- 1. N.Mathavan || AP – ECE – NSCET DC Defibrillators Defibrillator is an electronic device that creates a sustained myocardial depolarization of a patient's heart in order to stop ventricular fibrillation or atrial fibrillation. The instrument for administering the electric shock is called as defibrillator. Defibrillation is the application of electric shock to the area of the heart which makes all the heart muscle fibers enter their refractory period together, after that normal heart action may resume. If the heart does not recover spontaneously after delivering the shock to the heart using defibrillator then a pacemaker may be employed to restart the rhythmic contraction of the myocardium. 1. Types of Defibrillators o Internal Defibrillators (Surgical Type) o External Defibrillators (Therapeutic Type) 2. Types of defibrillator based on operation or Voltage delivered o AC defibrillator o DC defibrillator o Synchronized DC defibrillator o Square pulse DC defibrillator o Double square pulse DC defibrillator o Biphasic DC defibrillator Types of Defibrillators Internal Defibrillators (Surgical Type) It is used when chest is opened. Here large spoon shaped electrodes with insulated handle are used. Electrodes in the form of fine wires of Teflon coated stainless steel are used. There are AC and DC defibrillator methods but DC defibrillator is used today. Since the electrode comes in direct contact with the heart, the contact impedance is about 50 ohms.
- 2. N.Mathavan || AP – ECE – NSCET The Current passing through the heart is of the order of 1 to 20 amp. The Magnitude of the shock voltage is in the range from 50V to 1000V In internal defibrillation, the heart requires excitation energy of about 15 to 50 J. The duration of the shock is about 2.5 to 5 milliseconds. External Defibrillators (Therapeutic Type) It is used on the chest. Here paddle shaped electrodes are used. There are AC and DC defibrillator methods but DC defibrillator is used today. The contact impedance on the chest is about 100 ohms even after applying the gel. The Current passing through the heart is of the order of 10 to 60 amp. The Magnitude of the shock voltage is in the range from 1000V to 6000V In external defibrillation, the heart requires excitation energy of about 50 to 400 J. The duration of the shock is about 1 to 5 milliseconds. The bottom of the electrode consists of a copper disc with 3 to 5 cm diameter for pediatric patient and 8 to 10 cm diameter for adult patients with a highly insulated handle. Types of defibrillator based on operation or Voltage delivered There are six types of defibrillators based on the nature of the output voltage delivered. AC defibrillator One of the earliest forms of an electrical defibrillator is the AC defibrillator which applies several cycles of alternating current to the heart from the power line through a step-up transformer with various tapings on the secondary side. An electronic timer circuit is connected to the primary of the transformer. This timer connects the output to the electrodes for a pre-set time. The timing device may be a simple capacitor and resistor network or a monostable multivibrator, which is triggered by a foot switch or a push button switch.
- 3. N.Mathavan || AP – ECE – NSCET The duration of the counter shock may vary from 0.1 second to 1 second depending upon the voltage to be applied. For reasons of safety, the secondary coil of the transformer should be isolated from earth so that there is no shock risk to anyone touching an earthed object. For external defibrillation, the voltages are in the range from 250V to 750V. For internal defibrillation, the voltages are in the range from 60V to 250V. Disadvantages: 1. Large currents are required in external defibrillation to produce uniform and simultaneous contraction of the heart muscles fibers. 2. This current not only causes a violent contraction of the thoracic muscles but also results in occasional burning of the skin under the electrodes. 2. It produces atrium fibrillation while arresting the ventricular fibrillation. DC defibrillator This is also called Capacitive Discharge D.C. Defibrillators
- 4. N.Mathavan || AP – ECE – NSCET The DC defibrillator does not produce any undesirable side effect and at the same time it produces normal heart beat effectively. The 230V AC main supply is connected to a variable autotransformer (T1 & T2) in the primary circuit. The output of the autotransformer is fed as input to a step-up transformer to produce high voltage with a RMS value of about 8000 V. A half-wave rectifier rectifies this high AC voltage to obtain DC voltage, which charges the capacitor. The voltage to which C is charged is determined by the autotransformer in the primary circuit. An AC voltmeter across the primary is calibrated to indicate the energy stored in the capacitor. Five times the RC time constant circuit is required to reach 99% of a full charge that means that the time constant must be less than 2 s. With the electrodes firmly placed at appropriate positions on the chest, the clinician or technician discharges the capacitor by momentarily changing the switch S from position A to position B. The capacitor is discharged through the electrodes and the patients are protecting by a resistive load and the inductor L. The inductor L is used to shape the wave in order to eliminate a sharp, undesirable current spike that would occur at the beginning of the discharge. The wave is monophasic and the peak value of the current is nearly 20 A. Depending on the defibrillator energy setting, the amount of electrical energy discharged by the capacitor may range between 100 and 400 watts or joules When the electrodes are applied externally and the duration of the effective portion of the discharge is approximately 5 to 10 millisecond.
- 5. N.Mathavan || AP – ECE – NSCET Once the discharge is completed, the switch automatically returns to position A and the process can be repeated, if necessary. When the electrodes are applied directly to the heart, about 50 to 100 joules only is required for defibrillation. The energy stored in the capacitor is given by the equation Where, C is the capacitance and V is the voltage to which the capacitor is charged. Capacitors used in the defibrillator range from 10 to 60µF. Thus, the voltage for a maximum of 400 J ranges from 2 to 9 KV, depending on the size of the capacitor. Delay-Line Capacitive Discharge DC Defibrillator Even with DC defibrillation, there is a danger of damage to the myocardium and the chest walls, because peak voltages as high as 6000 V may be used. To reduce this risk, some defibrillators produce dual-peak waveforms of longer duration (approximately 10 mis) at a much lower voltage. In circuit diagram, the parallel combination of C1 and C2 stores the same energy as the single capacitor. However, its discharge characteristic is more rectangular in shape (1onger duration of approximately 10 mis) at a much lower voltage. With this type of waveform, effective defibrillation can be achieved in adults with lower levels of delivered energy - between 50 and 200 watts. E = ½ CV2
- 6. N.Mathavan || AP – ECE – NSCET Synchronized DC defibrillator DC defibrillator circuit consisting of defibrillator, electro cardio scope and pacemaker. The pacemaker is used in the case of emergency as a temporary pacing. It includes diagnostic circuitry which is used to assess the fibrillation before delivering the defibrillation pulse. Synchronizer circuitry which is used to deliver the defibrillation pulse at the correct time. So, as to eliminate the ventricular fibrillation or atrial fibrillation without inducing them. Working: ECG unit – To obtain the electrocardiogram of the patient who is going to receive defibrillation pulse. Switch - The medical attendant energizes the switch to deliver a defibrillation pulse. The QRS detector - It pass a signal if R wave is absent in the electrocardiogram. If R Wave is present it would not give any output. The AND gate 'B' - Delivers on signal to the defibrillator only when the 'R' wave is absent, As the two inputs of AND gate 'B' it receive the signal from the medical attendant & QRS detector ,if any one of the inputs is missing, then it would not give any output. By this way the defibrillator is inhibited and would not deliver the defibrillation pulse.
- 7. N.Mathavan || AP – ECE – NSCET Fibrillation detector - It Searches the ECG signal for frequency components above 150 Hz. If they are present, fibrillation is probable and the fibrillation detector gives an output signal. Thus when the AND gate B and C are simultaneously triggering the defibrillator, the defibrillation pulse is delivered. In the synchronization or Cardiovert mode – The defibrillator is synchronised with the ECG unit. Suppose a patient is suffered by atrial fibrillation. First, the doctor diagnoses it correctly and then the treatment is initiated using this circuit. The ECG signal in the instrument is given to QRS detector. Its output is used to time the delivery of the defibrillation pulse with a delay of 30 milliseconds. At this time, the ventricles will be in uniform state of depolarisation and the normal heart beat will not be disturbed. This delay of 30 milliseconds after the occurrence of R wave allows the attendant to defibrillate atrium without inducing ventricular fibrillation. Square wave defibrillator In this defibrillator, capacitor is discharged through the subject by turning on a series silicon controlled rectifier (SCR). When sufficient energy has been delivered to the subject a shunt SCR short circuits the capacitor and terminates the pulse. The output can be controlled by varying the voltage on the capacitor or duration of discharge. Here the defibrillation is obtained at low peak current and so there is no side effect.
- 8. N.Mathavan || AP – ECE – NSCET Advantages: The advantages of square wave defibrillator are, 1. It requires low peak current 2. It requires no inductor 3. It is possible to use physically smaller electrolytic capacitors. Double Square Pulse Defibrillator Double square pulse defibrillator is normally used after the open-heart surgery. If the chest is opened, only lower energy electric shock should be given. Instead of 800 - 1500V, employed in capacitor discharge DC defibrillators, Here 8-60 V double pulse is applied with a mean energy of 2.4 watt-second. When the first pulse is delivered, some of the fibrillating cells will be excitable and will be depolarized. However, cells, which are in refractory during the occurrence of first pulse, will continue to fibrillate. In order to, obtain a total defibrillation; the second pulse operates on latter group of cells. Thus, complete defibrillation can be obtained by means of selecting proper pulse- space ratio. Using double square pulse defibrillator, efficient and quick recovery of the heart to beat in the normal manner without any side effect like burning of myocardium or inducement of ventricular or atrial fibrillation. The double square pulse with the required pulse-space ratio can be produced with the use of digital circuits similar to those digital pacemaker circuits. Biphasic DC defibrillator Biphasic DC defibrillator is similar to the double square pulse defibrillator such that it delivers DC pulses alternatively in opposite directions. This type of waveform is found to be more efficient for defibrillation of the ventricular muscles.