Pacemakers and Defibrillators
Pacemakers and Defibrillators
Cardiac pacemakers and defibrillators are used to stimulate cardiac muscle directly. The pacemaker corrects for abnormalities in the heart rate (this can be fast or slow). Defibrillators are used to restore a fibrillating or tachycardic heart, to sinus rhythm. These are normally external, battery or mains powered, but can be internal devices, which are called Implantable Cardiac Devices (ICDs). Pacemakers that deal with bradycardia will be considered first. Normally a slow or irregular heart rhythm is caused by three types of heart block: ◆ First degree, where the delay at the AV junction is increased beyond the normal 0.2 s; ◆ Second degree, where a proportion of the depolarisation wave fails to pass through the AV junction; ◆ Complete block, where none of the depolarisation waves pass through the AV junction, and ventricular electrical activity is independent of supraventricular activity. In all these cases, the ventricles will beat at a slower or irregular rate. Dizziness or loss of consciousness may occur. The simplest pacemaker consists of three major components: batteries, the pulse generator, and the electrode leads. The pulse generator is required to provide a rectangular pulse. Typical parameters are the duration of 1 ms, a voltage of 5 V and capable of delivering a current of 10 mA. The power needed per second (if the pacemaker is on all the time) would be I 2R = 50 mW, for an electrode tissue resistance of 500 Ω. If the pacemaker is operating at 1 Hz (60 beats per minute), then the average power consumption would be 50 μW, as the pulse width is 1 ms (the pacemaker is on for 1/1000 of a second, and so the power consumption will be divided by 1000). A typical small battery has a capacity of 1 A h, so that this battery could supply the average current (10 μA) for about 11 years. The circuitry would also absorb power so that the battery life would drop to around 5 years. The batteries used are now commonly lithium iodide. The output pulse is applied to the tissue via an electrode. The electrode tip, which can screw in (or more unusually, is sown in), can be made of platinum, silver, stainless steel, titanium as well as various alloys.
Keywords: atrial triggered pacing, defibrillator waveforms, defibrillators, demand led pacing, fixed rate pacing, heart block, inhibit mode pacemakers, monophasic defibrillator circuits, pacemaker codes, physiological pacemakers, rate-adaptive pacemakers, rectilinear biphasic waveform, resistor–inductance–capacitance (RLC) circuits, truncated exponential biphasic waveform, underdamped biphasic waveform, ventricular pacing
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