Defibrillation circuit

You may be asked to draw a defibrillator circuit diagram in the examination in order to demonstrate the principles of capacitors and inductors. 

The inductor is used in a defibrillation circuit to modify the discharge waveform of the device so as to prolong the effective delivery of current to the myocardium. 

Figure 10.17 Classical defibrillator circuit. Typical values are C = 20 pF, L = 100 mH, R = 15 Q,...

The heart also acts as a voltage source that can cause current to flow with each beat and register a signal in the ICD s sensing circuitry. This information can then be used to inhibit pacing, and also provide input to the defibrillator circuit. 

Even with an appropriately positioned dual coil system and attempts using both shock polarities the DPT may rarely remain unacceptably high. In this instance the implanting physician has the option of taking the proximal coil out of the defibrillation circuit and adding a subcutaneous array or patch as a means of improving the DPT. 

When charging the defibrillator, the switch is positioned so that the 5000 V DC current flows only around the upper half of the circuit. It, therefore, causes a charge to build up on the capacitor plates. 

The basic biopotential amplifier described above, along with the specific design considerations for each biopotential, can yield a signal acquisition of acceptable quality in most laboratory settings. In practice, however, further enhancements are always necessary to achieve acceptable clinical performance in novel applications. These enhancements include circuits for reducing electric interference, filtering noise, reduction of artifacts, electrical isolation of the amplifier, and electrical protection of the circuit against defibrillation shocks [9]. 

Circuit enhancements for biopotential measurements, (d) Baseline restoration circuit the high-pass filter capacitor Cl is discharged by field-effect transistor F when activated manually or automatically by a baseline restoration pulse, (e) Electrical isolation transformer coupled using the transformer T (top) or optical using the diode D and the photodetector P (bottom). Note that the isolator separates circuit common on the amplifier side from the earth ground on the output side, (f) Electrical protection circuit resistance R limits the current, reverse-biased diodes D limit the input voltage, and the spark gap S protects against defibrillation pulse-related breakdown of the isolation transformer T. 

Fig. 8.3 Block diagram of an ICD sensing circuit. Raw signals are amplified, filtered to reject high and low frequency noises, rectified to eliminate polarity dependency, then compared to a voltage threshold. (From Olsen WH. Tachyarrhythmia sensing and detection. In Singer I, ed. Implantable cardioverter defibrillator. Armonk, NY Futura Publishing, 1994 75, with permission.)...

The electrical model of the respiration circuit of the Fig. 2 includes the defibrillator proteetion resistances (Rp) in each wire of the ECG cable and the electrode impedanee. The electrode impedance is modeled as a 51-kf2 resistor in parallel with a 47-nF capaeitor as specified in the ANSI/AAMI EC13 2002 standard. The total baseline impedance consists of the sum of Rp, Rb, and electrode impedance [9]. 

Glotzer TV, Gordon M, Sparta M, et al. Electromagnetic interference from a muscle stimulation device causing discharge of an implantable cardioverter defibrillator epicardial bipolar and endocardial bipolar circuits are compared. Pacing Clin Electrophysiol 1998 21 (10) 1996-1998. 

Implantable medical devices have been widely used to restore body functions, improve the quality of life, or save lives. Experts estimate that 8 to 10 percent of all Americans (some 20 million to 25 million people) [1], or about 1 in 17 people in industrialized countries [2], carry some form of implanted device. Many medical devices, such as the implantable cardiac defibrillator, cochlear implant, artificial vision prosthesis, neuromuscular microstimulator, and the like contain sophisticated electronic circuits. Such long-term implantable medical devices are susceptible to damage by body fluids over time. Hermetic packaging is required to protect the electronic circuitry of the implant from the harsh environment of the human body. 

A number of courts have evaluated a variety of different uses to determine whether they qualify as protected uses under 271(e)(1). In Telectronics, the Federal Circuit held that the demonstration of a potentially infringing defibrillator at a medical conference in order to obtain clinical investigators, even though some non-physicians were able to view the demonstration, qualified as a use reasonably related to the development of submission of information and therefore was a non-infringing activity under 35 U.S.C. 271(e). Id. at 1523. 

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