2.0: What is a defibrillator.
2.1: The monitor.
you need to be able to see what’s going on. This is of course one of the
reasons why our patients are monitored at the bedside: so you can see
what rhythm your patient is in. Defibrillators are built to travel– so they
have a built in monitor screen.
2.3: The capacitor.
Generally you need a battery to run any transportable medical device.
Due to rechargeable batteries these devices are so heavy. The battery
stores electricity,when machine is plugged in to AC. The capacitor fills
up with electricity when you push the button that selects charge .
2.4: The paddles and the pads:
Paddles
Self Adhesive Pads
Internal Paddles
paddles are considered “old-tech” – nowadays the thing to do is to slap on
sticky defibrillation pads that hook up to the machine – the same ones as
external pacing pads – then stand back, charge and discharge the machine
from a few feet away. The shock can be delivered to the heart by means of
electrode placed on chest of the patient(External defibrillation) or the electrode
may be held directly against the heart when the chest is open
(internal defibrillation). Higher voltage are required for external defibrillation
than for internal defibrillation.
2.5: Theory of operation.
Schematic diagram of a defibrillator
Above schematic show basic circuit diagram of DC Defibrillator. A Variable auto transformer forms the primary of a high voltage transformer. The output voltage transformer is rectified by a diode rectifier and is connected to vacuum type high voltage change over switch. In position 1, the switch is connected to one end of an oil filled micro farad capacitor. In this position, the capacitor charge to a voltage set by the positioning of the auto transformer. When the shock is delivered to the patient, a foot switch or a push button mounted on the handle of the electrode is operated. The high voltage switch change over to position 2 and the capacitor is discharged across the heart through the electrode. The inductor in the circuit slow down the discharge from capacitor by induced counter voltage. This give the output pulse a physiologically favorable shape. The dis advantage of using inductor is that any practical inductor will have its own resistance and dissipates part of the energy during the discharge process. The shape of waveform that appears across electrodes will depend upon the value of the capacitor and inductor used in the circuit. The discharge resistance which the patient represent for defibrillating pulse may be regarded as purely ohmic resistance of 50-100 Ω approximately for typical electrode size of 80 cm2. The typical discharge pulse of defibrillator is shown in fig.3 b. Using this design, external defibrillation uses: –50 to 100 Joules of energy when electrodes are applied directly to the heart –Up to 400 Joules when applied externally ,Capacitors used range from 10 to 50F .Voltage using these capacitors and max energy (400J) ranges from 1 to 3 kV . Energy loss result in the delivery of less than theoretical energy to the heart
Defibrillator: Rectangular-Wave
•Capacitor is discharged through the subject by turning on a series silicon- controlled rectifier.
•When sufficient energy has been delivered to the subject, a shunt silicon- controlled rectifier short-circuits the capacitor and terminates the pulse, eliminating a long discharge tail of the waveform
•Output control can be obtained by varying:
–Voltage on the capacitor
–Duration of discharge
•Advantages of this design:
–Requires less peak current
–Requires no inductor
–Makes it possible to use physically smaller electrolytic capacitors
–Does not require relays ·
Monophasic pulse width is typically programmable from 3.0 to 12.0 msec
Biphasic positive pulse width is typically programmable from 3.0 to 10.0 msec, while the negative pulse is from 1.0 to 10.0 msec ·
Studies suggest that biphasic pulses yield increased defibrillation efficacy with respect to Monophasic pulses.
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