The energy delivered by a defibrillator is measured in joules (the product of power (in watts) and duration (in seconds)). Power is the product of circuit voltage and current. Delivered energy may be up to 40 per cent less than actual stored energy because of losses between apparatus and electrodes. Defibrillation results from the passage of current through the myocardium, causing depolarization of a critical mass of myocardial tissue. The size of the current flowing depends on the transthoracic impedance which is measured in ohms:
current (amperes) = potential (volts)/impedance (ohms).
Typical impedance values for the human thorax range from 15 to 150 W (average 70-80 W). Impedance increases if the quality of contact between paddle and skin is poor (low pressure and small contact area), if conductive gels are not used, if the distance between the electrodes increases, and during inspiration ( Si^Qa..et,al
1988). It doubles when circulation stops. Impedance decreases with successive shocks if contact is maintained between paddle and chest. The optimal defibrillation current is believed to be in the region of 30 to 40 A over 4 to 12 ms.
Because individual impedance levels are unknown, the energy first chosen (200 J) may deliver inadequate current to defibrillate (high impedance) or too much current so that the myocardium is damaged (low impedance). Failure to defibrillate also occurs when transthoracic current follows alternative pathways in the chest, thus reducing current flow through the myocardium. Current-based defibrillators are being developed which measure individual impedance levels and then deliver the appropriate energy level to produce a chosen current. There is some evidence that a biphasic waveform may reduce defibrillation thresholds.
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