How an applied 50 Hz AC voltage may stimulate VF by external application to the body, and alternatively by a voltage applied directly to the myocardium

1. Macroshock

a) Current > 1 mA, which is the perception threshold

b) Most common household source of shock

c) Whole body becomes a conductor of current

d) VF may occur depending on

i) Points of body contact

ii) Magnitude of current passing via heart

e) mA may result in VF

f) Uncommon in OT due to:

i) Isolation transformers

ii) Isolation of electrical equipment

iii) Patient isolation from ground

iv) Grounding of equipment

v) Line isolation monitoring

2. Microshock

a) Current < 1 mA, which bypasses the skin to cause hazard

b) Microshock cannot occur unless the skin resistance has been bypassed

c) Cardiac patients are particularly susceptible:

i) Indwelling catheters leading directly into the heart

ii) Pacemaker wires

d) Resistance

i) Dry skin: 1 million W

ii) Wet skin: 1000 W

iii) Fluid filled CVC: 500 W

e) Results of microshock:

i) Ventricular Fibrillation:

a) current threshold » 100 mA

(1) dependent on size of catheter etc

ii) Dysrhythmias

3. Magnitude of current causing VF

a) Caused by passage of current through myocardium

b) If current passes from hand to feet only a fraction of this current passes via the heart

c) It is the fraction of the current passing through the myocardium or the current density (current per unit area) in the region of the myocardium that determines risk of VF

d) Much less current is needed to fibrillate the heart when the point of contact is the heart itself (eg CVC) than when the current is diffused across the whole thorax, and point contacts are much more likely to cause VF than large contact areas

e) VF produced by a given electrical current is inversely proportional to the area over which this current is applied

i) Eg for catheters: as catheter size decreases (resulting in increased current density) the total amount of current required to induce VF also decreases

4. Current duration

a) As the duration of the stimulus increases, the threshold decreases

i) Microseconds

a) Cardiac muscle can recover quickly

ii) > 1 second

a) Cardiac muscle becomes depolarised

5. Frequency of Current

a) 50 Hz is unfortunately the most lethat frequency, stimulation thresholds for tissues are higher at both lower and higher frequencies