Ventricular fibrillation and DC defibrillator

Ventricular fibrillation (VF)

VF is a condition in which the ventricular muscle fibres contract rapidly and out of step, so the ventricles quiver and do not pump blood. The cardiac output falls to zero and the patient dies within minutes unless the heart is defibrillated. The ECG shows irregular, chaotic waves without QRS complexes.

Is it caused by electric current? Yes

Electric current acts on the heart by stimulating the excitable cardiac cells. If a current passes through the chest, it can cause disordered excitation of these cells:

• A current of about 75 to 400 mA (50/60 Hz) through the body (macroshock) can send the ventricles into fibrillation.
• A current as low as about 100 µA applied directly to the heart (microshock, through a catheter or pacing wire) can cause VF.
• The danger is highest if the current falls in the vulnerable period (T-wave), when some cells have recovered and others have not, so re-entrant, uncoordinated circuits start.
• A frequency of 50/60 Hz is the most dangerous; higher frequencies (>1 kHz) are less so.
• Reasons for danger: leakage from faulty equipment, a broken ground, wet skin (low resistance), contact with the mains, or a catheter in the heart.

DC defibrillator

It gives a large DC pulse through the chest to depolarise all the heart cells at once, so that the natural pacemaker (SA node) can take control again.

 Mains -> [Step-up   ]-> [Rectifier] -> [HV capacitor C]
          [transformer]       ^               |
          [variable auto-     |         [Inductor L ~100 mH]
           transformer]  charge control       |
                                    [Discharge switch/relay]
                                              |
                                  Paddles (apex/sternum) -> chest
 [ECG monitor + synchroniser]--(R-wave trigger for cardioversion)

Function:

1. The AC mains is stepped up (to about 5 kV) and rectified to DC; the voltage control (autotransformer) selects the energy (about 50 to 360 J).
2. The capacitor stores energy E = ½CV². Example: C = 32 µF; for E = 360 J, V = √(2E/C) = √(720/32×10⁻⁶) ≈ 4.7 kV.
3. The paddles are placed on the chest (sternum and apex) with conductive gel, and the operator presses the discharge buttons.
4. The capacitor discharges through the inductor (which shapes a smooth damped sine pulse, the Lown waveform, lasting about 5 to 10 ms) and through the patient (resistance about 50 Ω).
5. A relay isolates the patient from the HV during charging; a safety discharge resistor dumps the stored energy if it is not used.
6. In synchronised mode (cardioversion), the shock is triggered by the R-wave to avoid the vulnerable period.