5. Defibrillation

two plates that are separated by an insulator and which will store electrons after the application of a potential difference ability to hold electric charge

4,000 and 6,000 V is applied across the capacitor to produce a store of electrons When the defibrillator is activated, this stored charge is released as a pulse of current across the patient’s heart inductance coil (inductor) in the output circuit decelerates the rapid discharge of the capacitor to give a shock that is slowed to 4–10 ms This duration gives the optimal chance of synchronous myocardial depolarization

The efficiency of the applied shock is greater if transthoracic impedance is minimized by the use of conductive gels, firm paddle pressure, and by defibrillation from front-to-back rather than from sternum to apex. (Impedance is the sum of all forces impeding electron flow in an AC circuit.)

In a defibrillator that uses a biphasic waveform, the current is reversed halfway through the discharge to move both in a positive and a negative direction. (There are also two biphasic waveforms: truncated exponential decay and rectilinear not only more effective than monophasic, but they also cause less myocardial injury. Their use is now becoming widespread

Atrial and ventricular fibrillation: in health, the sinus impulse is conducted evenly and concentrically to all parts of the atria and thence to the ventricles. fibrillation (AF) supervenes, the excitation and recovery of different parts of the atria becomes uncoordinated, with various areas at different stages of excitation and recovery. It is similar with ventricular fibrillation (VF). changing amplitude of the ECG reflects electrical activity, but depolarization is chaotic and unable therefore to generate any cardiac output.

: in AF, there is loss of the atrial contribution to ventricular filling, which is usually around 20%. In addition, the risk of thrombus formation is substantially increased. A fibrillating ventricle produces no cardiac output

Common causes of AF include ischaemic heart disease and acute critical illness, particularly sepsis ( mitral stenosis, Thyrotoxicosis) Alcohol

by myocardial disease, both ischaemic and myopathic, by hypoxia, by profound hypothermia, by electrolyte imbalance, by some drugs and by electrocution

refractory AF is treated by the application of a DC shock, which is synchronized to the peak deflection of the ‘R’ wave of ventricular depolarization on the ECG. The risk of inducing VF is very high during repolarization (as shown by the ‘T’ wave on the ECG). This is why the ‘R-on-T’ rhythm is particularly dangerous.

this can be treated either by mechanical defibrillation or electrical defibrillation. The application of mechanical energy in the form of a praecordial thump (sometimes known as ‘thumpversion’) may convert VF to a viable rhythm only if it is applied very early. It normally achieves only around 5–10 joules of mechanical energy. In electrical defibrillation, a defibrillator delivers a charge across the chest which causes simultaneous depolarization of myocardial cells. If the procedure is successful, there is a short refractory period after which there is resumption of normal pacemaker activity with myocardial contraction and a stable rhythm.