\chapter{Defibrillation}
The defibrillator generates a surge of current which passes through the heart and simultaneously depolarises the
bulk of the muscle, rendering it refractory to further depolarisation. The hope is that this breaks the cycle of
ventricular fibrillation and allows an organised rhythm to re-establish itself.\\
Since there is seldom direct access to the heart, current must be made to pass through the skin and tissues. This
requires a high voltage (in the order of 5kV) and a high current (only a small fraction will reach and pass through the
heart) be delivered.\\
In order to achieve this, the defibrillator builds a substantial charge in a capacitor and then delivers it when
required. The discharge characteristics of a capacitor (exponential decay) would result in a very high initial current
which would decrease very rapidly. Clinically, a more sustained current is desirable and so the output circuit
includes and inductor. This tends to oppose the initial surge in current, storing energy in its magnetic field. As the
capacitor discharges, the collapsing magnetic field sustains the defibrillator current.\\
Modern machines are biphasic meaning that the flow of current is reversed during the discharge phase. This is more
effective than monophasic defibrillation, allowing lower energies to be used and allowing for smaller devices.
\begin{figure}[h]
\centering
\includegraphics[scale=0.8]{images/defib1.jpg}
\label{Fig:Defib}
\end{figure}
\vspace{1in}
Concerning pacemakers: \footnote{TTFFT}
\begin{enumerate}
\item AOO is a fixed rate type of pacemaker
\item VVI is the most common type of synchronous pacemaker
\item failure to capture is never seen with hyperkalaemia
\item unipolar electrocautery is preferred in patients with a pacemaker
\item the ground plate of the electrocautery should be placed as far as possible from the pacemaker
\end{enumerate}
\vspace{0.5in}Regarding defibrillation:\footnote{TTTFT}
\begin{enumerate}
\item The greater the time interval between onset of ventricular fibrillation and defibrillation, the less the
success of defibrillation
\item 5-40 joules should be applied to the heart if the chest is open
\item Paddles should be of 13 cm in diameter in adults
\item Defibrillation is most effective when the electric shock is delivered during inspiration
\item The myocardium is refractory to defibrillation in hypothermia
\end{enumerate}
