No one would have believed a century ago when electricity was first dosvovered for the mankind by Benjamin Franklin with glorious purpose , would now be used as a drug for treating life threatening heart ailments !
Yes , electricity is a drug by definition.
It is administered percutaneously by focusing a beam of current into the heart.
- There is a dose , shape , energy and direction for this drug.
- Paddle size determine the energy.
- Paddle location determine the direction of current vector.
- Dose is selected by the physician.
Mechanism of DC shock / Defibrillation *
When heart suddenly behaves abnormally and start generating its own electricity and sends it through abnormal channels other than its natural paths , it becomes a dangerous arrhythmia .This propagation of wave front can occur in multiple directions in a chaotic manner , resulting in VT/VF and imminent death.
Like an air to air missile ,this abnormal wave front can be tackled only by an another electrical wave front . Nothing else will work.
* The difference between DC shock and defibrillation is only technical. If one gives a synchronised shock ( with qrs complex ) it becomes DC shock .If not , it is defibrillation
The success of defibrillation depends on many factors .
The following are most important.
- The critical myocardial mass must be depolarized by the current delivered.Sufficient amount of sodium channels /less of calcium currents need be activated for this to happen .(JACC 2008)
- The direction and the angle of current entry with reference to advancing end of abnormal wave front. is also important .
- Distance between the paddles.(Antero posterior paddles more effective than Apex /Sternal pads )
- Energy level (seems to be less important ! )
Two shock forms are used
- Monophasic shocks
- Biphasic shocks
A biphasic DC shock has replaced the traditional mono phasic sine wave shocks in most machines.
What is the fundamental difference between the two ?
- In bi phasic shocks , the current traverses the myocardium twice .
- So, it has a second chance to interrupt the critical tachycardia circuit , if the first one fails. In other words, biphasic shocks are technically equivalent to “two sequential low energy shocks” delivered in opposite polarity . This change in direction happens in micro seconds .
- The shape of biphasic DC current wave form can be a truncated sine wave or square wave .The maximum energy of DC shock in biphasic mode is 200 joules (In Monophasic it is 360joules) . All AEDs, ICDs, now use bi phasic shocks to conserve energy .
Final message
A biphasic shock waveform has a proven advantage . It has greater efficacy ( because it traverses the heart twice ) , requires fewer shocks with low delivered energy and hence less myocardial and dermal injury.
References
Even though there is general acceptance of superiority of bi phasic shocks , it is still considered by some , that there is no great difference in the overall outcome .
http://circ.ahajournals.org/cgi/content/full/94/10/2507
Bi phasic shocks in atrial fibrillation
Dr.S.Venkatesan MD 