Bi-phasic DC shocks : It is different !

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November 14, 2010 by dr s venkatesan
No one , would have   ever believed  a century ago , when the  electricity was invented  for the mankind (By Benjamin franklin   with  a  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 a 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 .
Bi phasic shocks in atrial fibrillation

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