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Posts Tagged ‘dc shock’

Ventricular tachycardia is  a major cardiac electrical disorder. Even though it  connotes a deadly meaning the prognosis and outcome vastly vary.It can be a benign arrhythmia in  structurally normal heart that present as occasional fasicular VT  or Exercise  induced RVOT , to dangerous ischemic polymorphic VT which rapidly degenerate to VF and SCD if not reverted . It is ironical we are  trained  to put all VTs in a single basket and  propagate fear psychosis among   physicians and patients .

Management of VT has certain broad principles.

  • Identify the cause
  • Whether  specific structural heart diseases present or not
  • Identify the mechanism if possible
  • Rule out transient metabolic cause as a trigger

Therapeutic targets

  • Stabilising the cell of origin
  • Passifying the scars
  • Interrupting bundle branches in  BBR  mediated tachycardia
  • Ischemia related  Focus – Re-perfusion
  • Reversing LV dysfunction

Management

General

  • Correct Cell hypoxia /Acidois
  • Pharmacological ( Class 1A/1B /1C , class 3 and Beta blockers , Magnesium  )
  • Role of  beta blockers for VT management is largely under recognised.It has an important role to play in both acute and chronic  VTs)

Electrical (DC shock ,Ablation and ICD)

  • DC shock is treatment of choice  all emergency VTs
  • Ablation  aims  at preventing episodes of VT .Ablation needs EP study and  expertise of  an electro physiologist.
  • ICDs  revert it only after the VT emanates from the focus . ICD can be implanted without knowing the focus .May not require a EP consult.

Surgical

CABG + Surgical scar excision , Aneurysectomy  might help in certain refractory VT.

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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://content.onlinejacc.org/cgi/reprint/52/10/828.pdf?ijkey=5a8f50ff2542182c857d4f3fe553aef8df6e3fd3

http://circ.ahajournals.org/cgi/content/full/94/10/2507

Bi phasic shocks in atrial fibrillation

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1768486/

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Acute coronary syndrome is the commonest cardiac emergency. STEMI and NSTEMI are the two clinical limbs of ACS. Generally they have distinct clinical, ECG, angiographic features.(Ofcourse,  with some degree of overlap) . It is  a  mystery , both clinical presentations differ so much inspite of the common denominator  , namely ,  an injured plaque with add on thrombus  within the coronary artery.

The mystery is since  decoded , the primary difference between these two entities is STEMI the occlusion occurs sudden and complete and in NSTEMI it occurs slow and incomplete

In STEMI ,  most of the clinical features and , need for emergent treatment , response to thrombolysis /PCI are dictated by the time dependent risk to myocardial loss .

Cardiac arrhythmias in ACS

It is a  much published  factoid   for  many decades  only one third of STEMI patients  reach the hospital alive ! The reason being , STEMI  is very much prone for primary VF.

Contrary  to this ,  almost all patients with NSTEMI reach the hospital alive ! How ?

Both are ACS, if ischemia is a powerful trigger for dangerous ventricular  arrhythmia’s ,  NSTEMI should also behave  similarly .

So what protects against arrhythmias in NSTEMI ?

  • We realise ,  by observational experience (Not EBM !)  It is the suddenness and totality of ischemia that trigger dangerous form of arrhythmia  .
  • Further, a balanced  ischemia in two contralateral segments (or global  ischemia) some how protects against development of ventricular  fibrillation .This may be due to preservation  of  electrical homogeneity  , and the spherical VT spiral waves are not sustainable.
  • In contrast , STEMI has a sudden  focal , ischemic  zone that initiates the VT and    ischemia free  contralateral segment  welcoming  and sustaining the  reentrant wavelet.
  • The observation of primarily single vessel disese in STEMI and multivessel disease in NSTEMI also give credence to this concept.
  • Further , ischemic preconditioning can exert an important anti arrhythmic  effect in NSTEMI as  patients with unstable angina have   slow, repetitive episodes of ischemia prior to the index event .
  • Post MI scar mediated VT/VF is independent of degree of overall ischemia
  • It is also established ,  a sub group of  STEMI pateints  who  had  preinfarction angina(  ie . a brief  period of UA/NSTEMI) have very low risk of SCD  supporting the concept of sensitising the myocardium against ventricular arrhythmias.

Final message

Even though , there is a convincing concept  of   ischemia induced  cardiac arrhythmia in literature , in real patients it is very difficult to link the two.

UA/NSTEMI is the most common  acute ischemic event but the incidence of VT/VF here,  is far less than one would expect.

In ACS , focal , total  ischemia is more likely to precipitate a VT/VF than multifocal and global ischemia.

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                              Cardioversion with DC shock  offers immediate cure in many of the dangerous ventricular and atrial tachycardias.  It is often  taught ,  any hemodynamically unstable tachycardia  refractory to  medical therapy respond to electrical cardioversion.  One should also  remember electricity is in fact be called  as a drug !  and it should be delivered in proper form and dose. Here it is the paddle size, paddle position and the axis of current flow all are important. Now we have bi phasic currents for better efficacy.

                             While it is true, most of cardiac arrhythmias respond to shock,  there are few which do not respond or respond very transiently.There are few arrhythmias  in which ,DC shock is not only ineffective but may precipitate a ventricular  fibrillation.

                            Generally arrhythmias of reentrant etiology respond well to DC shock were interuption of  electrical circuit by external current is easily possible. In arrhythmia’s of enhanced automaticity ,  and ectopic tachycardia  it is difficult  to extinguish  the tachycardia focus with DC shock .

Arrhythmias where DC shock is not going to work are

A. Mutifocal atrial tachycardia(MAT)

B. Digoxin induced arrhythmias.Patients who are on digoxin,  has  enhanced ventricular  automaticity.These patients if they  get a DC shock will unmask the  ectopic foci.

C. In elderly with atrial fibrillation and sinus node dysfunction it may be dangerous to shock them with out temporary pacing support as sinus node goes for prolonged sleep mode.

D.In electrical storm with VT ,  if more than three shocks are required within a minute,  the VT will most often going to be permanent and the  electrical therapy can be termed as a failure. These patients will require intensive pharmacological management( Including magnesium, bretyllium etc)

E. And finally , sinus tachycardia (whatever the rate)  is an absolute contraindication for DC shock.

 Verapmil is often effective in MAT  but correction of hypoxia and acidosis may be critical.For digoxin induced arrhythmias phenytoin may be tried.

What to do when the DC shock fails?

  • It will be a  tricky situation and one wonder what to do next when the so called  universal antidote for cardiac arrhythmia fails !
  • Cellular internal millieu  is altered  by hypoxia and acidosis .It may prevent the  effectiveness of cardioversion.So try to correct them .
  • Over dirve atrial  pacing  is one option for automatic tachycardia.
  • And now ablation of arrhythmic focus is possible with radio frequency waves  in some of these patients.( Diffiuclt as an emergency procedure)

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