It is one of the greatest innovation in medicine . . . that is . . . electric current being used as a drug to treat disorders of heart . Of course , it is not a surprising finding when we know heart is an electro mechanical organ , and electricity can be used to treat various disorders of heart by delivering it in an optimal dosage and site.
Devices that help administer electric current in cardiac disease.
- External cardiovertor and defibrillator
- Implanted defibrillator
- Anti bradycardia pacemaker
- Anti tachycardia pacing
- Cardiac resynchronisation device
What is the difference between cardioversion and pacing ?
Cardioversion is reverting a tachycardia with a electric shock that is delivered diffusely throughout the heart This electrical wavelets traverse the focus of tachycardia and the adjoining myocardium which is called critical electrical mass (Usually reentrant) .This depolarises the cells responsible for tachycardia and extinguishes the abnormal electrical activity.
Defibrillation is same as cardiversion except that it is a high energy shock and delivered without synchrony with qrs complex . In VF, we defibrillate in all others we cardiovert .
What are the disadvantages of cardioversion ?
Eventhough it is a very successful modality for treating cardiac arrhythmias it also has some issues.
- Cardioversion is not infallible. It rarely works in tachycardia due to enhanced automaticity (Multifocal atrial tacycardia , Automatic junctional tacycardia , Digoxin induced tacycardia it may even be dangerous !)
- Many times multiple shocks are required and may result in myocardial damage, stunning , and elevated cardiac enzymes.
- In susceptible patients, especially in elderly it may depress the natural pacemaker ie the sinus node and dangerous bradycardia
What is difference between cardioversion applied externally on the chest wall and intracardiac cardiversion as in Implantable cardiovertor defibrillator(ICDS) ?
The underlying principle is same except that the energy required is a fraction of that applied in the chest wall . The average energy required is up to 20 joules . while it requires up to 300 joules
What is anti tachycardia pacing ? Why this concept came into vogue ?
When it became clear , cardioversion may not work in all forms of tachycardia and risks of multiple shocks on the myocardium not be taken lightly , experts in those times (1970s) thought a pace maker lead in a optimal site can do the job of cardiovertor. .
Pacing rapidly in the tachycardia zone provide us an opportunity to enter the tachycardia circuit , interfering , interrupting and blocking the reentrant circuit (We call it entrainment) . If it is an automatic tachycardia pacing in close vicinity of the tachycardia focus result in a electrical line of barrier which acts as an exit block ( Like the lakshman reka ! in Ramayana )
The term ATP is used as a general term as anti tachycardia pacing .Over drive pacing can be used synonymously.
What is the main advantage of ATP ?
- Less injury as it avoids recurrent shocks .
- Can be administered as many time as required .
- Some tachycardias specifically respond to ATP only (Read below)
How to perform overdrive pacing ?
- Conventional temporary pacing lead can be used .
- Special leads may be required for paired pacing.
- The rate of pacing is generally 20 % more than the tachycardia rate.
- Though we call it over-drive pacing, in reality, pacing can be done at a lesser heart rate than tachycardia itself.This should be recognized because the term anti-tachycardia pacing doesn’t always mean pacing at more than the tachycardia rate. (Concept of Under-drive pacing Bond R, et al Underdrive pacing to terminate ventricular tachycardia Heart 2012;98:676)
- The duration of pacing maybe 30 seconds to 2 mts as necessary .
Can we use the external transcutaneous pacemaker paddles for overdrive pacing ?
Yes we can, it may be termed a non invasive external overdrive pacing .This mode is not popular among cardiologists not because it is ineffective , rather we have not fully realised it’s potential .
Different types of overdrive pacing
What is coupled pacing ?
It is a type of overdrive pacing where patient’s own spontaneous rhythm is used trigger a pacemaker stimulus and hence only alternate beats or pacing beats which is coupled with the pateint,s own rhythm it is called coupled pacing . This is different from from paired pacing in that only single pacemaker stimulus per cycle .
What is paired pacing ?
Two pacing stimulus are given.The first impulse is maintained constant and the second impulse is done with varying coupling interval to scan the entire cardiac cycle .It is expected at some point of paired pacing the second impulse would block the reentrant circuit.
What is random paired pacing ?
The atrium is delivered a pair of random stimulus ( . . Like a bite of snake !) is delivered into the atria .This can revert many of the reentrant atrial and ventricular re entrant tachycardia.
What is the unique value of sinus paired pacing ?
In patients with persistent sinus tachycardia, especially in patients with high MVo2 situations or dysfunctional ventricle we have no option to control the heart rate without depressing myocardial contraictility . Most of the negative chronotropic drugs have negative inotropic action also. In these situations pairing a pacemaker stimulus with a sinus impulse can produce a compenstatry pause and result in reduction in net heart rate as well as increased contractility due to post extrasystolic potentiation.
How does a catheter whip inside the atria terminate many of the procedure related tachycardias in cath lab ?
It is a common maneuver in cath lab , to forcibly whip the catheter for terminating many of the transient procedure related SVTs and outflow tract VTs . The arrhythmias get terminated either due to catheter hit induced mechano electrical cardioversion (5 joules ?) or the atrial subendocardial stretch due to the whip lash .
What are the tachycardias that may respond to overdrive pacing ?
It is logical to expect any of the reentrant tachycardia might respond to ATP. The exact success rate can not be established since this modality is not applied in vast majority of patients . Only if a patient is not responding to drugs or multiple DC shocks ATP is thought off . Of course ATP can not considered a first option unless othe patient is on a temporary pacer.
What is the caution for using ATP ? Why atrial overdrive pacing is preferred over ventricular overdrive pacing ?
Pacing a ventricle rapidly carries a risk of inducing ventricular fibrillation . So whenever possible ATP should be administered through an atrial lead. This may not be possible always as in the presence of AV block a VT can not be captured by atrial pacing .
It is also a fact many times when the ventricular overdrive pacing fails to revert a VT , an atrial overdrive pacing has been successful . This is due to the more uniform depolarization wave fronts , that reach the ventricle and reset the VT .
Currently ATP is useful in
- Recurrent atrial tachycardia
- Refractory ventricular tachycardia especially with enhanced automaticity (Early ischemic VT )
- Digoxin induced tachycardias
- Some cases of Tachy brady syndrome
In some of the modern pacemakers and in all ICDs ATP is a an important programmable parameter .In fact, using this mode liberally would conserve battery life .Many times a simple hemodynamically stable VTs are shocked by ICDs instead an ATP will do the job . It is a well recognised fact that ATP is underutilsed in ICDs .This issue needs to be addressed.
Final message
Pacemakers are not only meant to treat bradycardias but also tachycardia. Even though it is a well-known fact for over 3 decades, for some reason this simple and effective concept is not getting the attention of the current generation cardiologists which it definitely deserves!
Reference
- Overdrive Pacing for Ventricular Tachyarrhythmias: A Reassessment P. R. KOWEY andT. R. ENGEL
ANN INTERN MED November 1, 1983 99:651-656
- Pacing Techniques in the Treatment of Tachycardias I. WIENER ANN INTERN MED August 1, 1980 93:326-329
- Treatment of Recurrent Symptomatic Ventricular Tachycardia R. A. WINKLE, E. L. ALDERMAN, J. W. FITZGERALD, and D. C. HARRISON ANN INTERN MED July 1, 1976 85:1-7
- Treatment of Tachyarrhythmias by Pacing J. E. Batchelder andD. P. Zipes
Over-drive pacing : A practical approach
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Dr.S.Venkatesan MD 