The   following   questions   refer   to   implantable   cardiac   pacemakers   and implantable cardiac defibrillators.

a)         What is the effect of applying a magnet to these devices?

b)         What information can you gain from a chest X-Ray in a patient with an implantable cardiac device?

c)         What are the advantages of DDD pacing compared to VVI pacing?

d)         List 4 benefits of cardiac resynchronisation therapy.

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College Answer

a)         What is the effect of applying a magnet to these devices?

ICD:  it turns off antiarrhythmic programme but has no affect on backup pacemaker
Pacemaker: It defaults to asynchronous mode or a fixed rate. Rate depends on battery life.

b)         What information can you gain from a chest X-Ray in a patient with an implantable cardiac device?

•    Single v dual chamber
•    Biventricular or left ventricular (cardiac resynchronisation)
•    Lead displacement or injury
•    Number of devices present

c)         What are the advantages of DDD pacing compared to VVI pacing?

•  AV synchronisation maintained
•  Avoids pacemaker syndrome
•  Reduced incidence of AF
•  Possible decreased thrombotic events

d)         List 4 benefits of cardiac resynchronisation therapy.

•  improved LVEF,CO and haemodynamics
•  improved exercise tolerance
•  decreased NYHA class
•  decreased hospitilisation
•  improved quality of life

Discussion

a) is pretty straightforward as far as pacemaker questions go. Generally speaking, most of them will respond to magnet exposure by becoming asynchronous, i.e. they will start pacing without sensing. AICDs will stop defibrillating. This is good to know if you are palliating a patient with an implanted device, and you don't want to have the defib firing randomly in the last few minutes of their life.

Of course, there is no standard among the manufacturers regarding what precisely should happen when the magnet is applied. Medtronic and Boston Scientific models will pace asynchronously; St Jude will cycle through some pre-programmed protocol which typically involves recording and storing an ECG, and Biotronic devices will do something completely random, depending on model and battery life. A good article about these idosyncratic behaviours is available on Medscape.

b) is also pretty straightforward. Immediately, on a chest Xray you can see how many leads there are, and you can guess from the size of the battery whether the device is also set up to defibrillate. Fractured leads and leads which have become grossly dislodged will also become obvious.

c) is a little more complicated. VVI mode of pacing is an older mode and is not optimised to deliver the best cardiac output, because it completely ignores the atrial contribution. If the atrial kick disappears altogether, the VVI box will make no attempt to compensate for this. In fact, the retrograde conduction of ventricular pacing tends to cause the atrium to contract in a profoundly stupid fashion, during ventricular systole and against a closed valve. This loss of atrioventricular synchrony is called "pacemaker syndrome".

DDD on the other hand is more sensitive to the needs of the ailing myocardium. Not only aresymptoms of the patients better with DDD (supporting the concept of hemodynamic advantage) but the risk of AF is decreased because there is a constant and reliable single source of electrical stimulus in the atrium.

d) is even more complicated, and asks for some indepth understanding of CRT. A good review article of the technique is available. The benefits of CRT for heart failure are discussed in a 2007 meta-analysis; bottom line is that CRT improves NYHA grade, and it may improve mortality (though some think this may be due to the built-in AICD function and the prevention of sudden cardiac death by these devices). The problem is, you have to have LBBB, severe heart failure and several other hideous criteria in order to benefit, which means that only 5-10% of people actually qualify for this treatment.

Relevant Required Reading chapters include:

References

References

BYRD, CHARLES L., et al. "DDD pacemakers maximize hemodynamic benefits and minimize complications for most patients." Pacing and Clinical Electrophysiology 11.11 (1988): 1911-1916.

Channon, K. M., et al. "DDD vs. VVI pacing in patients aged over 75 years with complete heart block: a double-blind crossover comparison." QJM 87.4 (1994): 245-251.

Strik, Marc, et al. "Cardiac resynchronization therapy." Circ J 75.6 (2011): 1297-1304.

Tang, Anthony SL, et al. "Cardiac-resynchronization therapy for mild-to-moderate heart failure." New England Journal of Medicine 363.25 (2010): 2385-2395.

McAlister, Finlay A., et al. "Cardiac Resynchronization Therapy for Patients With Left Ventricular Systolic DysfunctionA Systematic Review." Jama 297.22 (2007): 2502-2514.