We find that one of the most intimidating things about implantable cardiac devices is figuring out what each one does, so we spent some time decoding the alphabet soup of pacemaker function letter codes (which can be found on the pacemaker/defibrillator cards that patients carry in their wallets). Every pacemaker has a five-letter code associated with it. Since 2002, the United States and the United Kingdom have used the same letter code for all pacemakers in order to keep things as simple as possible. (We guess the US and the UK have a special relationship after all!) The first three letters are the most commonly referred to letters. The first position tells you which chamber gets paced. The second letter indicates which chamber the pacemaker uses for sensing cardiac activity. The third letter is the “mode of response.” This one indicates how the pacemaker responds to the information it senses. The most common first three letters are VVI (that’s ventricular pacing, ventricle sensing, and inhibition of electric pacing) and DDD (dual chamber pacing, dual chamber sensing, and dual functionality for both the inhibition and the triggering of a pacemaker). While the fourth letter is a little boring (programming settings), the fifth letter in the code is probably our favorite: anti-tachyarrhythmia function.

Many pacers have no anti-tachyarrhythmia functions (denoted as a 0). Some pacemakers pace in response to tachyarrythmias (P). Other pacers can deliver a shock in response to tachyarrhythmias (S). Finally, a pacemaker can have a dual anti-tachyarrhythmia; that’s both shock and pace (D). That’s what we call an “everything but the kitchen sink” pacemaker.

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Dr. Faust is a senior emergency-medicine resident at Mount Sinai Hospital in New York. He tweets about #FOAMed and classical music @jeremyfaust.

Dr. Westafer is chief resident at the Baystate Medical Center at Tufts University in Springfield, Massachusetts. Follow her @LWestafer.