"You mean I was shocked?"
"Three weeks ago, on the 23rd, at 06:38AM and 12 seconds."
"But I didn't feel anything. Just a minute, let me check my calendar." She flips through her calendar, mind scrambling. "I was in Florida. That was the day I went to the spa. I felt fine."
"Remember when you got up that day?"
Hestitating. "I'm not sure. I never had a shock before. Are you sure this thing fired?"
"Yep. Here's what the device recorded:"
(Click to enlarge)
"Why did this happen?"
"I'm not sure. As you know you have atrial fibrillation and a weak heart muscle though you were never found to have coronary artery disease (blocked arteries) - that's why the defibrillator was installed."
"Well, if we look at the tracings, I notice two small clues as to what happened. See those little VS notations?"
"Well those occur every time one of your heart beats are sensed by the device. If there's an FS, that means a heart beat was detected that was above the ventricular fibrillation cutoff rate (in other words, it was going really fast). The TF also means that the heart rate fell into a very fast ventricular tachycardia zone. A VP notation means the device paced your heart. Now, see the 'FD' as the first label on the bottom of the seond line of tracings?"
"Well that means that enough fast heart beats occurred to satisfy the device's algorithm to call this rhythm ventricular fibrillation - the FD stands for 'Fib Detect.' Note how the device then stops labelling the sensed ventricular beats as FS's as it charges to deliver a shock(the labels turn back to VS's but the rate still looks really fast.) Once the charge ends, a CE label is displayed. The device looks for one more fast heart beat, and when it sees it, it delivers its charge, labeled as 'Charge Delivered,' or CD on the next sensed rapid ventricular beat to reset the rhythm. That's when the shock occurs that most people feel. Then we can see your own normal ventricular sensed (VS) beats after the shock is delivered."
"Yes ma'am, it is. Happy Valentine's Day."
"Oh my God, you're right! I can't believe it! I really just can't believe it!"
So what programming change was made to her device to prevent another shock? It was a single-chamber Medtronic ICD programmed with backup ventricular pacing at forty beats per minute.
Addendum: For those not accustomed to viewing the information produced by interrogating a defibrillator after a shock, the top two lines on each strip represent the signal seen (1) from the tip electrode to the can of the device, simulating an EKG lead, and the lower line (2) is the signal seen locally inside the heart between closely spaced electrodes on the lead (these lines are not "on" at first until a rapid rhythm is detected). The lowest line is the "marker channel" that demonstrates how each blip on the middle signal line was interpreted by the device.