For example, that 150 bpm rate might be perfectly appropriate for a patient doing strenuous exercise. It would just be an exercise-induced sinus tachycardia. In fact, in such a patient, it would be a problem if the ventricular rate did not respond that way!

On the other hand, a 150 bpm rate might be caused by a ventricular arrhythmia. It could be provoking symptoms and might advance to faster and more dangerous even life-threatening rates.

Sometimes a rapid ventricular rate is just a ventricular response to an atrial tachycardia. However, if we as clinicians need an ECG along with our judgment, training, and experience to discern which rhythm disorder is which, how does an ICD know what arrhythmias to treat?

ICDs were designed to treat dangerous ventricular tachyarrhythmias, such as ventricular tachycardia (VT) and ventricular fibrillation (VF). ICDs tend to see cardiac activity strictly in terms of programmable rate ranges. So how does an ICD know when a 150 bpm rate is a supraventricular tachycardia (SVT) or a VT?

The distinction is an important one, because ICD therapy is designed to treat VT and VF. Shocking an SVT may or may not convert the rhythm, but SVT is not life-threatening. An ICD should not be wasting its battery energy by delivering therapy for SVTs, which do not require treatment. This saves device service life, but more importantly, spares the patient painful and unnecessary shocks. The industry euphemism for ICD shocks for SVTs is called inappropriate therapy.

However, there is something even more important to the patient than avoiding inappropriate therapy. It s assuring appropriate therapy. Discriminators not only prevent unnecessary shocks, they assure that patients get the life-saving therapy they need for VT.

SVT discrimination is not about treating VF. When a patient experiences VF, therapy is delivered as quickly as possible. SVT discrimination only applies to VT, which in itself can be a life-threatening arrhythmia. However, when we talk about device therapy, VF remains rightfully in a class by itself.

The challenge for the device inventors has been to find a way to minimize or prevent inappropriate therapy while assuring appropriate therapy for every VT. But how does a device know what s what?

Put in simple terms, the ICD does it the same way we clinicians do. It looks at the ECG and asks itself some simple questions. In fact, SVT discrimination features in modern ICDs are really nothing more than the questions we would pose when analyzing an ECG.

If you had a patient with a rapid ventricular rate, you would want to check out the ECG to see what the atria are doing with respect to the ventricles. If you have an atrial rate that is going even faster than the ventricular rate, then the patient has atrial fibrillation (AF), and the ventricular rate is likely to be the heart s attempt to keep pace with the atria. This strongly suggests you have an SVT with a rapid ventricular response.

A 1:1 response, that is, every ventricular beat linked to an atrial event, would make you think that this might be a sinus tachycardia. You would want to find out what the patient was doing when the ECG was taken. A sinus tachycardia for a patient doing a treadmill test is different than a sinus tachycardia in a little old lady bedridden in a nursing home.

If you see that the ventricles are going much faster than the atria, then it is clear that the atria aren t driving the rate. This would have to be VT.