How do you differentiate between a supraventricular arrhythmia and a ventricular arrhythmia on an electrogram? In this lesson, find out what morphology discriminators are and how we can use them to help us discriminate between various arrhythmias.
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[00:00:00] QRS morphology is a discriminator that uses the appearance of a QRS to discriminate between benign rhythms and potentially dangerous ventricular arrhythmias. Now, the way it's able to do this is that, really, in the majority of SVTs or supraventricular arrhythmias, QRS will remain narrow. However, during ventricular arrhythmias, we might have a broad complex
[00:00:30] in VT or a very wild, chaotic, and unusual QRS complex in VF. The QRS discriminator looks to recognize, when the QRS has become so different, that a ventricular arrhythmia is assumed. So, if we think about VT, the reason we get a broad and wide complex, is normally because we have a re-entrance circuit going on. This may be occurring around some scar tissue from an old
[00:01:00] myocardial infarction, an old heart attack, but either way, this circuit, that is occurring, is taking slightly longer, to depolarize the myocardium, then the depolarization would occur if the normal sinus pathways had been used. So, through the AV node, into the His-Purkinje system. Now, this is how it works. It takes a baseline template of a normal heartbeat. So, immediately, it becomes apparent,
[00:01:30] this discriminator is not available to people with no underlying rhythm because we're unable to obtain a baseline, to use as a reference point. So, the template is recorded. Now, during an arrhythmia, another template is recorded and the two are compared against one another. Like most of these discriminators, we have a threshold, a cutoff point. This can be anywhere from 70% to as high as 90%–95%, depending on the
[00:02:00] manufacturer and on the patient. But the way it works is quite simple. In comparing the two templates, if they resemble each other by greater than 70%, then this is considered a normal rhythm or a supraventricular tachycardia. For that reason, therapy is withheld. Alternatively, if the rhythms are not 70% similar in our template, then therapy is delivered, as a ventricular event is assumed.
[00:02:30] This is a really great discriminator, and this is one of the newer discriminators available to us. And it's really important to us, as a discriminator now because it's able to discriminate where other discriminators may fall short. Let me explain what I mean. If we take an SVT and a VT together in a single-chamber defibrillator. We can see that, actually, the stability is the same, the heart rate is the same, and the onset
[00:03:00] could also be the same. They might have both started very abruptly. Now, the morphology discriminator, at that point, is the only discriminator available, that is able to discriminate between a VT and an SVT because it recognizes that the QRS is slightly different and actually, only has a 48% match score in this example, so therapy will be delivered. The SVT, in contrast, had a 97% template match, and therefore, therapy
[00:03:30] wasn't delivered and therapy was correctly withheld. Also, worth mentioning, is that many of these devices regularly update their template. And if they don't, a template should be recorded at every follow-up. Now, the reason is that the heart can remodel, and the baseline QRS can change. If we didn't keep our template up to date, then in some patients, eventually, their intrinsic QRS will become so different to when the template was recorded, that therapy could be inappropriately
[00:04:00] delivered during a sinus tachycardia or another SVT.