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5-3. Lesson 5 (cont) Ventricular Arrhythmias

Topics of Study:

  1. Premature ventricular complexes (PVCs)
  2. Aberrancy vs. ventricular ectopy
  3. Ventricular tachycardia
  4. Differential diagnosis of wide QRS tachycardias
  5. Accelerated ventricular rhythms
  6. Idioventricular rhythm
  7. Ventricular Parasystole


Premature Ventricular Complexes (PVCs)

image 04-39

PVCs may be unifocal (see above), multifocal (see below) or multiformed. Multifocal PVCs have different sites of origin, which means their coupling intervals (measured from the previous QRS complexes) are usually different. Multiformed PVCs usually have the same coupling intervals (because they originate in the same ectopic site but their conduction through the ventricles differ. Multiformed PVCs are common in digitalis intoxication.

image 04-33

PVCs may occur as isolated single events or as couplets, triplets, and salvos (4-6 PVCs in a row), also called brief ventricular tachycardias.

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PVCs may occur early in the cycle (R-on-T phenomenon), after the T wave (as seen above), or late in the cycle - often fusing with the next QRS (fusion beat). R-on-T PVCs may be especially dangerous in an acute ischemic situation, because the ventricles may be more vulnerable to ventricular tachycardia or fibrillation. Examples are seen below.

image 04-07

The events following a PVC are of interest. Usually a PVC is followed by a complete compensatory pause because the sinus node timing is not interrupted; one sinus P wave isn't able to reach the ventricles because they are still refractory from the PVC; the following sinus impulse occurs on time based on the sinus rate. In contrast, PACs are usually followed by an incomplete pause because the PAC usually enters the sinus node and resets its timing; this enables the following sinus P wave to appear earlier than expected. These concepts are illustrated below.

image 01-06

Not all PVCs are followed by a pause. If a PVC occurs early enough (especially if the heart rate is slow), it may appear sandwiched in between two normal beats. This is called an interpolated PVC. The sinus impulse following the PVC may be conducted with a longer PR interval because of retrograde concealed conduction by the PVC into the AV junction slowing subsequent conduction of the sinus impulse. test

image 04-31

Finally a PVC may retrogradely capture the atrium, reset the sinus node, and be followed by an incomplete pause. Often the retrograde P wave can be seen on the ECG, hiding in the ST-T wave of the PVC.

The most unusual post-PVC event is when retrograde activation of the AV junction re-enters the ventricles as a ventricular echo. This is illustrated below. The "ladder" diagram below the ECG helps us understand the mechanism. The P wave following the PVC is the sinus P wave, but the PR interval is too short for it to have caused the next QRS. (Remember, the PR interval following an interpolated PVC is usually longer than normal, not shorter!).

image 04-08

PVCs usually stick out like "sore thumbs", because they are bizarre in appearance compared to the normal complexes. However, not all premature sore thumbs are PVCs. In the example below 2 PACs are seen, #1 with a normal QRS, and #2 with RBBB aberrancy - which looks like a sore thumb. The challenge, therefore, is to recognize sore thumbs for what they are, and that's the next topic for discussion!

image 04-13

Aberrancy vs. Ventricular Ectopy

A most important question

Aberrant Ventricular Conduction: defined as the intermittent abnormal intraventricular conduction of a supraventricular impulse. The phenomenon comes about because of unequal refractoriness of the bundle branches and critical prematurity of a supraventricular impulse (see diagram of Three Fates of PACs). With such critical prematurity, the supraventricular impulse encounters one bundle branch (or fascicle) which is responsive, and the other which is refractory, and is consequently conducted with a bundle branch block or fascicular block pattern.

ECG clues to the differential diagnosis of wide QRS premature beats:

If the QRS in V1 is mostly positive the following possibilities exist:

image 01-20

If the QRS in V1 is mostly negative the following possibilities exist:

image 01-21image 01-21.5

Another QRS morphology clue from Lead V6:

The timing of the premature wide QRS complex is also important because aberrantly conducted QRS complexes only occur early in the cardiac cycle during the refractory period of one of the conduction branches. Therefore, late premature wide QRS complexes (after the T wave, for example) are most often ventricular ectopic in origin.

Ventricular Tachycardia

Descriptors to consider when considering ventricular tachycardia:

Differential Diagnosis: just as for single premature funny-looking beats, not all wide QRS tachycardias are ventricular in origin (i.e., they may be supraventricular tachycardias with bundle branch block or WPW preexcitation)!

Differential Diagnosis of Wide QRS Tachycardias

Although this is an ECG tutorial, let's not forget some simple bedside clues to ventricular tachycardia:

ECG Clues:

Accelerated Ventricular Rhythms

(see ECG below)

image 07-04

Idioventricular Rhythm

A "passive" escape rhythm that occurs by default whenever higher-lever pacemakers in AV junction or sinus node fail to control ventricular activation.

Ventricular Parasystole

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