FIGURE 73-1 Documented tachycardia with atrial and ventricular electrograms and marker channel annotations with measured intervals in milliseconds.

CASE EXPLANATION

• This is a case of 1:1 tachycardia resulting in ICD therapy. Evaluation of an arrhythmia resulting in a shock requires careful evaluation of the stored electrograms, interval plots, tachycardia initiation, response to antitachycardia pacing (ATP), and arrhythmia termination to determine if the therapy was appropriate or inappropriate.

• Inappropriate shocks are shocks that occur for rhythms other than ventricular tachycardia or ventricular fibrillation and occur in up to 30% of patients with ICD shocks.¹ They are most common due to supraventricular tachycardia (SVT), which gets classified by the device as ventricular tachycardia (VT). It is essential to determine the appropriateness of the shock to guide programming and medical therapy.

• The differential diagnosis of this event with 1:1 AV relationship includes ventricular tachycardia with retrograde atrial activation versus supraventricular tachycardia.

PATHOPHYSIOLOGY

The quintessential reason for implanting an ICD is to provide therapy for a life-threatening rhythm that if left untreated would result in sudden death. Differentiating VT from SVT is a key component of device algorithms to appropriately classify tachycardia events. Enhancements to tachycardia detection include rhythm stability, tachycardia onset, electrogram morphology, atrial to ventricular ratios, and atrial to ventricular timing. No algorithm is perfect in differentiating ventricular tachycardia from supraventricular tachycardia, but each detection enhancement aids in correctly identifying the rhythm.

• Rhythm stability helps differentiate atrial fibrillation with rapid ventricular response from ventricular tachycardia due to the variable R-R intervals seen in atrial fibrillation.

• Onset: Sinus tachycardia tends to gradually increase in rate; whereas other types of supraventricular tachycardia and ventricular tachycardia occur abruptly with a premature complex.

• Morphology templates: Newer algorithms store a baseline morphology of the intrinsic QRS. This is used during rhythm discrimination; SVT usually results in preserved QRS morphology, and VT results in a change in morphology due to the activation sequence. This algorithm is less reliable in patients with a baseline bundle branch block or when aberrancy develops during supraventricular tachycardia. Limitations to morphology algorithm, even when the template is regularly updated, include changes in the baseline QRS morphology, problems with alignment in the baseline and tachycardia morphology, and clipping of the signal.

• Atrial to ventricular ratios also assist. In general, A > V suggests SVT or atrial tachycardia, A = V suggest SVT or VT, and V > A suggests ventricular tachycardia.

DIAGNOSIS

The approach to evaluating an episode should be performed in a stepwise fashion.

• The episode text will outline the event, including the classification, duration, and therapy received as seen in Figure 73-2

Stay updated, free articles. Join our Telegram channel

Join