Bidirectional Ventricular Tachycardia: Challenges and Solutions

Spatially discordant alternans due to periodic pacing site alternation

BACKGROUND

Bidirectional ventricular tachycardia (BVT) is a rare type of ventricular tachycardia that is characterized by a beat-to-beat alternation in the QRS axis. Previous studies have shown that it is caused by alternating focal activities from 2 locations.

OBJECTIVE

This study proposes a novel mechanism for the formation of spatially discordant alternans (SDA) due to the periodic pacing site alternation that occurs in BVT.

METHODS

We used mathematical models of cardiac tissue to understand the dynamic and physiologic mechanisms underlying SDA formation.

RESULTS

We found that SDA was formed by periodic pacing site alternation. When tissue was paced from 2 locations alternately, the timing of pacing at distant locations varied, creating a long-short-long-short sequence of pacing periods and thus action potential durations. Importantly, the nodal lines were perpendicular to the wavefront, which is more arrhythmogenic than when nodal lines are parallel to the wavefront. A positive correlation was observed between the separation distance of the 2 sites and the alternans amplitude. SDA patterns can be predicted from the tissue geometry and pacing site locations.

CONCLUSION

Periodic pacing site alternation, which occurs in BVT, leads to arrhythmogenic SDA. The nodal lines associated with this phenomenon can be predicted on the basis of tissue geometry and focal locations.

Ventricular Tachycardia Due to Triggered Activity: Role of Early and Delayed Afterdepolarizations

Most forms of sustained ventricular tachycardia (VT) are caused by re-entry, resulting from altered myocardial conduction and refractoriness secondary to underlying structural heart disease. In contrast, VT caused by triggered activity (TA) is unrelated to an abnormal structural substrate and is often caused by molecular defects affecting ion channel function or regulation of intracellular calcium cycling. This review summarizes the cellular and molecular bases underlying TA and exemplifies their clinical relevance with selective representative scenarios. The underlying basis of TA caused by delayed afterdepolarizations is related to sarcoplasmic reticulum calcium overload, calcium waves, and diastolic sarcoplasmic reticulum calcium leak. Clinical examples of TA caused by delayed afterdepolarizations include sustained right and left ventricular outflow tract tachycardia and catecholaminergic polymorphic VT. The other form of afterpotentials, early afterdepolarizations, are systolic events and inscribe early afterdepolarizations during phase 2 or phase 3 of the action potential. The fundamental defect is a decrease in repolarization reserve with associated increases in late plateau inward currents. Malignant ventricular arrhythmias in the long QT syndromes are initiated by early afterdepolarization-mediated TA. An understanding of the molecular and cellular bases of these arrhythmias has resulted in generally effective pharmacologic-based therapies, but these are nonspecific agents that have off-target effects. Therapeutic efficacy may need to be augmented with an implantable defibrillator. Next-generation therapies will include novel agents that rescue arrhythmogenic abnormalities in cellular signaling pathways and gene therapy approaches that transfer or edit pathogenic gene variants or silence mutant messenger ribonucleic acid.

Feud, Flower, and Fatal Electrocardiograms

Oleander is a prevalent tropical plant used in many parts of India for deliberate self-harm. The active ingredients act in a mechanism similar to cardiac glycosides; hence, the toxicological profile is similar to digoxin toxicity. Cardiac toxicity occurs in the form of a heart block with concomitant ventricular arrhythmia. Identifying the distinct electrocardiographic pattern for early diagnosis and initiating emergency management is imperative. Here, we present two such interesting cases of oleander intoxication, one with Nerium oleander and the other with Thevetia peruviana.

Unusual Tachycardia After Cocaine Use

This case report presents the electrocardiogram findings of a patient in their 20s with severe midsternal chest pain of several hours’ duration after cocaine use.