Torsade De Pointes
Highlights
- The most appropriate way to classify PVT is by its association with normal or prolonged QT (or QTU) segment (View Highlight)
- The electrophysiologic mechanisms of these two types of PVT may be different. The term TdP should be reserved for use with the long QT syndrome (LQTS). However, not all patients with LQTS have PVT with a characteristic TdP configuration, and this classic configuration can be seen in some cases without a prolonged QT interval. (View Highlight)
- The change in QRS configuration during ventricular tachyarrhythmia can take several forms. During a very fast ventricular tachyarrhythmia, a periodic decrease in the amplitude of the entire QRS-T complex is seen with less distinct shifts in the QRS axis (Fig. 2A). In ventricular tachyarrhythmias with slower rates, the classic twisting of the QRS axis from a predominantly positive to a predominantly negative configuration with a variable number of transitional complexes and vice versa is commonly seen (Fig. 2B). Sometimes, a polymorphic QRS configuration is seen without any of the two previous characteristic patterns (as verified in multiple simultaneous leads, Fig. 2C, middle recording). Different patterns can be seen in different ventricular tachyarrhythmia episodes from the same patient (View Highlight)
- The congenital LQTS is caused by mutations in potassium (K+) and sodium (Na+) cardiac ion channel genes (View Highlight)
- It is estimated that LQT1, caused by KVLQT1 IKs potassium channel gene mutations, and LQT2, caused by HERG IKr potassium channel gene mutations, account for the majority (87%) of cases of LQTS with known genotype (View Highlight)
- Homozygous KVLQT1 and KCNE1 mutations are associated with congenital deafness (Jervell and Lange-Nielsen syndromes) and account for less than 1% of LQTS (View Highlight)
- A slow driving rate and a low extracellular K+ usually have a significant synergistic effect. The slow driving rate probably acts by reducing the Ip. Low extracellular K+ will decrease K conductance, which in turn will affect a number of currents, including Ik, Ik1, and If. (View Highlight)
- Any pharmacologic agent that can result in prolonged QT can induce LQTS (View Highlight)
- Prolongation of the repolarization phase acts as a primary step for the generation of early afterdepolarizations. Early afterdepolarization-induced triggered beats arise predominantly from the Purkinje network [16]. In the LQTS, prolonged repolarization is associated with increased spatial dispersion of repolarization [16,20]. The focal early afterdepolarization-induced triggered beats can infringe on the underlying substrate of inhomogeneous repolarization to initiate re-entrant excitation in the form of rotating scrolls. (View Highlight)
- The mechanism of the characteristic twisting morphology of TdP was explained on the basis of transient bifurcation of a predominantly single rotating scroll into two simultaneous scrolls involving both the right and left ventricles separately (View Highlight)
- One or more short-long cardiac cycles, usually the result of a ventricular bigeminal rhythm, frequently precede the onset of malignant ventricular tachyarrhythmias (View Highlight)
- The bigeminal beats consistently arose from a subendocardial focal activity (SFA) from the same or different sites, whereas TdP was caused by encroachment of the SFA on a substrate of dispersion of repolarization to induce re-entrant arrhythmias. (View Highlight)
- In the presence of a multifocal bigeminal rhythm, TdP followed the SFA that had both a critical site of origin and a local coupling interval in relation to the underlying pattern of dispersion of repolarization that promoted re-entry. In the presence of a unifocal bigeminal rhythm, the following mechanisms for the onset of TdP were observed: (1) a second SFA from a different site infringed on the dispersion of repolarization of the first SFA to initiate re-entry, and (2) a slight lengthening of the preceding cycle lengths resulted in increased dispersion of repolarization at key sites caused by differential increase of local repolarization at midmyocardial zones compared with epicardial zones. The increased dispersion of repolarization resulted in de novo arcs of functional conduction block and slowed conduction to initiate re-entry. Thus, the transition of a bigeminal rhythm to TdP was caused by well-defined electrophysiologic changes with predictable consequences that promoted re-entrant excitation. (View Highlight)
- tachycardia-dependent T-wave alternans occurs in patients with the congenital or the idiopathic form of the LQTS and may presage the onset of TdP (View Highlight)
- The arrhythmogenicity of QT/T alternans was primarily caused by the greater degree of spatial dispersion of repolarization during alternans than during slower rates not associated with alternans (View Highlight)
- In the presence of a critical degree of dispersion of repolarization, propagation of the activation wavefront could be blocked between these zones to initiate re-entrant excitation and polymorphic ventricular tachyarrhythmia. Two factors contributed to the modulation of repolarization during QT or T alternans, resulting in greater magnitude of dispersion of repolarization between midmyocardial and epicardial zones at critical short cycle lengths: (1) differences in restitution kinetics at midmyocardial sites, characterized by a larger ΔARI and a slower time constant (τ) compared with epicardial sites, and (2) differences in the diastolic interval that would result in different input to the restitution curve at the same constant cycle length. The longer ARI of midmyocardial sites resulted in shorter diastolic intervals during the first short cycle and, thus, a greater degree of ARI shortening (View Highlight)
- Clinical observations suggest that the onset of TdP in LQT3 patients with a mutant Na+ channel may occur at rest or during sleep rather than during exercise, possibly in association with relative bradycardia (View Highlight)
- patients with mutant K+ channels, especially LQT1, usually have syncope or cardiac arrest under stressful conditions, possibly because of an arrhythmogenic effect of catecholamine or differences in the rate and degree of accomodation of the QT interval to cycle length shortening (View Highlight)
- From an electrophysiologically mechanistic point of view, autonomic manipulations can be arrhythmogenic in the LQTS by means of two interrelated mechanisms: (1) by enhancing or suppressing the generation of early afterdepolarizations and their conduction in the heart, and (2) by enhancing or suppressing the dispersion of repolarization. The latter mechanism is essential for the occurrence of re-entrant excitation. (View Highlight)
- Besides the LQTS, the Brugada syndrome [38] and the catecholaminergic PVT [39••] occur in structurally intact hearts with genetic mutations of the SCN5A gene that encodes the cardiac Na+ channel in the former syndrome and the cardiac ryanodine receptor gene (RyR2) in the latter. (View Highlight)