Tachyarrhythmias, Pre-excitation & Inherited Channelopathies

How tachyarrhythmias start

Reentry. Most clinically important tachycardias are reentrant. A circuit needs two limbs with different conduction speeds and refractory periods. A premature impulse blocks in one limb because it is still refractory, travels slowly down the other, and returns up the first limb once it has recovered, producing a self-sustaining loop. AVNRT, AVRT, atrial flutter and VT around an infarct scar all work this way.

Enhanced automaticity. A steeper phase 4 slope lets a focus fire faster than the SA node. Catecholamines, ischemia and hypokalemia promote this.

Triggered activity. Afterdepolarizations are oscillations that can reach threshold. Early afterdepolarizations (EADs) occur during phases 2–3 when repolarization is prolonged, as with a long QT or bradycardia, and they set off torsades de pointes, typically after a pause. Delayed afterdepolarizations (DADs) arise after full repolarization when the cell is overloaded with Ca2+ and the sarcoplasmic reticulum releases it spontaneously. Causes include digoxin toxicity (Na+/K+-ATPase inhibition), catecholamine excess and CPVT. Bidirectional VT is the signature of DAD-driven rhythms, seen with digoxin toxicity and CPVT.

Supraventricular tachycardias

Atrial fibrillation. AF is the most common sustained arrhythmia, and its prevalence rises steeply with age. Most episodes start from ectopic foci in the myocardial sleeves of the pulmonary veins, which is why catheter ablation isolates the pulmonary vein ostia. Drivers include hypertension, heart failure, mitral valve disease with left atrial enlargement, CAD, hyperthyroidism (check TSH in new AF), alcohol binges ("holiday heart"), obesity, obstructive sleep apnea and cardiac surgery.

Consequences of AF. Without organized contraction, blood stagnates, especially in the left atrial appendage. Thrombus there can embolize, most often to the brain (commonly MCA territory) but also to mesenteric or limb arteries. The CHA2DS2-VASc score guides anticoagulation with a DOAC or warfarin; warfarin is required with moderate–severe mitral stenosis or a mechanical valve. Losing the atrial kick removes the jugular a wave and any S4, and persistently rapid rates can cause tachycardia-induced cardiomyopathy. Rate control uses β-blockers, verapamil/diltiazem or digoxin.

Atrial flutter. A macroreentrant wave circles the tricuspid annulus and passes through the cavotricuspid isthmus between the tricuspid valve and the inferior vena cava. The atria fire at about 300/min, producing sawtooth flutter waves best seen in II, III and aVF. With the usual 2:1 AV conduction, the ventricles run at about 150/min, so a regular narrow tachycardia at exactly 150 should prompt a search for flutter waves. Ablating the isthmus is usually curative, and stroke prevention follows the same rules as for AF.

Multifocal atrial tachycardia. Besides COPD with hypoxemia, MAT occurs with theophylline, hypokalemia and hypomagnesemia. Treat the cause, adding magnesium or verapamil; cardioversion is ineffective. The same pattern under 100/min is called wandering atrial pacemaker.

AVNRT. AVNRT is the most common paroxysmal SVT. In the typical form the impulse goes down the slow pathway and up the fast one, so atria and ventricles depolarize almost together. Vagal maneuvers come first, then IV adenosine; slow-pathway ablation is curative.

Adenosine. Adenosine stimulates A1 (Gi-coupled) receptors in the AV node, increasing K+ efflux (hyperpolarization) and reducing Ca2+ current, which causes a few seconds of AV block. Its half-life is under 10 seconds. It causes flushing, chest tightness, dyspnea or bronchospasm, and a sense of impending doom. Theophylline and caffeine blunt its effect and dipyridamole potentiates it. It terminates AV node–dependent reentry and, in flutter, slows the ventricles long enough to reveal the flutter waves.

Pre-excitation: Wolff–Parkinson–White

Anatomy and ECG. An accessory AV connection, the bundle of Kent, conducts without the AV node's delay, so part of the ventricle is activated early and slowly through muscle. The resting ECG shows a PR under 120 ms, a slurred QRS upstroke (delta wave) and a widened QRS. Accessory pathways are associated with Ebstein anomaly. WPW syndrome means pre-excitation plus symptomatic tachyarrhythmia.

AVRT. In about 90–95% of WPW tachycardias the circuit is orthodromic: down the AV node and back up the accessory pathway. The QRS is narrow and the delta wave disappears because the ventricles are activated only through the normal system. Because this circuit uses the AV node, vagal maneuvers or adenosine can terminate it. Antidromic AVRT runs down the pathway and up the node, producing a wide QRS, and is much rarer.

Pre-excited AF. When AF occurs in a patient with an accessory pathway, the ventricles can be driven very fast through the pathway, giving an irregular, wide-complex tachycardia whose QRS shape changes beat to beat.

In pre-excited AF, do not give AV-nodal blockers: adenosine, β-blockers, verapamil/diltiazem or digoxin. Blocking the node removes competition and speeds conduction down the pathway, which can lead to VF. Use procainamide (or ibutilide), synchronized cardioversion if the patient is unstable, and then catheter ablation of the pathway.

Ventricular arrhythmias

Monomorphic VT. VT is three or more consecutive wide (≥120 ms) ventricular complexes at over 100/min; uniform complexes define monomorphic VT, and it is sustained if it lasts more than 30 seconds. The usual substrate is reentry around healed infarct scar. AV dissociation, fusion beats and capture beats confirm VT over SVT with aberrancy. Pulseless VT is defibrillated, unstable VT with a pulse gets synchronized cardioversion, and an ICD provides secondary prevention.

Torsades de pointes. Torsades is polymorphic VT whose QRS amplitude twists around the baseline. EADs trigger it on a prolonged QT, often after a short-long-short sequence of beats. Acquired causes include class IA and III antiarrhythmics, macrolides, fluoroquinolones, haloperidol, methadone, ondansetron, hypokalemia, hypomagnesemia, hypocalcemia and bradycardia; congenital long QT syndrome is the inherited cause. IV magnesium sulfate is first-line even when serum magnesium is normal. Potassium is corrected, and for acquired, pause-dependent torsades the heart rate is raised with isoproterenol or overdrive pacing to shorten the QT. Isoproterenol is avoided in congenital LQTS, where adrenergic surges are the trigger.

Ventricular fibrillation. VF is chaotic ventricular activity with no identifiable QRS complexes and no cardiac output, often degenerating from VT or acute ischemia. It is the classic mechanism of sudden cardiac death, and in adults the usual substrate is coronary artery disease. Treatment is immediate CPR and unsynchronized defibrillation.

Inherited arrhythmia syndromes and sudden death in the young

Congenital long QT. Loss of a repolarizing K+ current or gain of the late Na+ current prolongs the action potential and predisposes to EAD-triggered torsades. LQT1 is the most common form. Romano-Ward syndrome is the autosomal dominant, purely cardiac form. Jervell and Lange-Nielsen syndrome is autosomal recessive, caused by biallelic KCNQ1 or KCNE1 mutations, and combines a very long QT with congenital sensorineural deafness, because IKs is also needed to secrete K+ into the endolymph. Management: β-blockers (nadolol, propranolol), avoidance of QT-prolonging drugs and hypokalemia, and an ICD when risk is high.

Brugada syndrome. The type 1 pattern is coved ST elevation of at least 2 mm with T-wave inversion in V1–V2. It can be concealed until fever or a Na+-channel blocker challenge (procainamide, flecainide, ajmaline) unmasks it. Survivors and high-risk patients get an ICD, fever is treated promptly, and quinidine or isoproterenol can suppress arrhythmic storms.

ARVC and CPVT. In ARVC, defective desmosomes (plakophilin-2, desmoplakin, plakoglobin, desmoglein-2) let myocytes detach, fibrofatty tissue replaces the RV wall, and the RV dilates. The epsilon wave is a small notch just after the QRS. Naxos disease is an autosomal recessive plakoglobin form with woolly hair and palmoplantar keratoderma. In CPVT, leaky RyR2 channels release SR Ca2+ under adrenergic stimulation, causing DADs. The resting ECG is normal, so diagnosis rests on exercise testing and genetics.

Sudden death in the young. Hypertrophic cardiomyopathy is the most common cause in young US athletes. Other causes are channelopathies (LQTS, Brugada, CPVT), ARVC, anomalous origin of a coronary artery, myocarditis, WPW with pre-excited AF, and commotio cordis, in which a blow to the chest during the upstroke of the T wave triggers VF. When the heart is structurally normal at autopsy, a channelopathy is likely, and first-degree relatives should be screened with ECG and genetic testing.