What is the most likely underlying condition that predisposed a patient to a dangerous arrhythmia, given a history of syncope and dizziness, with vital signs showing tachycardia (elevated heart rate), normotension (normal blood pressure), and normal temperature?

Most Likely Underlying Condition: Long QT Syndrome

The most likely underlying condition predisposing this patient to a dangerous arrhythmia following syncope is long QT syndrome (LQTS), particularly given the clinical presentation of syncope in a young patient with subsequent rhythm disturbance. 1

Clinical Reasoning

Why Long QT Syndrome is Most Likely

LQTS is a critical diagnosis to identify in young patients presenting with syncope, as it predisposes to life-threatening ventricular arrhythmias (torsade de pointes) and sudden cardiac death. 1 The American Heart Association specifically emphasizes that the primary purpose of syncope evaluation is to identify potentially life-threatening diseases such as LQTS, which can be fatal if missed. 1

Key features supporting LQTS in this scenario:

• Syncope is the hallmark presenting symptom of LQTS, occurring in the majority of affected individuals, often during childhood or teenage years 2, 3
• The subsequent "dizzy" episode with rhythm change on monitor is highly suggestive of a ventricular arrhythmia, which is the mechanism by which LQTS causes symptoms 2, 4
• Tachycardia (pulse 105) may represent either sinus tachycardia post-event or the beginning of a tachyarrhythmic episode 3
• LQTS-related syncope frequently occurs with physical, emotional, or auditory arousal, and can be misinterpreted as seizure disorder 5, 3

Why Other Options Are Less Likely

Pre-excitation (Wolff-Parkinson-White syndrome): While WPW can cause syncope through rapid supraventricular tachycardia or atrial fibrillation with rapid ventricular response, syncope occurs in only up to 20% of patients with supraventricular tachycardias and is less commonly the presenting feature compared to palpitations. 6 The European Heart Journal notes that supraventricular arrhythmias are less frequently implicated as causes of syncope than ventricular arrhythmias. 1

Right bundle branch block: RBBB alone does not predispose to dangerous arrhythmias or syncope unless associated with other conduction abnormalities (bifascicular block) suggesting risk for complete heart block. 1 The guidelines indicate that isolated RBBB is not diagnostic of syncope etiology. 1

Short PR interval: While this may suggest pre-excitation, it is not independently associated with life-threatening arrhythmias in the absence of delta waves indicating an accessory pathway. 1

Diagnostic Approach

Critical ECG Findings to Assess

Immediately measure the QTc interval on the ECG. 1 A QTc >0.44 seconds is diagnostic of LQTS, with QTc ≥0.50 seconds indicating higher risk. 3 Look for:

• Prolonged QT interval with bizarre T-wave morphology 5, 4
• T-wave abnormalities in precordial leads 5
• Genotype-specific ECG patterns if LQTS is confirmed 4

Risk Stratification

Three independent risk factors predict subsequent cardiac events in LQTS: 3

1. QTc duration (higher values = higher risk)
2. History of prior cardiac events (syncope or cardiac arrest)
3. Heart rate (lower resting heart rate associated with higher risk)

By age 12 years, 50% of LQTS probands have experienced syncope or death, with post-diagnosis syncope rates of 5.0% per year and LQTS-related death rates of 0.9% per year. 3

Management Priorities

Immediate Actions

This patient requires continuous cardiac monitoring until LQTS is excluded or treated. 1 The American Heart Association states that patients with inherited long QT who present with unstable ventricular arrhythmias should have electrocardiographic monitoring until stabilization. 1

Avoid QT-prolonging medications and correct electrolyte abnormalities (particularly hypokalemia and hypomagnesemia), as these can precipitate torsade de pointes in LQTS patients. 1, 2

Definitive Treatment

Beta-blockers (particularly propranolol) are first-line therapy for LQTS and have been shown to be most effective in preventing ventricular arrhythmias. 5 However, note that beta-blockers can cause bradycardia, which may paradoxically prolong QT further. 7

Genotype-specific treatment approaches are emerging, with different LQTS subtypes (LQT1-6) potentially responding differently to specific interventions. 2, 4

Critical Pitfalls to Avoid

Do not dismiss syncope as a seizure disorder without ECG evaluation. 3 LQTS-related syncope is frequently misdiagnosed as epilepsy, particularly when seizure-like activity occurs due to cerebral hypoperfusion during arrhythmia. 4

Do not assume a normal baseline ECG excludes dangerous arrhythmia. 1 However, any ECG abnormality (including prolonged QT) is an independent predictor of cardiac syncope and increased mortality. 1

Screen for family history of unexplained sudden cardiac death, as LQTS is inherited and affects multiple family members. 1, 3