General Medical Sciences ECG Arrhythmia practice questions
Cardiac arrhythmias arise from abnormalities of impulse formation (automaticity, triggered activity) or impulse conduction (reentry, block) within the heart's conduction system. Interpreting an ECG requires systematically assessing rate, rhythm regularity, P-wave/QRS relationship, QRS width, and QT interval to localize the abnormality and identify life-threatening patterns. Recognizing hallmark features of common arrhythmias guides both diagnosis and time-sensitive management decisions.
A 24-year-old woman presents to the emergency department with sudden-onset palpitations and mild lightheadedness that began 20 minutes ago. Blood pressure is 118/76 mmHg, and she is alert with no chest pain. ECG shows a regular narrow-complex tachycardia at 182/min with no discernible P waves. What is the most appropriate initial step in management?
A 68-year-old man with known heart failure with reduced ejection fraction (LVEF 30%) presents with palpitations. ECG shows atrial fibrillation with an irregularly irregular ventricular response of 140/min. Blood pressure is 105/68 mmHg and he has no signs of hemodynamic compromise. Which agent is most appropriate for rate control?
A 70-year-old man with a history of a prior myocardial infarction presents with palpitations and mild dyspnea. Blood pressure is 100/64 mmHg. ECG shows a wide-complex tachycardia at 160/min with a QRS duration of 160 ms, AV dissociation, and an intermittent fusion beat. Which of the following is the most likely diagnosis?
A 55-year-old woman hospitalized for pneumonia and receiving IV azithromycin develops recurrent brief episodes of dizziness. Telemetry captures a polymorphic ventricular tachycardia with a QRS axis that appears to twist around the isoelectric baseline; her QTc prior to the episode was 540 ms. Between episodes she remains hemodynamically stable. What is the most appropriate immediate treatment?
A 78-year-old man reports several episodes of syncope over the past two weeks. ECG shows P waves occurring at a regular rate that is entirely independent of the QRS complexes, with a narrow QRS escape rhythm at 38/min and no consistent PR relationship. What is the most appropriate definitive management?
ECG Arrhythmia, answered
What is the fastest way to tell ventricular tachycardia apart from SVT with aberrant conduction on a wide-complex tachycardia?
Look for AV dissociation, fusion beats, or capture beats, which strongly favor ventricular tachycardia because they show the ventricles being driven independently of the atria. A history of structural heart disease, especially a prior myocardial infarction, also shifts the likelihood heavily toward ventricular tachycardia, and when in doubt it is safer to treat a wide-complex tachycardia as ventricular in origin.
What is the difference between AVNRT and AVRT?
AVNRT (AV nodal reentrant tachycardia) involves a reentrant circuit contained entirely within or around the AV node itself, using two functionally distinct nodal pathways. AVRT (AV reentrant tachycardia) uses an accessory pathway outside the AV node, such as in Wolff-Parkinson-White syndrome, to complete the reentrant loop between the atria and ventricles. Both typically present as regular narrow-complex tachycardias, but AVRT can become dangerous if atrial fibrillation develops in a patient with a pre-excited pathway.
Why is IV magnesium given for torsades de pointes even when the patient's magnesium level is normal?
Magnesium suppresses the early afterdepolarizations that trigger torsades de pointes independent of whether a true magnesium deficiency exists, so it works as a membrane-stabilizing antiarrhythmic in this specific rhythm rather than purely as electrolyte replacement. It is given empirically in this setting because it is fast, generally safe, and effective even in normomagnesemic patients.
Why doesn't atropine reliably fix every type of bradyarrhythmia?
Atropine works by blocking vagal (parasympathetic) tone, so it is most effective for bradycardia caused by increased vagal input at the level of the sinus node or AV node. When the block is infranodal, such as in some cases of complete heart block below the AV node, the conduction tissue is not primarily vagally mediated, so atropine often fails to improve the rate and a pacing strategy is needed instead.
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