Chapter 6: Arrhythmias

INTRODUCTION

• Cardiac arrhythmia involves a group of conditions in which the heartbeat is irregular, too slow, or too fast. Supraventricular arrhythmias occur above the ventricles, and ventricular arrhythmias occur within the ventricles.

PATHOPHYSIOLOGY

Supraventricular Arrhythmias

Atrial Fibrillation and Atrial Flutter

• Atrial fibrillation (AF) has extremely rapid (400–600 atrial beats/min) and disorganized atrial activation. There is loss of atrial contraction (atrial kick), and supraventricular impulses penetrate the atrioventricular (AV) conduction system to variable degrees, resulting in irregular ventricular activation and an irregularly irregular pulse. The AV junction will not conduct most of the supraventricular impulses, causing the ventricular response to be considerably slower than the atrial rate.

• Atrial flutter has rapid (270–330 atrial beats/min) but regular atrial activation. Ventricular response usually has a regular pattern and a pulse of 300 beats/min. This arrhythmia is less common than AF but has similar precipitating factors, consequences, and drug therapy approach.

• The predominant mechanism of AF and atrial flutter is reentry, which is usually associated with organic heart disease that causes left atrial distention (eg, ischemia or infarction, hypertensive heart disease, and valvular disorders). Additional associated disorders include acute pulmonary embolism and chronic lung disease, resulting in pulmonary hypertension and cor pulmonale, and states of high adrenergic tone such as thyrotoxicosis, alcohol withdrawal, sepsis, and excessive physical exertion.

Paroxysmal Supraventricular Tachycardia

• PSVT arising by reentrant mechanisms includes arrhythmias caused by AV nodal reentry (AVNRT), AV reentrant tachycardia (AVRT) due to an accessory pathway, sinoatrial (SA) nodal reentry, and intraatrial reentry.

Ventricular Arrhythmias

Premature Ventricular Complexes

• Premature ventricular complexes (PVCs) can occur in patients with or without structural heart disease (SHD). PVCs may be elicited by abnormal automaticity, triggered activity, or reentrant mechanisms. PVCs often occur in healthy individuals and have little, if any, prognostic significance in this situation. PVCs occur more frequently and in more complex forms in patients with SHD than in healthy individuals, and patients with some PVC forms (multifocal or couplets) are at higher risk of sudden cardiac death.

Ventricular Tachycardia

• Ventricular tachycardia (VT) is defined by three or more repetitive PVCs occurring at a rate of >100 beats/min. It is a wide QRS tachycardia that may result acutely from severe electrolyte abnormalities (hypokalemia, hypomagnesemia), hypoxia, drug toxicity (eg, digoxin), or (most commonly) in patients presenting with acute myocardial infarction (MI) or myocardial ischemia complicated by heart failure (HF). The chronic recurrent form is almost always associated with SHD (eg, idiopathic dilated cardiomyopathy or remote MI with left ventricular [LV] aneurysm).

• Sustained VT requires intervention to restore a stable rhythm or persists for a relatively long time (usually >30 seconds). Nonsustained VT self-terminates after a brief duration (usually <30 seconds). Incessant VT refers to VT occurring ...