Cardiac arrhythmia is an abnormality of impulse generation, impulse propagation, or a combination of both. Normal conduction and cardiac rhythm is initiated by the sinoatrial (SA) node. The electrical current then travels through the conduction network and enters the ventricle via the atrioventricular (AV) node and bundle of His. From the bundle of His, the electrical activity moves into the branch-like system called the Purkinje system. As the current flows through the myocardium, the excitation coordinates the contraction of the atria and ventricles. After stimulation, each group of cells experiences a refractory period in which it cannot be excited. As the electrical current meets refractory tissue, the stimulation ceases allowing for the process to begin again.
Conduction of electrical impulses through the myocardium is represented on an electrocardiogram (ECG) as waves of depolarization and repolarization. As depolarization occurs, the heart’s myocytes become positive and contract. Recovery immediately follows as the myocytes return to their resting negative charge during repolarization. The initial P wave on an ECG represents depolarization leading to contraction of the atria, or top chambers of the heart. The QRS complex on an ECG represents depolarization of the ventricles and subsequent ventricular contraction. Finally, the T wave represents ventricular repolarization. One cardiac cycle consists of atrial systole, then ventricular systole, and finally a resting phase (Figure 9-1).
The cardiac cycle. Reproduced with permission from Hume JR, Grant AO. Chapter 14. Agents Used in Cardiac Arrhythmias. In: Katzung BG, Masters SB, Trevor AJ. eds. Basic & Clinical Pharmacology, 12e. New York, NY: McGraw-Hill; 2012:chap 14.
Atrial fibrillation (AF) is disorganized atrial activation and subsequent uncoordinated atrial contraction. AF can be symptomatic, lead to hemodynamic compromise and result in increased morbidity and mortality. AF is characterized as recurrent (having presented with two or more episodes), paroxysmal (if the recurrent AF terminates spontaneously), persistent (if it is sustained for at least 7 days), or permanent (in which attempts to convert to sinus rhythm have failed). It is the most common cardiac arrhythmia and the onset is associated with increasing age, male sex, and presence of cardiovascular disease. AF can be asymptomatic or symptomatic (palpitations, shortness of breath, and fatigue). The ECG of AF is an irregularly, irregular supraventricular rhythm, with no discernable P waves, and a variable ventricular rate between 120 and 180 beats/min.
Paroxysmal Supraventricular Tachycardia
Paroxysmal supraventricular tachycardia (PSVT) is a tachyarrhythmia with an abrupt onset and termination which results from an arrhythmia originating or involving supraventricular tissue. Many patients with PSVT are asymptomatic while others have symptoms including palpitations, fatigue, light-headedness, chest discomfort, and dyspnea.
Premature Ventricular Complexes
Premature ventricular complexes (PVCs) are a common, most often benign, arrhythmia originating from ...