Arrhythmias

Introduction

Arrhythmia is the term used to describe an abnormal heart rhythm. The most common arrhythmias are atrial or ventricular ectopic beats. Tachycardia occurs when the heart rate is greater than 100 beats per minute (bpm), and bradycardia is defined as a rate below 60 bpm. The management of cardiac arrhythmias depends on the presentation of the patient, his or her haemodynamic stability, underlying heart disease (if any) and the exact type of arrhythmia. Patients with asymptomatic stable arrhythmias in the absence of underlying heart disease usually do not require emergency treatment. However, patients with symptomatic arrhythmias, especially when associated with underlying heart disease, require more urgent therapy. The key objective is restoration of adequate cardiac output to maintain cerebral perfusion as well as a stable rhythm using interventions least likely to cause harm.

Pathophysiology and pathogenesis

An understanding of cardiac arrhythmia requires knowledge of the normal conduction system ( Fig. 5.4.1 ). In the normal heart, electrical impulses start from the sinoatrial (SA) node and conduct via the atria to the atrioventricular (AV) node. The electrical impulses then conduct down the bundle of His to the right and left bundle branches and subsequently via the Purkinje fibres to the ventricular myocardium.

Different mechanisms—such as re-entry, enhanced automaticity and triggered activity—can result in arrhythmias. Generally arrhythmias are thought of as abnormal impulse generation or abnormal impulse conduction. Abnormal impulse generation in the SA node, as in sick sinus syndrome, can result in failure of impulse formation. Abnormal impulse conduction in the AV node can result in failure of electrical conduction from the atrium to the ventricles, resulting in various degrees of AV block. Ectopic impulses in the atria result in atrial ectopics or atrial tachycardia. Accessory pathways between the atrium and ventricle can result in supraventricular tachycardia (SVT). Abnormalities in ventricular conduction can result in bundle branch blocks or a variety of intraventricular conduction abnormalities. The most dangerous arrhythmias arise from the ventricles, as these, especially in the presence of underlying structural heart disease, may be associated with sudden death.

Principles of assessment and management

Patients with arrhythmias may present with symptoms due to the arrhythmia, such as palpitations or syncope, or they may be asymptomatic and have the arrhythmia noticed during routine examination or investigations. All patients should initially be managed in an area where cardiac and other physiological monitoring is available.

The management of arrhythmias should begin with attention to the airway, breathing and circulation. Management of cardiac arrest is discussed in Section 1 . For stable patients, the usual process of history, physical examination and investigations (particularly ECG), is appropriate. Family history of sudden cardiac death should raise the suspicion of ventricular arrhythmia. For unstable patients, urgent intervention is required, with restoration of a stable cardiac rhythm and cerebral perfusion being the priorities. Unstable patients are those with hypotension, dyspnea, chest pain or a decreased level of consciousness. Clinical information—for example, that from a patient with a history of renal failure or heart failure taking spironolactone—should raise the suspicion that a wide complex tachycardia is due to hyperkalemia.

Intravenous access should be obtained and blood drawn for investigations such as full blood counts, electrolytes and cardiac markers (if indicated). A 12-lead ECG is essential, and a chest x-ray may be helpful. Other specific investigations—such as serum digoxin level, thyroid function tests and theophylline levels—may sometimes be indicated, depending on the arrhythmia and the clinical context.

Bradyarrhythmias

Bradycardia is defined as a heart rate of less than 60 bpm. It is important to take into account the patient’s underlying clinical state when bradyarrhythmias are being treated. ECG diagnosis of bradyarrhythmias can be simplified by the algorithm in Fig. 5.4.2 . Note that denervated transplanted hearts will not respond to atropine; thus, if treatment is required, pacing, catecholamine infusion or both should be used.