Implantable cardioverter defibrillators and cardiac resynchronisation therapy for arrhythmias and heart failure

Arrhythmia is a condition where the heart contracts irregularly, or at a faster or slower pace than normal. It is caused by an abnormality in the myocardial tissue, or in the electrical conduction system of the heart. Arrhythmias that arise from ventricles (ventricular arrhythmias) can occur unexpectedly and can cause sudden death when insufficient blood is pumped out by the heart to sustain life. Ventricular arrhythmias include ventricular tachycardia and ventricular fibrillation. In ventricular tachycardia, the ventricles beat faster than normal (at between 120 and 200 beats per minute). In ventricular fibrillation, electrical impulses rapidly start firing from multiple sites in the ventricles, resulting in an uncoordinated, irregular rhythm.

Ventricular arrhythmias most commonly occur in people with underlying heart disease. Approximately 75% to 80% of the 70,000 sudden cardiac deaths in England and Wales in 2010 could be attributed to ventricular arrhythmias. The average chance of survival of adults after an out-of-hospital episode of ventricular arrhythmia has been reported to be as low as 7%. However, with appropriate treatment, recent studies have reported 5‑year survival of 69% to 100% in people who had survived a cardiac arrest.

Many patients presenting with arrhythmias, with or without symptoms, are treated with antiarrhythmic drug therapy. However, antiarrhythmic drugs may not be optimally effective and need careful and frequent adjustment. This can be confusing for patients and may lead to missed doses, taking the wrong dose or overdose. Many antiarrhythmic drugs result in tiredness, inability to perform day-to-day activities and dependence on carers, and consequently increase the risk of depression. Antiarrhythmic drugs also have many side effects on a range of organs including the thyroid, liver and lungs.

Chronic prophylactic antiarrhythmic drug therapy aims to suppress the development of arrhythmias, but does not stop an arrhythmia once it has started. People who survive a first episode of life-threatening ventricular arrhythmia are at high risk of further episodes. For preventing further life-threatening events in survivors of previous serious ventricular arrhythmias, people are usually treated with implantable cardioverter defibrillators (ICDs). Preventing sudden cardiac death in someone who has never had a cardiac arrest or ventricular arrhythmia is challenging because it requires identifying a person with substantial level of risk. Many risk factors for sudden cardiac death have been reported such as age, hereditary factors, having a high risk for coronary artery disease, inflammatory markers, hypertension, left ventricular hypertrophy, conduction abnormalities (for example, left bundle branch block), obesity, diabetes and lifestyle factors. There is currently no optimal strategy for risk stratification.

Heart failure is caused by any structural or functional cardiac disorder that impairs the heart's ability to function efficiently as a pump to support circulation. It causes breathlessness, fatigue and fluid retention. Clinically it is classified using the New York Heart Association (NYHA) functional class system, ranging from class 1 (no limitation of physical activity or symptoms, but heart failure symptoms in the past) to class 4 (symptomatic at rest and discomfort from any physical activity). Heart failure is also classified based on which heart function or which side of the heart is most affected: some patients have heart failure due to left ventricular systolic dysfunction, which is associated with a reduced left ventricular ejection fraction (left heart failure or biventricular failure); while others have only right heart failure with a preserved left ventricular ejection fraction. The scope for this appraisal focuses on left and biventricular heart failure.

Heart failure is a chronic condition predominantly affecting people over the age of 50 years. The incidence of heart failure in the UK is 140 per 100,000 men and 120 per 100,000 women. Approximately 900,000 people in England and Wales have heart failure, of which at least half have left ventricular systolic dysfunction. The incidence and prevalence of heart failure increases with age and the average age at first diagnosis is 76 years. People with heart failure are at risk from sudden cardiac death; this is the most common cause of death in people with mild to moderate heart failure.

Treatment of heart failure aims to improve life expectancy and quality of life. NICE's guideline on chronic heart failure in adults (now replaced by NICE's guideline on chronic heart failure in adults: diagnosis and management) recommends pharmacological treatment initially. However, as the condition becomes more severe, cardiac function and symptoms may no longer be controlled by pharmacological treatment alone, and can be improved by the implantation of a cardiac rhythm device which can sense and stimulate the atria and right and left ventricles independently. These devices are known as cardiac resynchronisation therapy pacing (CRT‑P) devices or cardiac resynchronisation therapy defibrillator (CRT‑D) devices.