2 Clinical need and practice
2.1 Abnormal heart rhythms (dysrhythmias) are caused by disturbances in electrical impulse generation or by abnormal conduction between chambers of the heart – principally within the sinus node, atrioventricular (AV) node and the His–Purkinje network. Dysrhythmias may be fast (tachyrhythmias) or slow (bradyrhythmias), and regular or irregular.
2.2 Symptoms of bradycardia include faints, falls, dizziness and confusion (manifestations of hypotension), palpitations, fatigue on exertion, difficulty with breathing (dyspnoea) and chest pain. Common pathological conditions that cause bradycardia are sick sinus syndrome, atrioventricular block or a combination of the two.
2.3 Sick sinus syndrome is an irreversible dysfunction of the sinus node, a small area of the right atrium in which a small group of cells spontaneously depolarise and act as the heart's natural pacemaker. Sick sinus syndrome is characterised by impaired impulse formation, which is often the result of chronic fibrotic degeneration or calcification of the sinus node and/or the surrounding atrial tissues.
2.4 Atrioventricular block is a failure in the conduction of electrical impulses from the atria to the ventricles. This may be caused by conduction defects at the AV node (situated between the atria and ventricles), bundle of His and/or bundle branches. The AV node captures waves of depolarisation from the atria, which are then transferred to the ventricles via the bundles of His and the Purkinje system (branches of the conducting system). Atrioventricular block may be intermittent or permanent, and it can progress from minimal asymptomatic conduction delay to the ventricles (first‑degree), to partial (second‑degree) atrioventricular block, or complete (third‑degree) atrioventricular block, in which there is no conduction between the atria and ventricles. Although partial atrioventricular block is usually asymptomatic, it carries a high risk of progression to complete block.
2.5 The diagnosis of sick sinus syndrome and atrioventricular block is based on the correlation of symptoms with electrocardiographic findings (electrocardiogram [ECG] and ambulatory ECG or Holter monitoring). The prognosis of individuals with sick sinus syndrome or atrioventricular block is variable and difficult to predict because it may depend on the presence and severity of comorbidities (such as ischaemic heart disease) and the underlying cause of the conduction defect.
2.6 The prevalence of sick sinus syndrome is thought to be about 0.03% of the whole population, and increases with age. Estimates of the prevalence of atrioventricular block (based on clinical studies) range from 0.015% to 0.1%, although it is common for people to have coexisting abnormalities of both the sinus node and the AV node.
2.7 Pacemakers are indicated for use in the treatment of symptomatic bradycardia, and they control or replace the heart's intrinsic electrical activity. Some patients require intermittent pacing, whereas patients whose intrinsic heart rate is slow for most of the time require a pacemaker to pace most of their heartbeats.
2.8 Pacing systems are electrical devices that consist of a small battery‑powered generator and one or more pacing leads that are in contact with the inner wall of the right atrium and/or the right ventricle. The pacemaker senses whether an intrinsic depolarisation has occurred. When this has not occurred, the pacemaker generates an electrical impulse, which is delivered to the heart muscle via the pacemaker leads to initiate contraction.
2.9 Pacemakers may be broadly classified as single‑ or dual‑chamber devices, depending on whether leads are applied to one or two heart chambers. A range of additional features is also available, such as rate modulation (which allows the pacing rate to increase in response to physical activity or metabolic demand).
2.10 The British Pacing and Electrophysiology Group (BPEG) and the North American Society of Pacing and Electrophysiology have developed nomenclature that describes the different types of pacemakers (see Appendix D).
2.11 Pacemaker syndrome refers to a group of symptoms that includes nausea, palpitations, chest pain, fatigue, breathlessness, pre‑syncope and syncope. The underlying cause of pacemaker syndrome is not fully understood. It is thought to be caused by loss of the heart's natural AV sequence, causing simultaneous contraction of the atria and ventricles. Under these circumstances, blood in the atrial chamber is not efficiently expelled into the ventricles, which results in large reductions in systolic blood pressure and cardiac output. There are difficulties in the diagnosis of pacemaker syndrome because of the overlap of the symptoms of pacemaker syndrome with many symptoms typical of cardiac disease, and with symptoms arising from comorbidities, particularly in elderly patients. Pacemaker syndrome may develop in patients with functional atria who receive a single‑chamber ventricular pacemaker. Severe pacemaker syndrome can be eradicated by an upgrade to a dual‑chamber pacemaker, although this may be associated with an increased risk of perioperative complications. Patients with mild pacemaker syndrome often adapt over time to the condition and do not require a pacemaker upgrade.
2.12 Dual‑chamber pacing and single‑chamber atrial pacing (in patients with sick sinus syndrome without atrioventricular block), as opposed to single‑chamber ventricular pacing, are considered to be 'physiological' pacing modes because AV synchrony is maintained and the frequency of contractions of the atria and ventricles varies with metabolic demand, mimicking the heart's natural rhythm.
2.13 In the UK, about 26,000 pacemakers are implanted each year. In 2003, about 60% of implants were dual‑chamber pacemakers, 40% were single‑chamber ventricular pacemakers, and 1% were single‑chamber atrial pacemakers. Pacemakers may be implanted in patients of any age, although the average age of the recipients of pacemakers was 76 years in 2003.