18.5 — Arrhythmias

Almost everyone has arrhythmias. Extra beats — the sensation of a skipped or thumping beat — occur in the majority of healthy people, and on 24-hour monitoring they are found in nearly everyone.

Most are harmless. The clinical task is separating those from the small number that are not, and the separating features are mostly things a person can report.

Chapter 7.3 covered the conduction system and the ECG. This chapter is the practical version.

When a palpitation matters

Reassuring features:

Isolated skipped or extra beats, often noticed at rest or in bed. Worse with caffeine, alcohol, tiredness or stress.Disappearing on exercise — which is characteristic of benign extra beats, because the faster underlying rhythm suppresses them. No other symptoms.A normal ECG and a structurally normal heart.

Concerning features:

Fainting or near-fainting with the palpitation — the single most important red flag. Chest pain or breathlessness.Palpitations during exercise rather than at rest.A family history of sudden death under 40.Known heart disease.And very fast, sustained, regular palpitations.

"Do you faint with them?" and "do they happen when you exercise?" are the two questions that do most of the work.

Atrial fibrillation

By far the commonest sustained arrhythmia — affecting around 2 percent of adults, and over 10 percent of people over 80.

The atria depolarise chaotically at 400 to 600 per minute instead of contracting in an organised way (Chapter 7.3).

No P waves, and an irregularly irregular rhythm with no discernible pattern.

Three consequences:

Loss of the atrial kick — costing 10 to 30 percent of cardiac output, which matters most in a heart that was already struggling.

A fast and irregular ventricular rate.

And the important one: stagnant blood in the non-contracting atrium clots.

Atrial fibrillation causes roughly a fivefold increase in stroke risk and accounts for 20 to 30 percent of all ischaemic strokes — and those strokes are on average larger and more disabling, because the clot is bigger.

Which makes anticoagulation the central treatment, and it reduces stroke risk by around two thirds.

The decision is made with a risk score combining age, heart failure, hypertension, diabetes, previous stroke, vascular disease and sex — and the threshold for treating is low, because the benefit is large.

Direct oral anticoagulants — apixaban, rivaroxaban and others — have largely replaced warfarin. No monitoring, fewer food and drug interactions, and less intracranial bleeding.

Warfarin remains necessary for mechanical heart valves and severe mitral stenosis, where the newer drugs have not been shown to work.

And aspirin is not adequate for stroke prevention in atrial fibrillation — a common and harmful misconception, since it provides little protection and still carries bleeding risk.

Rate or rhythm control:

Rate control — accepting the fibrillation and slowing the ventricular response with a beta-blocker or a calcium channel blocker.

Rhythm control — restoring sinus rhythm with drugs, electrical cardioversion, or catheter ablation, which burns or freezes the tissue around the pulmonary veins where the triggering beats usually originate.

And the balance has shifted. Older trials found the two strategies equivalent. More recent evidence, particularly the EAST-AFNET trial, found early rhythm control within a year of diagnosis reduced cardiovascular events — so ablation and rhythm control are offered earlier than they were.

Ablation success rates are around 70 to 80 percent for paroxysmal atrial fibrillation, sometimes requiring a repeat procedure.

And the reversible contributors matter more than they are given credit for: alcohol — a clear dose-related trigger, and reducing intake measurably reduces recurrence; obesity — weight loss substantially reduces the burden; sleep apnoea; hyperthyroidism (Chapter 12.3); and untreated hypertension.

A trial of structured weight loss and risk factor management produced arrhythmia-free survival comparable to some procedures, which is worth knowing before assuming ablation is the only route.

Supraventricular tachycardia

Sudden-onset, regular, fast — typically 150 to 250 beats per minute — starting and stopping abruptly.

Often in young, otherwise healthy people, and frequently dismissed as anxiety for years.

The mechanism is usually a re-entry circuit — an electrical loop that captures the rhythm.

And a person can frequently stop it themselves.

Vagal manoeuvres (Chapter 7.4): the modified Valsalva — blowing hard against resistance for 15 seconds while sitting, then lying flat with legs raised immediately afterwards — is substantially more effective than the standard version and converts around 40 percent.

Cold water on the face works through the dive reflex (Chapter 8.6), and is particularly effective in infants.

If that fails: intravenous adenosine, which briefly blocks the AV node. It works within seconds, and it produces a few seconds of intense chest tightness and a feeling of doom — which is why patients are warned beforehand.

Catheter ablation is curative in over 95 percent for the common types, and it is offered readily because it removes the condition rather than managing it.

Wolff–Parkinson–White syndrome — an accessory pathway bypassing the AV node.

And it matters because of a specific danger. If atrial fibrillation occurs in someone with an accessory pathway, the impulses can bypass the AV node's rate-limiting function and reach the ventricles at extreme rates, which can degenerate into ventricular fibrillation.

Which is why AV node-blocking drugs are contraindicated in that situation — they push conduction down the accessory pathway. Ablation is the definitive treatment.

Bradycardia and heart block

Sinus bradycardia — normal in athletes and during sleep. Also caused by beta-blockers, hypothyroidism, and raised intracranial pressure.

Sick sinus syndrome — the node failing, often alternating fast and slow rhythms.

Heart block:

First degree — every impulse conducts, slowly. Usually harmless. Second degree — some impulses fail. One pattern is benign, one is not.Third degree (complete) — atria and ventricles beating independently, with the ventricles driven by an escape pacemaker at 20 to 40 per minute. Requires a pacemaker, usually urgently.

Symptoms: fatigue, breathlessness, dizziness, and fainting.

And a specific pattern worth knowing: fainting with no warning at all, with rapid recovery, in an older person, is a Stokes–Adams attack from transient complete heart block — and it is distinguishable from a vasovagal faint, which has warning symptoms and slower recovery.

Pacemakers are highly effective and the procedure is minor.

Ventricular arrhythmias

These are the dangerous ones.

Ventricular ectopics — extra beats from the ventricle. Common and usually benign in a structurally normal heart, and significant in a damaged one.

Ventricular tachycardia — fast, wide-complex, from the ventricle. Poor filling and poor output, and it can degenerate into fibrillation.

Ventricular fibrillation — completely disorganised, no coordinated contraction, no output. This is cardiac arrest.

And the treatment is defibrillation — which does not restart the heart but stops it, allowing the sinus node to resume (Chapter 7.3).

Shockable rhythms: ventricular fibrillation and pulseless ventricular tachycardia.Non-shockable: asystole and pulseless electrical activity.

Which is why the defibrillator analyses the rhythm before advising a shock, and why shocking a flat line — as television consistently depicts — does nothing.

Sudden cardiac death in the young

Rare, devastating, and partly preventable.

Causes include hypertrophic cardiomyopathy — the commonest, an inherited thickening of the heart muscle; arrhythmogenic cardiomyopathy; long QT syndrome (Chapter 7.3); Brugada syndrome; anomalous coronary arteries; and myocarditis.

And most are inherited, which means a death in a young person should trigger screening of first-degree relatives.

Warning signs that should never be dismissed in a young person:

Fainting during exercise — as opposed to after it or in a hot room, which is usually vasovagal. Exertional syncope is a red flag until proven otherwise.

Chest pain on exertion.

A family history of sudden death under 40, or of unexplained drowning or car accidents.

Palpitations with exercise.

These warrant an ECG and an echocardiogram, and the cost of investigating is trivial compared with the alternative.

Screening athletes is done routinely in some countries and not others, and the debate is genuine — it turns on false positive rates, the consequences of disqualifying healthy athletes, and cost-effectiveness rather than on whether the conditions exist.

Long QT and drugs

Worth its own note because it is the arrhythmia most likely to be caused by medical treatment (Chapter 7.3).

A prolonged QT interval allows a chaotic ventricular rhythm to start.

Inherited forms exist, and drug-induced prolongation is far commoner: some antiarrhythmics, several antipsychotics, some antibiotics including macrolides and fluoroquinolones, some antidepressants, and some antiemetics.

And low potassium and low magnesium make it worse.

Combining two QT-prolonging drugs in someone with low potassium is a recognised route to cardiac arrest, and it is exactly the kind of interaction that electronic prescribing systems are designed to flag.

Living with an arrhythmia

Most are manageable, and several are curable.

Reduce the triggers: excessive caffeine, alcohol, dehydration, sleep deprivation, and stimulants including some decongestants.

Treat what contributes: thyroid disease, sleep apnoea, obesity, hypertension, and electrolyte abnormalities.

Wearable devices — smartwatches now detect atrial fibrillation reasonably reliably, and they generate both genuine early diagnoses and a substantial volume of false alarms. A device-detected abnormality should be confirmed on a proper ECG before any conclusion is drawn.

And the encouraging summary: supraventricular tachycardia is curable in over 95 percent. Atrial fibrillation stroke risk is reduced by two thirds with a tablet. Complete heart block is fixed by a pacemaker. And defibrillators prevent sudden death in those at risk.

What the next page fixes

Chapter 18.6 covers what happens when a clot reaches the brain — stroke, where the phrase "time is brain" is even more literal than "time is muscle".