Rhythm abnormalities: supraventricular arrhythmias

01Definition and overview

Supraventricular tachycardias are rhythms faster than 100 a minute whose mechanism involves the atria, the atrioventricular node or the His bundle. The ventricles are activated through the normal conduction system, so the QRS is usually narrow, under 120 ms. It is broad when there is bundle branch block or when the ventricles are activated through an accessory pathway. Atrial ectopic beats come from the same tissue.

AVNRT and AVRT are often called paroxysmal supraventricular tachycardia, because they start and stop suddenly.

Atrial fibrillation is described by its time course:

An accessory pathway that conducts from atria to ventricles in sinus rhythm activates part of the ventricle early. This pre-excitation gives a short PR interval (under 120 ms) and a slurred upstroke to the QRS, the delta wave. Pre-excitation with arrhythmias is the Wolff–Parkinson–White syndrome.

02Etiology and pathophysiology

Causes of atrial fibrillation

Atrial fibrillation that appears with an acute, reversible trigger such as sepsis, surgery or a bout of drinking still often recurs and carries a similar stroke risk, so these patients are followed up rather than discharged as cured. The atrial tachyarrhythmias share these causes.

How atrial fibrillation starts and persists

1. 1

   Triggers

   Rapidly firing foci, mostly in the sleeves of muscle that extend into the pulmonary veins, set off bursts of fibrillation. This is why electrically isolating the pulmonary veins treats it.
2. 2

   A substrate

   Stretch, hypertrophy, fibrosis and inflammation of the atria, from age, pressure and volume overload, slow and fragment conduction, so many wandering wavelets of re-entry can keep the arrhythmia going.
3. 3

   Atrial fibrillation begets atrial fibrillation

   The fast rate shortens the atrial refractory period and adds fibrosis. Episodes lengthen, and paroxysmal fibrillation tends to become persistent, faster when risk factors are left untreated.
4. 4

   The consequences

   The atria quiver without contracting, so blood stagnates in the left atrial appendage, where more than 90 per cent of the thrombi form, and clots can travel to the brain. Ventricular filling loses the atrial contraction, which matters most in a stiff ventricle or mitral stenosis. A persistently fast ventricular rate can weaken the ventricle, a tachycardia-induced cardiomyopathy.

The atrioventricular node lets through only part of the atrial impulses. Without treatment the ventricular rate is usually 120 to 180 a minute, slower in older people with nodal disease or on rate-slowing drugs.

Atrial flutter

Typical flutter is a single circuit in the right atrium. It runs around the tricuspid annulus, usually counter-clockwise, and through a narrow corridor of tissue between the tricuspid valve and the inferior vena cava, the cavotricuspid isthmus. The atria beat at about 300 a minute, and the node usually conducts every second wave, so the ventricular rate is close to 150. A drug that slows the flutter circuit, such as flecainide, can allow every wave to conduct, giving a ventricular rate near 200 or more, so these drugs are given with a drug that slows the node.

Re-entry at the atrioventricular junction

• AVNRT. The node has a fast pathway with a long refractory period and a slow pathway with a short one. An early atrial beat finds the fast pathway refractory, travels down the slow one and returns up the fast one, and the circuit repeats. Atria and ventricles are activated together, so the P wave is hidden in the QRS.
• AVRT. An accessory pathway, a strand of muscle left when the atria and ventricles failed to separate fully in the fetus, joins them across the valve ring. In most attacks the impulse goes down the node and back up the pathway (orthodromic), giving a narrow QRS. Less often it goes down the pathway and back up the node (antidromic), giving a broad QRS. A pathway that conducts only backwards is invisible in sinus rhythm (concealed).
• Pre-excited atrial fibrillation. If atrial fibrillation develops in a patient whose pathway conducts quickly forwards, the pathway, unlike the node, does not filter the impulses. The ventricles can be driven at 250 to 300 a minute or more, and this can degenerate into ventricular fibrillation.

03Clinical features

• Palpitations. In AVNRT and AVRT they are fast and regular, start and stop abruptly, and may end with a Valsalva manoeuvre. In atrial fibrillation they are irregular.
• Rapid regular pulsation in the neck during AVNRT, as the atria contract against closed valves.
• Breathlessness, chest pain, dizziness, fatigue and reduced exercise tolerance.
• Syncope in about 1 in 10 patients with a paroxysmal tachycardia, usually at the start of an attack or in a pause as it ends. In older patients with aortic stenosis or cerebrovascular disease, even a moderately fast rate can cause hypotension and syncope.
• Polyuria during or after an attack, from the release of atrial natriuretic peptide.

Atrial fibrillation causes no symptoms in about a third of patients and is found by chance or after a stroke, which is why the pulse or an ECG is checked whenever people aged 65 or over see a doctor. Others have palpitations, breathlessness, fatigue or chest pain, or present in heart failure. On examination the pulse is irregularly irregular, the apical rate counted with a stethoscope can exceed the radial pulse (a pulse deficit), the loudness of the first heart sound varies, and the jugular venous pulse has no a wave. Flutter with 2:1 block gives a regular pulse near 150.

The complications of atrial fibrillation are stroke and systemic embolism, heart failure, tachycardia-induced cardiomyopathy, and cognitive decline.

04Diagnosis and management

Tests

• A 12-lead ECG during the arrhythmia is the key test. Atrial fibrillation is confirmed by a 12-lead ECG or at least 30 seconds of a single-lead tracing, which can come from a handheld device or a smartwatch.
• An ECG in sinus rhythm, looking for pre-excitation.
• Ambulatory monitoring for intermittent symptoms: a 24-hour to 7-day recording, an event recorder, or an implantable loop recorder.
• Blood tests: thyroid function, electrolytes including magnesium, full blood count, glucose, and kidney and liver function, which guide the dose of anticoagulants and antiarrhythmic drugs.
• Echocardiography for valve disease, ventricular function and atrial size.
• An electrophysiology study, usually as part of catheter ablation.

A broad-complex tachycardia is treated as ventricular tachycardia unless it is certain to be supraventricular, especially in a patient with known heart disease.

Acute management of a tachycardia

An unstable patient, with shock, syncope, myocardial ischaemia or severe heart failure, needs synchronised electrical cardioversion at once, under sedation or anaesthesia.

For a stable regular narrow-complex tachycardia:

1. 1

   Vagal manoeuvres

   The modified Valsalva manoeuvre: the patient, sitting half upright, strains for 15 seconds, for example by blowing into a 10 mL syringe hard enough to move the plunger, then lies flat at once with the legs raised. It restores sinus rhythm in about 4 in 10, against about 1 in 6 with the standard manoeuvre. Carotid sinus massage is an alternative, avoided with a carotid bruit or a previous stroke.
2. 2

   Adenosine

   A rapid intravenous bolus of adenosine into a large vein, followed by a saline flush: 6 mg, then 12 mg, then 18 mg if needed, with the ECG recording. It blocks the node for a few seconds, which ends AVNRT and AVRT and unmasks flutter or atrial tachycardia. Flushing, chest tightness, breathlessness and a sense of doom pass within seconds. It can cause bronchospasm and is avoided in severe asthma. When the resting ECG shows pre-excitation, it is given only with a defibrillator at hand, because it can trigger atrial fibrillation conducted fast down the pathway.
3. 3

   If adenosine fails or cannot be given

   Intravenous verapamil or diltiazem, or a beta-blocker, not combined. Verapamil is never given for a broad-complex tachycardia. If drugs fail, synchronised cardioversion.

Acute atrial fibrillation and flutter

• Triggers are treated: sepsis, thyrotoxicosis, alcohol, electrolyte disturbance.
• Rate control with a beta-blocker, or with diltiazem or verapamil if the ejection fraction is above 40 per cent. With heart failure and a lower ejection fraction, a beta-blocker and digoxin; amiodarone in the critically ill.
• Restoring sinus rhythm is optional in a stable patient. Many recent-onset episodes stop by themselves: in a trial of a wait-and-see approach with rate control alone, about 7 in 10 patients reverted within 48 hours.
• Cardioversion can be electrical, a synchronised shock under sedation, or pharmacological, with flecainide or propafenone in a heart without significant structural disease, vernakalant (not after a recent acute coronary syndrome, or with heart failure with a reduced ejection fraction or severe aortic stenosis), or amiodarone when there is heart failure or structural disease.

Long-term management of atrial fibrillation

European guidance organises care in four steps, AF-CARE:

Preventing stroke

The CHA₂DS₂-VA score estimates the risk:

Female sex, the Sc of the older CHA₂DS₂-VASc score, no longer counts, because it raised the risk only in older women who already qualify.

• A score of 2 or more: oral anticoagulation is recommended. A score of 1: it is considered. A score of 0 needs none.
• Hypertrophic cardiomyopathy and cardiac amyloidosis call for anticoagulation whatever the score.
• Direct oral anticoagulants (apixaban, rivaroxaban, edoxaban, dabigatran) are preferred to warfarin: they work as well, cause fewer intracranial bleeds and need no INR checks. They are given at full dose unless the criteria for a reduced dose are met.
• Warfarin is used instead with a mechanical heart valve or moderate to severe mitral stenosis, where direct anticoagulants are unsafe or unproven. The INR target is 2 to 3 for atrial fibrillation itself, but a mechanical valve sets its own, higher for a mitral valve or an older valve type (about 3).
• Aspirin and other antiplatelet drugs do not prevent strokes in atrial fibrillation and are not used for this.
• Anticoagulation follows the risk score, not the rhythm: it continues after successful cardioversion or ablation, and in paroxysmal fibrillation as much as in permanent.
• Bleeding risk is reduced by correcting what can be corrected: uncontrolled blood pressure, alcohol excess, anaemia, needless antiplatelet drugs or NSAIDs, unstable INRs. Bleeding scores such as HAS-BLED find these factors but are not used on their own to withhold anticoagulation.
• When anticoagulants cannot be taken long term, the left atrial appendage can be closed with a device placed through a vein. In patients having cardiac surgery it is closed surgically, in addition to anticoagulation.
• Short episodes found only on a pacemaker or implanted monitor (device-detected subclinical atrial fibrillation) carry a lower risk; anticoagulation may be considered when the stroke risk is high and the bleeding risk low.

Rate control

• The initial target is a resting heart rate below 110 a minute, made stricter if symptoms persist.
• Ejection fraction above 40 per cent: a beta-blocker, diltiazem or verapamil, or digoxin, alone or combined.
• Ejection fraction 40 per cent or below: a beta-blocker, digoxin or both; diltiazem and verapamil are avoided.
• When drugs fail, ablation of the atrioventricular node with a pacemaker controls the rate permanently. The patient is then pacemaker-dependent, and anticoagulation continues.

Rhythm control

Rhythm control is considered in every suitable patient, chiefly for symptoms, and is favoured when the fibrillation is recent, the atrium is not dilated, the arrhythmia may be weakening the ventricle, or the patient prefers it. Beyond symptoms, a large trial showed that starting rhythm control within a year of diagnosis, in patients with stroke risk factors, reduced a combination of cardiovascular death, stroke and admission for heart failure or an acute coronary syndrome. In heart failure caused by the arrhythmia, restoring sinus rhythm can bring the ventricle back, and ablation is recommended.

• Antiarrhythmic drugs. Flecainide and propafenone suit a heart without coronary disease, heart failure or severe hypertrophy, and are combined with a nodal blocker. Dronedarone and sotalol are alternatives with specific limits. Amiodarone is the most effective and safe in heart failure, but it harms the thyroid, lungs, liver, skin and eyes over time. Some patients carry a single oral dose of flecainide or propafenone, first tried under monitoring, to take at the start of an attack (the pill in the pocket). See the antiarrhythmic drugs.
• Catheter ablation. The pulmonary veins are electrically isolated by heat, cold or pulsed electric fields. It keeps sinus rhythm better than drugs and is now a first-line option for symptomatic paroxysmal atrial fibrillation, and the preferred option after a drug has failed. Serious complications occur in a few per cent, including tamponade and stroke. Patients at risk of stroke take anticoagulation for at least 3 weeks before it. Recurrence is common, and anticoagulation afterwards still follows the stroke risk.
• Surgical ablation can be added during other cardiac surgery.

Atrial flutter

• Acute: the rate is harder to control than in atrial fibrillation, and electrical cardioversion, which needs little energy, works very well. The anticoagulation rules are those of atrial fibrillation.
• Long term: catheter ablation of the cavotricuspid isthmus cures typical flutter in more than 90 per cent and is first-line for recurrent flutter. Atrial fibrillation often develops later, so anticoagulation still follows the stroke risk score.

AVNRT and AVRT

• Patients learn the modified Valsalva manoeuvre for their own attacks.
• Catheter ablation is first-line for recurrent symptomatic attacks. Ablation of the slow pathway cures AVNRT in about 95 per cent, with a risk of complete heart block needing a pacemaker of under 1 per cent. Ablation of an accessory pathway succeeds in about 95 per cent.
• Patients who prefer drugs or are unsuitable for ablation take verapamil, diltiazem or a beta-blocker (for AVRT only when the resting ECG shows no pre-excitation), or flecainide or propafenone if the heart is structurally normal.

Wolff–Parkinson–White

• Symptomatic patients have the pathway ablated.
• Pre-excitation found by chance is assessed for risk, especially in athletes and in jobs where a sudden collapse would endanger others. A pathway that conducts fast (a shortest pre-excited RR interval in atrial fibrillation of 250 ms or less), or several pathways, are ablated. Pre-excitation that disappears abruptly on exercise suggests a low-risk pathway.
• In pre-excited atrial fibrillation, adenosine, verapamil, diltiazem, beta-blockers, digoxin and intravenous amiodarone are avoided. The patient is cardioverted if unstable; if stable, ibutilide, procainamide or flecainide is given, then the pathway is ablated.

Other atrial rhythms

• Sinus tachycardia: the cause is treated, not the rate. Inappropriate sinus tachycardia, a fast resting rate without a cause, mostly in young women, may need ivabradine or a beta-blocker.
• Focal atrial tachycardia: rate or rhythm control, or ablation of the focus. With block it suggests digoxin toxicity, and digoxin is stopped.
• Multifocal atrial tachycardia: treatment of the lung disease and hypoxia, and correction of potassium and magnesium; a selective beta-blocker, or verapamil or diltiazem, if the rate must be slowed.
• Atrial ectopic beats: reassurance; less caffeine and alcohol; a beta-blocker if they are troublesome.

05Epidemiology

Atrial fibrillation is the commonest sustained arrhythmia, affecting about 2 to 4 per cent of adults and rising steeply with age to about 1 in 10 people over 80. About 1 in 3 people of European ancestry develop it after the age of 55. It is commoner in men at any age, but because women live longer the totals are similar. Its prevalence in Europe is expected to roughly double by 2060, with an ageing population and more obesity and diabetes. About 1 in 5 strokes is linked to atrial fibrillation. It follows about a third of cardiac operations.

Paroxysmal supraventricular tachycardia affects about 2 to 3 people per 1,000, with the first attack usually between 12 and 30. AVNRT is the commonest form in adults and is about twice as common in women, while AVRT is relatively commoner in children and teenagers. Pre-excitation appears on about 1 to 3 per 1,000 ECGs. Atrial flutter is far less common than atrial fibrillation and often occurs with it.

06Prognosis

Atrial fibrillation raises the risk of stroke about five-fold, and strokes caused by it are larger and more often fatal or disabling. It also raises the risk of heart failure, dementia and death. Anticoagulation prevents about two-thirds of these strokes and lowers mortality, and direct oral anticoagulants cause fewer brain haemorrhages than warfarin. Without attention to risk factors, paroxysmal fibrillation tends to become persistent. Tachycardia-induced cardiomyopathy usually recovers once the rate or rhythm is controlled.

AVNRT and AVRT are benign: they cause distress but rarely danger, and ablation cures most patients. In Wolff–Parkinson–White syndrome the risk of sudden death is small, but it can be the first sign of the disease in a young person, which is why high-risk pathways are ablated. Ablation cures typical flutter, but many patients develop atrial fibrillation in the following years.