Conductive disturbances. Treatment of conductive disturbances

01Definition and overview

The conduction system generates and spreads each heartbeat. The sinus node, high in the right atrium, fires spontaneously. The impulse spreads across the atria to the atrioventricular node, which delays it so the ventricles fill, then passes through the His bundle into the right and left bundle branches (the left dividing into anterior and posterior fascicles) and the Purkinje fibres.

Conduction disturbances are slowing or block anywhere along this path:

Sinus node dysfunction and atrioventricular block slow the heart (bradyarrhythmias). Bundle branch and fascicular blocks widen or shift the QRS, and they matter mainly as markers of heart disease and of the risk of complete block.

02Etiology and pathophysiology

Causes

Why the level of block matters

1. 1

   Block in the node

   Block in the atrioventricular node is usually caused by vagal tone, drugs, ischaemia of an inferior infarction or inflammation, and it often recovers. The His bundle takes over with a narrow-complex escape rhythm of 40 to 60 a minute, which is fairly reliable and responds to atropine.
2. 2

   Block below the node

   Block in the His bundle or the bundle branches is usually caused by fibrosis or a large anterior infarction and seldom recovers. The escape rhythm comes from the ventricles: broad, slow (20 to 40 a minute) and unreliable, and atropine does not help.
3. 3

   The consequence

   When an infranodal escape rhythm pauses, the patient loses consciousness without warning, a Stokes–Adams attack, and may die. This is why Mobitz II, high-degree and complete block are paced even without symptoms; the danger is greatest when the escape is broad.

Prolonged right ventricular pacing activates the ventricles in the same abnormal sequence as left bundle branch block. In some patients this weakens the left ventricle, a pacing-induced cardiomyopathy.

03Clinical features

• Many conduction disturbances cause no symptoms and are found on an ECG.
• Slow rates and pauses cause fatigue, dizziness, presyncope, syncope, breathlessness, reduced exercise tolerance, confusion in older people, and heart failure.
• Stokes–Adams attacks: sudden collapse without warning, pallor, sometimes brief jerking, then a flush as the circulation returns, with rapid recovery.
• Chronotropic incompetence, a heart that does not speed up with effort, causes breathlessness and fatigue on exertion only.
• Tachy-brady syndrome: episodes of atrial fibrillation or flutter that end in a long pause, causing palpitations then dizziness or syncope.

In complete heart block the pulse is slow and regular and does not rise with effort, the jugular venous pressure shows intermittent cannon waves when the atria contract against closed valves, and the loudness of the first heart sound varies. Right bundle branch block widens the splitting of the second sound, and left bundle branch block reverses it.

04Diagnosis and management

The ECG

Left bundle branch block makes the ECG hard to read for ischaemia, so left bundle branch block with symptoms of ongoing ischaemia is managed as a possible STEMI. Alternating right and left bundle branch block signals severe disease of the whole conduction system.

Other tests

• Blood tests: potassium, thyroid function, digoxin levels, Lyme serology where exposure is possible, and troponin when ischaemia or myocarditis is suspected.
• Ambulatory ECG monitoring, chosen by how often the symptoms occur: a 24-hour to 7-day recording, an external event recorder, or an implantable loop recorder for rare unexplained syncope.
• An exercise test for chronotropic incompetence and for block that appears on exertion, which points below the node.
• Echocardiography for structural heart disease, and cardiac MRI in a patient under about 60 with unexplained atrioventricular block, looking for sarcoidosis, myocarditis or a cardiomyopathy.
• An electrophysiology study in selected patients with syncope and bundle branch block: an HV interval of 70 ms or more indicates pacing.

Acute management of bradycardia

Bradycardia is dangerous when it causes shock, syncope, myocardial ischaemia or heart failure, or when there is a high risk of asystole: a recent pause of more than 3 seconds, Mobitz II, or complete block with a broad QRS.

• Reversible causes are treated at the same time: stopping the slowing drugs, treating hyperkalaemia, glucagon for a beta-blocker overdose, digoxin-specific antibody fragments for digoxin toxicity, reperfusion of an infarction.
• Atropine intravenously, 0.5 to 1 mg depending on the guidance followed, repeated to a total of 3 mg. It works on vagal and nodal causes. It is not given in high-degree block with a broad QRS, where it does not work and may worsen the block, nor after heart transplantation, where it can cause complete block or sinus arrest; aminophylline is used there instead.
• If it fails or is not suitable: an infusion of isoprenaline or adrenaline, and transcutaneous pacing, which is uncomfortable and needs sedation.
• A temporary transvenous pacing wire, placed early in an unstable patient through a central vein into the right ventricle, until the cause resolves or a permanent pacemaker is fitted.

After an inferior myocardial infarction, atrioventricular block usually resolves within days, so a permanent pacemaker is considered only if it persists for at least about 5 days. Block after cardiac surgery is also watched for several days, and after transcatheter aortic valve implantation for 24 to 48 hours. In Lyme carditis the block usually resolves with intravenous ceftriaxone, and a temporary wire bridges the gap.

Permanent pacing

Pacemakers are implanted under local anaesthetic, with the generator under the skin below the clavicle and leads passed through the cephalic or axillary vein. They are described by a code:

The R means rate-responsive: a sensor raises the rate with activity. Leadless pacemakers, implanted in the right ventricle through the femoral vein, avoid leads and a pocket. Pacing the conduction system itself, at the His bundle or the left bundle branch area, keeps a near-normal activation and is increasingly used when much ventricular pacing is expected, to prevent pacing-induced cardiomyopathy. Patients with a reduced ejection fraction who need ventricular pacing receive cardiac resynchronisation instead. A patient who needs pacing and has cardiac sarcoidosis or a lamin A/C variant receives an implantable defibrillator, because of the high risk of ventricular arrhythmia.

Complications include pneumothorax, a pocket haematoma, lead displacement in the first weeks, tricuspid regurgitation, and infection, which needs removal of the whole system. A single-chamber ventricular pacemaker in sinus rhythm can cause pacemaker syndrome: dizziness, fatigue and pulsation in the neck as the atria contract against closed valves. Patients are followed in a device clinic, often remotely; batteries last about 7 to 12 years; most modern systems are safe in MRI under set conditions.

05Epidemiology

First-degree block is found in about 1 to 2 per cent of young adults and becomes commoner with age. Right bundle branch block affects about 1 per cent of adults, left bundle branch block fewer, and both rise with age. Mobitz I is common in athletes and during sleep. Complete heart block is rare, mostly in older people, and congenital complete block occurs in about 1 in 15,000 to 20,000 births. European countries implant a median of about 740 pacemakers per million people a year, from under 50 to over 1,000 between countries, and more than 80 per cent go to people over 65; atrioventricular block and sinus node dysfunction are the commonest reasons.

06Prognosis

Sinus node dysfunction is not life-threatening in itself: pacing relieves the symptoms but does not prolong life, and atrial fibrillation and stroke are the main later risks. First-degree block and Mobitz I in a healthy heart are usually benign. Complete heart block with blackouts was often fatal before pacemakers, about half of such patients dying within a year; with pacing, survival approaches that of people of the same age, and the outlook depends on the underlying heart disease. Isolated right bundle branch block in a normal heart has a good prognosis, while left bundle branch block marks a higher cardiovascular risk. Bifascicular block progresses to complete block in only about 1 to 2 per cent a year without symptoms, more often with syncope or a prolonged HV interval.