Patients are seldom aware that their heart rate is slow, but they can certainly be aware of the effects of a bradycardia. Marked sinus bradycardia is characteristic of athletic training but bradycardia is also a contributory cause of: the symptom of fainting in vasovagal attacks; the reduced cardiac output and syncope associated with heart block; and the hypotension and heart failure in patients with an inferior myocardial infarction. A slow heart rate can also be a major contributor to angina. An ECG is therefore an essential part of the investigation of any patient with a slow pulse rate, and indeed of any patient with dizziness, syncope or breathlessness. The causes of sinus bradycardia have been discussed in Chapter 1 (see p. 5 and Box 1.2, p. 7). Escape rhythms have been discussed in Chapter 2 (p. 77). They are usually asymptomatic, but symptoms occur when the automaticity that generates the escape rhythm is inadequate to maintain a cardiac output. A bradycardia may cause the symptom of syncope; some of the possible underlying causes are listed in Box 5.1. Disordered sinoatrial (SA) node function can be familial or congenital and can occur in ischaemic, valvular, hypertensive or infiltrative cardiac disease. It is, however, frequently idiopathic. Abnormal function of the SA node may be associated with failure of the conduction system. Patients with SA disease may be asymptomatic, but all the symptoms associated with bradycardias – dizziness, syncope and the symptoms of heart failure – can occur. Atrial and junctional tachycardias often occur together with sinus node dysfunction, when the patient may present with palpitations. The abnormal rhythms seen in the sick sinus syndrome are listed in Box 5.2. The ECGs in Figs 5.1 and 5.2 are from a young man who had a normal ECG with a slow sinus rate when asymptomatic, but intermittently became extremely dizzy when he developed a profound sinus bradycardia. The ECG in Fig. 5.3 shows an ambulatory record from a young woman who complained of short-lived attacks of dizziness. When she had these, the ECG showed sinus pauses. Fig. 5.4 shows the other variety of sinus pause – sinus arrest. The ECG in Fig. 5.5 shows an example of a ‘silent atrium’, when the heart rhythm depends on the irregular depolarization of a focus in the atrioventricular (AV) node. The combination of sick sinus syndrome and episodes of tachycardia is sometimes called the ‘tachycardia–bradycardia syndrome’, and Fig. 5.6 shows the rhythm of a patient with this syndrome. This patient was asymptomatic at times, when his ECG showed a ‘silent atrium’ with a slow and irregular junctional (AV nodal) escape rhythm, but he complained of palpitations when he had an AV nodal tachycardia. Fig. 5.7 shows the ECG from a patient who, when asymptomatic, showed first degree block and right bundle branch block suggesting a potential for higher degree AV block. He complained of fainting attacks, but an ambulatory recording showed that this in fact due to sinus arrest with a very slow AV nodal escape rhythm, giving a ventricular rate of 15 bpm (Fig. 5.8). This is an example of the combination of conduction system disease and sick sinus syndrome. Possible causes of sick sinus syndrome are listed in Box 5.3. A slow ventricular rate can accompany atrial flutter or atrial fibrillation because of slow conduction through the AV node and His bundle systems (Figs 5.9 and 5.10). This may be the result of treatment with drugs that delay AV nodal conduction, such as digoxin, beta-blockers or verapamil, but can occur because of conducting tissue disease. Complete block associated with atrial fibrillation is recognized from the regular and wide QRS complexes which originate in the ventricular muscle (Fig. 5.11). Symptoms are not caused by first degree block, second degree block of the Wenckebach or Mobitz type 2 varieties, left anterior hemiblock or the bundle branch blocks. Second degree block of the 2 : 1 or 3 : 1 type will cause dizziness and breathlessness if the ventricular rate is slow enough (Fig. 5.12). Young people tolerate slow hearts better than old people do. Complete (third degree) block characteristically involves a slow rate, but this may be fast enough to cause only tiredness or the symptoms of heart failure. Fig. 5.13 shows the ECG of a 60-year-old man who, despite a heart rate of 40 bpm, had few complaints. If the ventricular rate is very slow the patient may lose consciousness in a ‘Stokes–Adams’ attack, which can cause a seizure and sometimes death. The ECG in Fig. 5.14 is from a patient who was asymptomatic while his ECG showed sinus rhythm with first degree block and right bundle branch block, but who then had a Stokes–Adams attack with the onset of complete block (Fig. 5.15). The possible causes of heart block are summarized in Box 5.4.
The ECG when the patient has a bradycardia
Mechanism of bradycardias
Sinoatrial disease – the ‘sick sinus syndrome’
Atrial fibrillation and flutter
AV block
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The ECG when the patient has a bradycardia
5
Long PR interval and RBBB pattern in lead V1
Fig. 5.7
Atrial flutter with variable block
Flutter waves in lead II
Fig. 5.9
Prolonged QT interval in lead V4
Fig. 5.10
P waves in lead V1
Fig. 5.12
P waves in lead VL
Fig. 5.13
Long PR interval and RBBB pattern in lead V1
Fig. 5.14
Complete block and Stokes–Adams attack
P waves
Fig. 5.15
