Definition of sinus bradycardia
Sinus bradycardia fulfills the criteria for sinus rhythm but the heart rate is slower than 50 beats per minute. ECG criteria follows:
- Regular rhythm with ventricular rate slower than 50 beats per minute.
- P-waves with constant morphology preceding every QRS complex.
- P-wave is positiv in limb lead II.
Normal (physiological) causes of sinus bradycardia
Sinus bradycardia (SB) is considered a normal finding in the following circumstances:
- During sleep.
- Well-trained individuals display SB at rest due to high vagal tone. These individuals have developed a highly efficient left ventricle, capable of generating sufficient cardiac output at low heart rates.
- During vasovagal syncope (e.g during intense emotional stress)
- During vagal maneuvers (Valsalva maneuver, carotid sinus [baroreceptor] stimulation).
- Its not uncommon to discover SB in healthy young individuals who are not well-trained. This is also a normal finding.
Abnormal (pathological) causes of sinus bradycardia
In all other situations, sinus bradycardia should be regarded as a pathological finding. There are numerous pathological conditions that cause sinus bradycardia. The most important causes are as follows:
- Myocardial ischemia/infarction – Particularly ischemia or infarction located to the inferior wall of the left ventricle. This type of bradycardia is due to diminished automaticity (pacemaker function) in the sinoatrial node or conduction defects (e.g second-degree AV block) as a result of ischemia/infarction.
- Sinus Node Dysfunction (SND) – Sinus node dysfunction implies that the cells of the sinoatrial node are defect and fail to generate electrical impulses.
- Side effects of drugs (notably beta blockers, digitalis, verapamil, diltiazem, amiodarone, klonidin) – These drugs affect the pacemaker cells in the sinoatrial node. They may also induce conduction defects (e.g AV block).
- Increased intracranial pressure (manifests with sinus bradycardia and hypertension).
ECG example of sinus bradycardia
Figure 1 shows sinus bradycardia at paper speed 25 mm/s.
Treatment of sinus bradycardia
Benign causes of sinus bradycardia (SB) do not require treatment. In all other situations it is necessary to find the underlying cause and direct treatments towards it. The most common causes are sinus node dysfunction, side effects of medications or ischemia/infarction. When the bradycardia causes hemodynamic symptoms it should be treated. Treatment of bradycardia is discussed separately. Note that SB due to ischemia located to the inferior wall of the left ventricle is typically temporary and resolves within 1–2 weeks. SB in infarction/ischemia is discussed separately.