Sinus bradycardia

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).
  • Hypothyroidism.
  • Hypothermia.
  • Hyperkalemia.

 

ECG example of sinus bradycardia

Figure 1 shows sinus bradycardia at paper speed 25 mm/s.

Figure 1. Sinus bradycardia. Paper speed 25 mm/s. Calculate the rate by dividing 300 by the number of large boxes between two cycles (e.g between two R waves). As seen in the figure, there are approximately 6.5 large boxes between two R waves. 300/6.5 equals 46 beats/min.

Figure 1. Sinus bradycardia. Paper speed 25 mm/s. Calculate the rate by dividing 300 by the number of large boxes between two cycles (e.g between two R waves). As seen in the figure, there are approximately 6.5 large boxes between two R waves. 300/6.5 equals 46 beats/min.

 

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.

 

Next chapter

Chronotropic incompetence

 

Related chapters

Normal automaticity and pacemaker cells in the heart

Mechanisms of cardiac arrhythmias

Normal sinus rhythm

Sinus tachycardia & Inappropriate Sinus Tachycardia

Management (Treatment) of Bradycardias

 

View all chapters in Cardiac Arrhythmias.

Become an ECG expert! 1 day left to save 90%REGISTER
error: Contact us for permission to use contents. Permission will be granted for non-profit sites.

Free ECG Pocket Guide!

Join our mailing list to get the Pocket Guide to ECG Interpretation!

 

Great! You will now be forwarded to download page!