Low Pulse – Causes of a Slow Heart Rate (Bradycardia)

What is Bradycardia?

Bradycardia is the medical term for a low pulse, which indicates a slow heart rate. Normally, the heart rate is between 60 and 100 beats per minute (bpm) when at rest. The rate of most adults is between 65 bpm and 85 bpm.

Very fit adults may have a heart rate that is lower than 60 bpm but this is never lower than 40bpm except in rare cases. Bradycardia may also be recorded in a person who is sleeping but the heart rate will rise once the person awakes and increase further when standing.

Any pulse reading that is recorded at less than 60 bpm indicates a heart that is beating too slowly. This means that the cardiac output is low – the heart is pumping out less blood than it should.

How does bradycardia occur?

The heart rate is determined by the impulses emanating from the sinoatrial node (sinus node). These impulses travel through the internodal pathways to the atrioventricular node (AV node). Here the impulses are delayed so that the heart atria, which receive blood, do not contract at the same time as the heart ventricles, which push out blood.

It is the ventricular contract that is felt as the pulse at teh various parts of the body – radial (wrist), brachial (elbow), carotid (neck), femoral (thigh), popliteal (knee), posterior tibial (ankle) and dorsalsis pedis (foot). In bradycardia, the sinus node is either generating impulses at a slower rate (sinus bradycardia) or AV node is not passing out the impulses effectively (AV block).

Symptoms of Slow Heart Rate

A slow heart rate in athletes and when asleep or very relaxed is due to reduced sinus node impulses (sinus bradycardia). This is also known as asymptomatic sinus bradycardia because a person does not feel unwell with such a slow heart rate and it is not due to any disease process.

Pathological bradycardia may result in a number of symptoms including :

  • Fatigue
  • Dizziness
  • Fainting
  • Blurred vision

Causes of a Low Pulse Rate

The following conditions may cause sinus bradycardia or affect the AV node.

  • Hypothyroidism
  • Hypothermia
  • Coronary artery disease
  • Myocardial infarction (heart attack)
  • Heart infections – endocarditis, myocarditis
  • Drugs like beta blockers, verapamil, digoxin
  • Raised intracranial pressure
  • Obstructive jaundice
  • Sick sinus syndrome (diseases affecting the sinus node)
  • Hypersensitivity of the carotid sinus (carotid artery)
  • Heart block (AV block)