Intrinsic Pacemakers

The SA node is the primace pacemaker of the heart, but depending on the condition of the heart, other areas of the heart can become the dominant pacemaker of the heart. We can tell what the dominant pacemaker is by looking at the P wave and the QRS complex.

SA Node Impulses: "Sinus" rhythms

Impulses from the SA node generate "sinus" beats, and have been the primary ECG rhythm seen so far. They characteristically have:

  • Narrow QRS complexes
  • Upright P waves before the QRS complex

The SA node naturally paces between 60 to 100 bpm.

Atrial Impulses: "Atrial" rhythms

Impulses from the atria (notsurprisingly) generate "atrial" beats. They often look like sinus beats (with narrow QRS complexes and upright Ps), and are often difficult to distinguish without a more thorough cardiac assessment, unless there are clear signs that the P waves are different. This includes:

  • Multiple P wave shapes
  • P waves that look different or strange (e.g. notched, "sawtooth" like, etc.)
  • A chaotically irregular P wave line A single atrial impulse will pace between 60 to 100 bpm.

AV Node Impulses: "Junctional" rhythms

Impulses from the AV node generate "junctional" beats. They have:

  • Narow QRS complexes
  • Inverted P waves before or after the QRS complex, or no P wave at all.

These P waves form because the atrial impulse is heading backwards from the AV node towards the atria. This is called retrograde conduction and so these inverted P waves are sometimes also called retrograde P waves.

The AV node will pace between 40 to 60 bpm.

Ventricular Impulses: "Ventricular" rhythms

Impulses from the ventricles (i.e. the bundle branches or Purkinje fibres) generate "ventricular" beats. They notably have:

  • Wide QRS complexes
  • No P waves

The ventricles will pace between 20 to 40 bpm.

Classification of Rhythms by Pacemakers

Regular rhythms can often be classified by heart rate and which pacemaker generated the impulse. The pacemaker rates are summarized here:

There are a few important principles with naming these rhythms in this manner:

  • All rhythms are named after the predominant pacemaker that generated the impulse.
  • A sinus rhythm going at its natural pace is simply a "sinus rhythm" or "normal sinus rhythm."
  • A junctional or ventricular rhythm going at its natural pace are termed "escape rhythms" since they naturally only occur if the SA node (or higher node) fails.
  • Tachycardia is always a heart rate greater than 100 bpm.
  • Accelerated refers to a heart rate going faster than it's natural pacemaker rate, but below 100. Because sinus rhythms pace up to 100 bpm, accelerated sinus rhythms are not a thing.
  • Bradycardia is always a heart rate less than 60 bpm that's not already named by one of the above rhythms (e.g. escape or accelerated).

Using the above principles, we can name the following sinus, junctional, and ventricular rates as per the following illustration.