what is the difference between Premature Atrial Contraction and Atrial Fibrillation. I know that both have irregular heart beat but have normal QRS complex. Both have abnormal or absence P waves but a normal T waves.

Can PAC be considered as an isolated event? and AFib be considered as chronic event?


1 Answer 1


Atrial Fibrillation is a dysrhythmia while PAC is an early depolarization coming from the atria. An occasional PAC can be normal for some people. In that case their heart rhythm would be a sinus arrhythmia instead of a normal sinus rhythm.

In AF, the atrias don’t contract/pump at all, but they quiver/twitch due to the many simultaneous ectopic foci that are being discharged from random cells; similar concept to Ventricular Fibrillation. Due to the atrias not fully depolarizing, you won’t see any P waves on the EKG. If/when an electrical discharge reaches the AV node, it goes through the normal electrical condition system from there, forming a narrow QRS complex. AF is the only irregularly irregular rhythm, meaning that the QRS complexes have no regularity at all, and have no pattern to their irregularity.

Video example of AF.

Rhythm Strip Rhythm strip of AF

As for PACs, the atrias depolarize earlier than expected creating what looks like an early beat. The next beat though will be later than expected. The pacemaker cells “reset”, returning to their pattern after this early beat.

Example of PAC

Both AF and PACs can be acute or chronic depending on the person.

  • $\begingroup$ i see~ thanks for the response! $\endgroup$
    – Larry To
    Jul 18, 2019 at 2:49
  • $\begingroup$ another question, so does it mean you cannot have PAC in two consecutive peaks right? because otherwise clinically how would you know the atria depolarized earlier or just because the patient's heart rate is increasing $\endgroup$
    – Larry To
    Jul 18, 2019 at 3:09
  • $\begingroup$ You can have what’s called a run of PACs(PJCs and PVCs as well). Remember that what is seen on the EKG is the electrical activity, not the actual contractions. On the cardiac monitor the HR will increase a bit. There is a debate on whether to count premature beats as part of the heart rate or not. You can tell if someone has a premature beat by feeling the pulse, but to know the type of premature beat, you’ll need an EKG. Clinically, premature beats are not an immediate life threat because they can be caused by something as simple as stress. A run of PVCs (aka run of V Tach) is dangerous. $\endgroup$
    – Gantz
    Jul 18, 2019 at 18:58
  • $\begingroup$ I hope this answered your question, as I was a bit confused about the second part. $\endgroup$
    – Gantz
    Jul 18, 2019 at 18:59
  • $\begingroup$ Not quite haha. So you know there are bigeminy and trigeminy for PVC, because you can have consecutive premature ventricular contraction. But my question can you have consecutive "premature atrial contraction"? Because wouldn't it be difficult to distinguish a consecutive PAC and an increased heart rate? $\endgroup$
    – Larry To
    Jul 19, 2019 at 9:46

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