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Monday, October 31, 2011

Heart Rhythms, Part 5: Premature Beats

There are 3 main types of premature beats. PAC's, PJC's and PVC's.


PAC's are Premature Atrial Contractions. While the SA node normally regulates the heartbeat during a sinus rhythm, PACs occur when another region of the atria depolarizes before the sinoatrial node and thus triggers a premature heartbeat. 

Clear as mud? In English, another part of the upper chambers of the heart prematurely fire off an electrical impulse causing an early beat. On the EKG it will have a deformed P wave (because it's not originating in the SA node), will have a narrow QRS complex, and be earlier than the other beats. After the PAC there were be a compensatory pause, a delay, after the premature complex, before the next regular complex. 

What does it mean? Nothing. Lots of perfectly healthy people have PAC's. Don't stress about it. I don't.


PJC are Premature Junctional Contractions. While the SA node normally regulates the heartbeat during a sinus rhythm, PJCs occur when AV node depolarizes before the sinoatrial node and thus triggers a premature heartbeat. 

Yes, it sounds a lot like a PAC. Very similar, just with a different origin. On the EKG they will look like a PAC except that there may not be a P wave, the P wave may be inverted, or after the QRS complex. Just like with a junctional rhythm.

Once again, these can happen with healthy hearts. Not too much to worry about.


PVC is a Premature Ventricular Contraction. While the SA node normally regulates the heartbeat during a sinus rhythm, PVCs occur when ventricles depolarize before the sinoatrial node or the AV node and thus triggers a premature heartbeat. At times PVCs can be felt as chest palpitations or a "missed heart beat" but they also occur in healthy hearts. Since the ventricles are contracting before they have time to fill with blood a PVC is not as efficient at pumping blood.

When PVCs happen more that once they can fall into a number of categories If every other beat is abnormal, you can describe it as bigeminal. If every third beat is aberrant, it is trigeminal; every fourth would be quadrigeminal. 

If you have 2 PVCs back to back they are considered couplets. 3 are considered triplets. More than 3 PVCs in a row is considered a run of V-Tach. Unlike PACs, PJCs and the occasional PVC, runs of V-Tach is something to be concerned about.

Some possible causes of PVCs include:

  • Ischemia
  • Certain medicines such as digoxin, which increases heart contraction
  • Myocarditis
  • Cardiomyopathy, hypertrophic or dilated
  • Myocardial contusion
  • Hypoxia
  • Hypercapnia (CO2 poisoning)
  • Smoking
  • Alcohol
  • Drugs such as cocaine
  • Caffeine
  • Tricyclic antidepressants
  • Magnesium and potassium deficiency
  • Calcium excess
  • Thyroid problems
  • Chemical (electrolyte) problems in the blood
  • Heart attack
  • Adrenaline excess
  • Lack of sleep/exhaustion
  • Stress
When PVCs all look the same on the EKG they are considered monomorphic or unifocal. This happens because the PVCs are all origintaing in the same location. If the PVCs have more than one look to them they are considered polymorphic or multifocal. This means they are stemming from multiple focal points in the ventricles.

Heart Rhythms Part 1: Basic Anatomy
Heart Rhythms Part 2: Sinus Rhythms
Heart Rhythms Part 3: Junctional Rhythms
Heart Rhythms Part 4: Ventricular Rhythms
Heart Rhythms Part 5: Premature Beats
Heart Rhythms Part 6: Heart Blocks
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