ATRIAL PREMATURE CONTRACTIONS
A premature atrial complex (PAC) is a premature beat arising from ectopic pacemaker tissue within the atria.
There is an abnormal P wave, usually followed by a NORMAL QRS complex.
Origin of Ectopic Beats:
● Groups of pacemaker cells throughout the conducting system are capable of spontaneous depolarization
● The rate of depolarization decreases from top to bottom: fastest at the sinoatrial node; slowest within the
ventricles
● Ectopic impulses from subsidiary pacemakers are normally suppressed by more rapid impulses from above
● However, if an ectopic focus depolarizes early enough — before the arrival of the next sinus impulse — it may
“capture” the ventricles, producing a premature contraction
● Premature contractions (“ectopic”) are classified by their origin — atrial (PAC), junctional (PJC) or ventricular
(PVC)
Causes of Premature Atrial Contractions (PAC)
1. Anxiety
2. Sympathomimetics
3. Beta-agonists
4. Excess caffeine
5. Hypokalemia
6. Hypomagnesaemia
7. Digoxin toxicity
8. Myocardial ischemia
Classification of PACs
PACs may be either:
● Unifocal – arising from a single ectopic focus; each PAC is identical.
● Multifocal – arising from two or more ectopic foci; multiple P-wave morphologies.
PACs often occur in repeating patterns:
● Bigeminy — every other beat is a PAC
● Trigeminy — every third beat is a PAC
● Quadrigeminy — every fourth beat is a PAC
● Couplet – two consecutive PACs
● Triplet — three consecutive PACs
A premature atrial complex (PAC) is a premature beat arising from ectopic pacemaker tissue within the atria.
There is an abnormal P wave, usually followed by a NORMAL QRS complex.
Origin of Ectopic Beats:
● Groups of pacemaker cells throughout the conducting system are capable of spontaneous depolarization
● The rate of depolarization decreases from top to bottom: fastest at the sinoatrial node; slowest within the
ventricles
● Ectopic impulses from subsidiary pacemakers are normally suppressed by more rapid impulses from above
● However, if an ectopic focus depolarizes early enough — before the arrival of the next sinus impulse — it may
“capture” the ventricles, producing a premature contraction
● Premature contractions (“ectopic”) are classified by their origin — atrial (PAC), junctional (PJC) or ventricular
(PVC)
Causes of Premature Atrial Contractions (PAC)
1. Anxiety
2. Sympathomimetics
3. Beta-agonists
4. Excess caffeine
5. Hypokalemia
6. Hypomagnesaemia
7. Digoxin toxicity
8. Myocardial ischemia
Classification of PACs
PACs may be either:
● Unifocal – arising from a single ectopic focus; each PAC is identical.
● Multifocal – arising from two or more ectopic foci; multiple P-wave morphologies.
PACs often occur in repeating patterns:
● Bigeminy — every other beat is a PAC
● Trigeminy — every third beat is a PAC
● Quadrigeminy — every fourth beat is a PAC
● Couplet – two consecutive PACs
● Triplet — three consecutive PACs
