Premature atrial contractions (PACs) on an EKG strip

How to identify PACs in five steps

Ask the same five questions of every strip. The right-hand column is what each one finds on the strip above.

1. 1Rate

   That of the underlying rhythm. Count all QRS complexes in 6 seconds, including the early ones.

   ~70 bpm

2. 2Rhythm

   Regular, interrupted by the early beat and the pause after it.

   Regular, with early beats

3. 3P waves

   Present before the early beat but shaped differently: flatter, more pointed, notched or inverted. It may be buried in the preceding T wave.

   Different on the early beat

4. 4PR interval

   Normal on the sinus beats. On the PAC it may be a little shorter or longer.

   0.16 s

5. 5QRS

   Narrower than 0.12 s on every beat, including the early one.

   0.07 s

What is happening in the heart

An irritable spot in the atria fires before the SA node’s next impulse. Because the impulse begins in a different place, it spreads across the atria in a different direction and draws a P wave of a different shape. From the AV node onward the route is the normal one, so the QRS looks like all the others. The early impulse also discharges and resets the SA node, which restarts its timing from that moment; that is why the pause afterward is not fully compensatory.

More PAC strips: no two are the same

A rhythm is a set of rules, not a picture. These three strips all meet the rules for PACs, with different numbers.

What PACs get mistaken for

PACs vs. premature ventricular contractions (PVCs)

A PVC is also early, but it is wide and odd-looking, has no P wave in front of it, and is followed by a full compensatory pause. A PAC has a P wave, a normal narrow QRS, and a shorter pause.

PACs vs. sinus arrhythmia

In sinus arrhythmia the spacing changes gradually and every P wave is identical. A PAC is one abrupt early beat with a P wave that does not match the others.

PACs vs. atrial fibrillation

Frequent PACs make a strip look irregular enough to suggest atrial fibrillation. If you can find a P wave before every QRS, it is not AFib.

Causes of PACs

• Caffeine, nicotine and alcohol
• Stress, fatigue and lack of sleep
• Hypoxia and lung disease
• Stretching of the atria in heart failure or valve disease
• Low potassium or low magnesium
• Digoxin toxicity
• Often nothing at all, in a healthy heart

Why it matters

Occasional PACs are common and harmless. They become worth reporting when they are frequent or new, because an irritable atrium can be the forerunner of atrial fibrillation, atrial flutter or SVT. A PAC that comes very early may not conduct to the ventricles at all, and that blocked PAC is one of the commonest causes of an unexpected pause.

What is done for PACs

• No treatment for occasional PACs in a patient without symptoms.
• Note how often they occur and whether that is increasing.
• Look for a trigger: stimulants, low oxygen saturation, electrolyte results.
• Report frequent or new PACs, especially in a patient with heart disease.
• If they cause troublesome palpitations, a beta blocker may be prescribed.

Study summary, not a protocol. Follow current American Heart Association guidelines and your facility’s policies. See the medical disclaimer.

Common mistakes with PACs

• Calling a PAC a PVC because it is early. Width and the P wave decide, not timing.
• Missing the early P wave when it is hidden in the T wave before it; compare that T wave with its neighbors.
• Mistaking a blocked PAC for a sinus pause or a heart block.

Questions about PACs