Premature Atrial Contractions
Premature atrial contractions (PACs) are beats which are initiated in the atria or upper chambers of the heart, prematurely, which cause the SA node (the natural pacemaker of the heart) to be interrupted. The terms SVEs (supraventricular extrasytoles) and PJCs (premature junctional contractions) are also often used when describing these beats but the distinctions are beyond the scope of this discussion.
PACs are one of the two most common heart rhythm abnormalities observed, the other being PVCs (premature ventricular contractions). They are frequently benign and require no treatment. However, in some cases they may be so frequent (over 15-20/minute) that they may cause the heart to beat inefficiently enough to cause symptoms which may need to be addressed. Occasionally, patients who have PACs may also have atrial fibrillation at other times.
PACs may occur singly, in pairs, in short runs or every other beat (bigeminy) and also may be aberrant or non-conducted. Examples of all of these are shown below. Patients who have these types of rhythm abnormalities may often refer to them as palpitations, skipped beats, hard beats, irregular beats, missing beats or extra beats. They may also complain of feeling dizzy or lightheaded or experience chest pain. Some patients may have no symptoms at all.
Just to reiterate, PACs are premature beats or beats occurring earlier than they should. Many patients describe them as skipped beats, because when they check their pulse, they don’t feel anything for a moment. However, your heart is not actually skipping or missing a beat. What is happening is that when a beat occurs prematurely, the normal volume of blood has not yet returned to your heart from the previous beat. So, even though your heart contracts, not enough blood has returned from the previous beat for it to pump the normal amount of blood. Because of reduced blood being pumped, it may feel like you have skipped a beat, but you have not, although the beat was certainly not as effective as a normal beat.
Patients frequently experience more of these palpitations at night or when they are relaxing. This is because when the natural pacemaker of the heart (the SA node) slows down, as it frequently will when you are relaxed, these ectopic (out of the wrong place) foci (point of origins) do not get reset soon enough to stop them.
The above is an example of a single, isolated PAC. The heart rate in this example is 79 beats per minute. The first three beats are normal, but the fourth beat occurs early. You can see the P wave right after the T wave of the previous beat. This beat occurs at a rate of 106 beats per minute. We know it is a PAC (instead of a PVC) because a P wave immediately precedes it and the QRS is a normal shape which means the beat originated in the upper chambers (atria) of the heart.
Pairs and brief runs of PACs
The above is an example where the underlying rhythm is Normal Sinus Rhythm at 75 beats per minute. In addition, it contains a PAC pair, a single PAC and a 3 beat run of PACs. The first beat is normal, then we have the PAC pair, then a normal beat, then a single PAC, then 2 normal beats, then a 3 beat run of PACs followed by a normal beat. If a run of PACs contains more than 3 beats it is called a PSVT or PAT.
The above is an example of bigeminal PACs. Bigeminal PACs are PACs which occur every other beat. Patients with bigeminal PACs that continue for a while (over 30 seconds), may feel lightheaded or faint. This is because as explained above, the premature beats are not pumping blood very effectively and the body is not receiving enough oxygen.
Aberrantly conducted PACs
The above example is a variation of a single PAC (the third beat) called an aberrant PAC. In this case, the beat does not look the same as the normal beats (which is usually the case with PACs), but through a number of clues, we know it is coming from above the ventricles. The reason it looks different is that the part of the conduction system that carries the impulse from the atria to the ventricles (the bundle branches) has not yet fully recovered from the previous beat (because this beat occurred early), causing the beat to be slowed down while traveling through one of the bundle branches. This slowing of the signal is what causes the shape to change since one of the ventricles begins to contract before the other ventricle.
The above example is a nonconducted PAC. The first three beats are normal, but if you look closely at the end of the T wave after the third beat (pointed out by the arrow), you will see a little notch not present in the other T waves. This is actually a P wave, but it does not get conducted to the ventricles, which in effect creates a skipped beat, since the ventricles do not contract. The atria contract, causing a P wave, but the ventricles do not. This is a nonconducted PAC. In this case it has likely occurred because the P wave came so early that the rest of the conduction system (the AV node and the bundle branches) did not have time to recover from the previous beat and could not transmit the impulse.