Premature atrial contraction

The heart has two upper chambers, which are called the atria, and a premature atrial contraction, or PAC, is when the atria contract earlier than normal in the cardiac cycle.

So normally, the sinoatrial, or SA, node sends an electrical signal called a depolarization that propagates out through the walls of the heart and causes both upper chambers to contract. Then, that signal moves to the atrioventricular, or AV, node, where it’s delayed for a split second. Then, the signal travels down into the ventricles, or lower chambers, where it moves down the bundle of His into the left and right bundle branches and into each ventricle’s Purkinje fibers, causing them to contract as well. This trip is called a depolarization wave, and in a healthy heart, it makes sure that the upper chambers contract before the lower chambers contract. On an electrocardiogram, or ECG, which measures the electrical activity of the heart via electrodes that are placed on the skin, the atrial depolarization and its contraction are seen as a P-wave, the ventricular contraction is seen as a QRS complex, and the ventricular repolarization and its relaxation are seen as a T-wave.

If an atrial cell outside of the SA node initiates a depolarization, that’s called an atrial ectopic focus. Now, this could be initiated by a cell that’s part of the conduction system, or it could just be initiated by another cardiac muscle cell. This typically happens when atrial cells are irritated and stressed by electrolyte imbalances, drugs like cocaine or methamphetamines, ischemic damage like a heart attack, or anything that increases sympathetic activity, like anxiety. Another type of atrial ectopic focus is a reentrant loop, which is when there’s some tissue that doesn’t depolarize properly, which could happen in scar tissue after a heart attack. As a result of this damage, the depolarization wave ends up circling around and around that tissue. A reentrant loop basically starts sending out depolarization waves to the rest of the heart tissue each time the wave goes around.

So, in short, a particularly stressed out atrial cell or a reentrant loop can send out depolarization waves that contract the atria too quickly, and that’s an early, or premature, atrial contraction.

One important effect of premature depolarization is that the ectopic focus will depolarize the cells of the SA node, which causes the SA node to skip a cycle. So, let’s say the SA node is working as usual, but, all of a sudden, an ectopic focus fires off a depolarization early. Now, if we measure out the interval between each SA node fire, we see that the SA node should have fired here, but it doesn’t because it was depolarized by the ectopic focus. This means that the SA node essentially skips one cycle of sending its own signal. So, the sinus node waits to fire for the same interval that it normally would, but it also fires based on the ectopic beat. This means that that you end up with an interval with two sinus beats and an ectopic beat in the middle, and that interval is shorter than an interval that’s three sinus beats in a row. This shorter interval is called a noncompensatory pause, where the distance from sinus P-wave to the ectopic P-wave to the next sinus P-wave is less than the distance from the sinus P-wave to sinus P-wave to sinus P-wave.

The location of the ectopic focus in the atria determines how the heart contracts and therefore what the ECG looks like. If the ectopic focus is at the bottom of the atria, then the P-wave will be upside-down. This is because instead of flowing from the top of the atria to the bottom, which is how depolarization normally spreads, the electricity flows in the opposite direction, i.e. from the bottom of the atria to the top.

Also, the closer the ectopic focus is to the AV node, the shorter the PR interval, which is the time from the P-wave to the QRS complex, will be. This is because the wave will have a shorter distance to travel. If the heart is beating fast enough, the P-wave from the premature atrial contraction might happen at the same time as the T-wave from the previous, normal contraction. In this case, the two waves will combine, making that T-wave look peaked, which means that it’s just a bit more lumpy than usual, kind of like a camel’s hump.

The timing of the PAC will also determine how the heart contracts and therefore what the ECG will look like. When the depolarization wave from an atrial ectopic focus gets to the AV node, there are a few things that could happen. If the AV node is receptive, the signal will be conducted down to the ventricles, resulting in a normal QRS complex. However, if a depolarization wave from an atrial ectopic focus gets through the AV node but is sent through the ventricles abnormally, like when a bundle branch is in refractory, this will lead to an aberrant signal. Most often, the aberrancy is due to a right bundle branch block because that’s the bundle branch that has a longer refractory period. In this situation, the depolarization wave conducts down the left bundle branch and then through the ventricles aberrantly, resulting in an abnormally wide QRS complex.