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.