Supraventricular arrhythmias: Pathology review

Melissa is a 21 year old college student who is having the time of her life at a party. It’s late, and unfortunately she has class the next morning, so she drinks a ton of coffee to sober up. On her way out, Melissa collapses to the floor, but wakes up after a couple of seconds. On her way to the emergency room, she tells the paramedics that she’s “aware of her heartbeat”. Then comes Taylor, a 32 year old female who is brought to the emergency room by her partner because she suddenly collapsed for a couple of minutes while cooking dinner. Taylor is now awake, and she tells you that right before collapsing she was feeling dizzy and like her heart was racing, but now she’s fine. They are both placed on different monitors. Melissa’s heart rate is 200 beats per minute and regular, and this is Melissa’s ECG. On the other hand, Taylor’s heart rate is 80 beats per minute and regular, so everything seems fine. However, her ECG shows this.

All right, so both Melissa and Taylor experienced palpitations and syncope, and their ECGs reveal they both have some form of arrhythmia. The best way to approach arrhythmias is to first: know what a normal ECG looks like, and second: have a good classification system to narrow down the diagnosis.

First, let’s review the normal electrical conduction pathway in the heart, and how it looks like on an ECG. An ECG tracing specifically shows how the depolarization wave flows through the heart during each heartbeat. The normal electrical activity of the heart starts in the sinoatrial or SA node and is then conducted through the atrium, creating the P wave on ECG. From the atrium, electrical activity goes to the atrioventricular, or AV node, after which it goes through the Bundle of His, then the right and left branches of the Bundle, and finally through the Purkinje fibers, which deliver the current to the right and left ventricles. On an ECG, this will create the QRS complex, which represents the depolarization of the ventricles; and finally the T wave, which represents the repolarization of the ventricles. To help identify an irregular rhythm you can look at the morphology of the waveform and make sure that there is a P wave before every QRS complex, and a QRS complex after every P wave.

Now let’s take a look at the heart rate. The resting heart beats at a rate between 60 to 100 times per minute, and each of those beats starts off with depolarization of the sinoatrial node, and so we call it a normal sinus rhythm. For your exams, you should be able to figure out the heart rate on an ECG. To do that, you can count the number of boxes between R waves. Each small box represents 0.04 seconds, and each big box is five small boxes, so each big box is 0.2 seconds. One quick way to estimate the heart rate on an ECG, is to remember that the heart rate is 300, 150, 100, 75, 60, 50 depending on whether there’s 1, 2, 3, 4, 5, or 6 boxes between R waves. It's also important to know that there is normally a delay in conduction at the AV node and the Bundle of His, which gives some time for ventricular filling before the ventricle contracts. On the ECG, this is represented by the PR interval, which should be less than 5 small boxes, or 200 milliseconds.