ECG intervals

An electrocardiogram is also known as an ECG; the Dutch and German version of the word, elektrokardiogram, is shortened to EKG. It is a tool used to visualize, or “gram,” the electricity, or “electro,” that flows through the heart, or “cardio.” Specifically, an ECG tracing shows how the depolarization wave, which is a wave of positive charge, moves during each heartbeat by providing the perspectives of different sets of electrodes. This particular set of electrodes is called lead II; one electrode is placed on the right arm and the other on the left leg. Essentially, when the wave’s moving toward the left leg electrode, you get a positive deflection. This big, positive deflection corresponds to the wave moving down the septum. When reading an ECG, there are a few key elements to keep in mind; one of them is looking at the intervals.

In a typical waveform, there’s a p-wave, QRS complex, and t-wave. In addition, there are certain intervals, including the PR interval, the QRS complex itself, and the QT interval.

The PR interval spans from the beginning of the p-wave to the beginning of the QRS complex, and it represents the time from the beginning of atrial depolarization to the beginning of ventricular depolarization. It’s normally 0.12-0.20 seconds, which is three to five little boxes, since each little box is 0.04 seconds. Therefore, the PR interval shown is about four boxes or 0.16 seconds.

Any deviation in the normal depolarization pathway from the SA node to the ventricles can change the PR interval. For example, consider if the atria are depolarized by an ectopic atrial focus, such as an irritable atrial cell outside of the SA node. If it was farther away from the AV node, it’d result in a longer PR interval, because the signal has to travel a greater distance. Alternatively, if it was really close to the AV node, the PR interval might be super short. Another example is first degree heart block, which is when the electrical signal travels more slowly through the AV node than it normally does, causing the PR interval to lengthen beyond 0.2 seconds.

The QRS complex represents depolarization of the ventricles; it’s normally less than 100 milliseconds or two and a half little boxes. Just like the PR interval, the QRS duration can differ in its path from the av node to the ventricles. For example, if an ectopic ventricular focus, such as an irritated ventricular cell, fires off, the resulting depolarization wave will move slowly from muscle cell to muscle cell, instead of traveling quickly through the electrical conduction system. Therefore, it takes a longer time to depolarize the ventricles, and the QRS is wider. It’s considered intermediate if it’s 100 to 120 milliseconds, and prolonged if it’s over 120 milliseconds, or three little boxes.