The PR Interval
PR Interval and Segment
The PR interval
The PR interval is a measurement of the time it takes from the start of the p wave to the start of the QRS complex, typically 3-5 mm in duration. This interval is representative of the physiologic delay caused by transmission of impulses through the AV node. We can think of the PR interval as the answer to the following question: “How long does it take for the ventricles to start depolarizing, from the moment that the atria start depolarizing?”
The PR segment
The PR segment is a different concept from the PR interval. While the PR interval is a measurement representing a period of time, the PR segment refers to the flat, usually isoelectric line between two points on the ECG — the end of the p wave and the start of the QRS complex. This represents the electrical activity in the heart happening after the atria finish depolarizing, but before the ventricles start depolarizing.
Normal and abnormal PR intervals
The above is a diagram of a few different PR segments and intervals.
In the first example, you see a normal PR interval and isoelectronic (flat) PR segment.
In the second example, you see a normal PR segment, but a down-sloping PR segment – this may be normal, and we will discuss this phenomenon a bit later.
In the third example, we have a short PR interval and almost non-existent PR segment. It appears that the QRS takes off right after the p wave finishes. This is an example of a pathology known as Wolff-Parkinson-White syndrome, which will be discussed in detail in another module.
In the fourth example, we see a prolonged PR interval with a long, isoelectronic PR segment. This is a pathology known as a first degree heart block, which will also be discussed in a future module.
Now, to get back to the second example. I’ve explained above that the PR segment represents the electrical activity in the heart between the end of atrial depolarization and the beginning of ventricular depolarization. But what actually happens in this interval?
One thing is that the action potentials are slowly moving through the AV node (the AV nodal delay), such that no myocardium is being activated during this period of time. This is why the PR segments appears mostly isoelectronic in most cases. However, another thing that’s happening here is that the atria are starting to repolarize, creating a separate Ta wave (defined below) that’s hidden in the PR segment, which can be responsible for the down-sloping seen in example 2.
Ta Wave
The Ta wave is a deflection on the ECG not commonly talked about, but which represents atrial repolarization.
As you can see above, atrial repolarization (represented in blue) occurs after atrial depolarization (in orange), and follows the same direction as atrial depolarization.
Thinking back to the basics of an ECG, if a repolarization wave is moving towards the bottom left, it means that positive currents in the atria are actually moving towards the top right (opposite to the direction of repolarization).
Because of this, the flow of positive currents travels in the opposite direction of lead II, meaning the Ta wave has a negative polarity in lead II. For the same reason, in any given lead, the Ta wave is typically the opposite polarity as the p wave.
The above is what the Ta wave looks like, with and without the QRS complex.
Notice that the PR segment has a downsloping appearance because of the Ta wave. Therefore, even though PR segments tend to look isoelectric in many cases, they are not truly isoelectric.
The reason that the PR segments look flat to our eyes in many cases is because, since repolarization occurs much slower than depolarization in general, the Ta wave tends to be wider and flatter than the p wave. The p wave is usually not a huge wave to start with, so it is normal for the Ta wave to be so wide and flat that it is not perceivable.
When does a Ta wave become more noticeable?
Sympathetic stimulation of the heart will speed up atrial repolarization by increasing ion permeability, possibly making the Ta wave narrower and deeper and therefore more noticeable.
Note: the deep Ta wave is a little exaggerated for the purpose of illustration.
Atrial injury can also make Ta waves more noticeable, and this will be discussed in a future module.
Summary
The normal PR interval is roughly 3-5 mm in width.
The PR segment is usually isoelectric, but depending on the prominence of the Ta wave, may take on a down-sloping appearance as a normal variant.
In lead II, the Ta should be negative, wide, and low-amplitude (nearly flat).
Ta waves can be made more prominent by sympathetic activation or atrial injury.