The presence or absence of any P waves is another major point in your decision tree. The presence of P waves tells you that the rhythm in question has some atrial or supraventricular component.
You’d think that being able to identify a P wave would be an easy thing to pick up. But you’d be wrong. P waves can hide in other waves, mimic or disguise themselves as other waves, or be completely invisible in certain leads. Finding them takes attention to detail and a good eye for minor variations between the various complexes.
4. Are all the P waves the same?
The presence of P waves that are identical means that they are being generated by the same pacemaker site. Identical P waves should have identical PR intervals unless an AV nodal block is present (more later). If the P waves are not identical, consider two possibilities: (1) There is an additional pacemaker cell firing, or (2) there is some other component of the complex superimposed on the P wave, such as a T wave occurring at the same moment as the P wave. The presence of three or more different P-wave morphologies with different PR intervals defines either wandering atrial pacemaker or multifocal atrial tachycardia, both described later in this chapter.
5. Does each QRS complex have a P wave?
What we are getting at in this question is the relationship of the P waves and the QRS complexes. Does each P wave trigger a QRS complex? Are any P waves missing? Are the ratios of P waves to QRS complexes 1:1 or greater? Conduction that occurs at a ratio of 1:1 is normal. When you have more P waves than QRS complexes, you need to think of the AV blocks. Conduction ratios in those cases can be 2:1, 3:1, 3:2, or any combination that you can dream of. Variable blocks can also occur at various conduction ratios on the same strip.
6. Is the PR interval constant?
There are many variables that can affect the PR interval because there are so many things occurring simultaneously or overlapping on each other during that short time. We discussed some of them in Chapter 4, Vectors and the Basic Beat, and will discuss more in Chapter 8, Normal Sinus Rhythm.
In addition to the influences that can affect the individual components of the PR interval, the PR interval can have wide variations based on the distance from the initiating pacemaker to the ventricular myocytes themselves. The greater the distance of travel, the longer the PR interval. This situation typically occurs when ectopic pacemakers initiate the QRS complexes rather than the sinus node itself. Examples of these include premature atrial contractions (PACs), premature junctional contractions (PJCs), and premature ventricular contractions (PVCs) to name a few.
The route that the depolarization wave takes to arrive at its final destination adds distance and, therefore, time to the length of the PR interval. Patients with structural heart disease can have obstructions that make the depolarization take the long way around a defect.
Finally, AV blocks can cause lengthening of the PR interval as the impulses travel through a diseased AV node. Any external influence that affects the AV node’s function or transmission rates also alters the PR interval. Some of these influences (including pharmaceutical agents, hormones, recreational drugs, decreased oxygenation, coronary artery disease, and various systemic diseases, to name a few) can also affect conduction rates through the AV node.