P Waves in Focal AT with Block

Take a look at Figure 16-1 and Figure 16-3, looking closely at the P waves. You will see that each P wave is a very discrete entity. The P waves are surrounded by segments that are at the baseline for the strip. The only exception to this rule is when they are buried within another wave. They can be buried in the QRS complex, the ST segment, or the T waves.

Figure 16-3 The P waves in focal AT with block. Note that the P waves are discrete entities and there is a return to the baseline during the P-P intervals. There are two buried P waves in this example. One occurs immediately after the QRS complex and the other is well within the ST segment.

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Description

Let’s repeat this critically important diagnostic point. The P waves in focal AT with block are discrete entities with a return of the baseline to normal between each wave. The importance of this point will become clear when we discuss atrial flutter and how to tell them apart.

When evaluating P waves, it is also critical to keep in mind that they vary in morphology based on each lead’s vantage point for viewing the P-wave vector. The clinical implication of this statement is that you never know which is the best lead to view the P waves for any one particular patient. Each patient, and each P-wave vector, is different. In one patient, the P waves may be upright and clearly visible in lead I and isoelectric and electrocardiographically invisible in aVF. In another patient, they may be upright and visible in lead aVF and isoelectric and invisible in lead I. In general, however, the best single lead to identify and evaluate P waves is in lead V₁. Let’s take a closer look at this problem.