The ST segments and T waves in Afib usually follow their normal physiologic pattern. On the ECG, however, the fusion with the underlying f waves will be more pronounced due to the lower voltages associated with them, thereby altering their appearance. In fact, there are many strips of coarse Afib where the ST segments and/or the T waves are completely obscured.
Additional Information
Ashman’s Phenomenon
While we’re on the topic of regularity, irregularity, and the morphology of the QRS complex, we should review a particular set of circumstances that alters the appearance of an occasional QRS complex in a patient with Afib. It is a situation known as Ashman’s phenomenon.
As we saw in Chapter 6, Electrocardiography and Arrhythmia Recognition, when comparing the two bundle branches, the right bundle branch has a longer period of time during which it remains refractory to a subsequent impulse. Therefore, the right bundle branch is the one that will probably “be caught with its pants down” when a premature impulse comes along. That is why most aberrantly conducted complexes have a right bundle branch block pattern.
What we did not mention before, however, is that this refractory period is even longer when the complexes are farther apart on the ECG; that is, they occur at a slower rate. To understand what we mean by that statement, let’s go back to the “caught with its pants down” comment. When the rate of conduction is slow, the right bundle branch lets its guard down, climbs up on the hammock, and begins to take a snooze. It does this because it knows that it has a while to go before the next impulse arrives and it is forced to go back to work. Suddenly, a premature impulse comes along ahead of schedule. The right bundle branch has to wake up and climb off the hammock before it can even begin to put its pants back on and conduct the impulse. That is just too long for a complex to wait. Instead, the electrical impulse moves around the right bundle branch and the aberrancy in conduction develops.
Ashman’s phenomenon refers to the process by which a premature complex is more likely to be transmitted aberrantly immediately after a long pause with a long R-R interval (Figure 20-6). To put it another way, if you have a long interval followed immediately by a short interval (a premature complex), the premature complex will more likely be conducted aberrantly.
Remember that a single lead strip oftentimes can be confusing in deciding between the irregularly irregular rhythms, especially between coarse atrial fibrillation and multifocal atrial tachycardia. This is because, many times, coarse rapid Afib appears like low-voltage atrial P waves. The best way to distinguish between the two is to obtain multiple leads or a 12-lead ECG where the morphological differences will be more obvious.
Additional Information
Digitalis Effect
There is one specific kind of ST-segment change that we should review at this point because of the frequency with which it occurs in Afib—that is the changes caused by digitalis. Digitalis, and its family of medications, is frequently used for rate control in Afib. Toxicity is associated with various forms of arrhythmias, most notably focal atrial tachycardia with block (although this is not the most commonly seen arrhythmia in digoxin toxicity) and a variety of clinical signs and symptoms. The ST-T wave changes that we will be discussing here appear even in the absence of toxicity.
Digitalis commonly causes a scooping or ladle-like appearance to the ST segments. Traditionally, the term “scooped-out” appearance is used because the ST segments look like the indentation made when you scoop out some ice cream (Figure 20-7). It also looks like a ladle (Figure 20-8). Whatever your preference, the key thing is that when you see a scooped-out segment on someone in Afib, you should think of digitalis effect.
Figure 20-7 Scooped-out appearance of the ST segments caused by digitalis effect.
Atrial fibrillation is very dependent on left atrial size. When the left atrial mass exceeds a certain limit, the arrhythmia is more common. Likewise, maintenance of the rhythm and chronicity is based on left atrial size. Age is the next determinant. This list is not inclusive of all possible causes.
