Bundle branch block (BBB)

This is an intra-ventricular conduction delay, and therefore should not influence the heart rate (i.e., does not result in bradycardia for example). The atrial depolarization process is normal and so is the conduction across the atrioventricular node (AV-Node). This is why both the P-wave morphology (amplitude and duration) and PR-interval (also known as PQ-interval) should both be normal and constant. Only once the ventricular depolarization process begins, conduction is delayed, and therefore the QRS complex (at least in Lead-II) simply lasts longer than normal (> 60 milliseconds in dogs and > 40 milliseconds in cats).

While the QRS complex in a BBB is reminiscent of that in a VPC, it is not premature, and does not originate in the ventricular myocardium. Rather, it is a supraventricular complex by origin. Typically a sinus beat with a P-wave preceding it, of which PR-interval should be both constant and normal in duration unless there is a concomitant first-degree AV-block. Conduction only starts to be delayed once the propagating depolarization front penetrates the ventricular myocardium below.

When concomitant atrial fibrillation (AF) is present, the absence of discernable P-waves along with the presence of wide-looking QRS complexes can sometimes be diagnostically challenging, and an irregularly irregular ventricular tachycardia is hard to rule out.

When the BBB is within the left ventricular myocardium, the QRS dominant wave is positive (an R-wave) in Lead I and often in Lead aVF. In contrast, when it is within the right ventricular myocardium, the dominant QRS wave is typically negative and late (an S-wave) in these very same leads. Typically, because the depolarization process is abnormal, the following repolarization process is abnormal as well, resulting in a wide and tall/deep T-wave starting just as soon as the QRS complex ends, with a slurred or sometimes absent ST-segment between the two. In addition, the T-wave is often “discordant” rather than “concordant” to the QRS complex, namely opposite in polarity to the dominant wave within the QRS complex. In other words, with a left-BBB (L-BBB), the dominant QRS deflection is an R-wave (positive by definition) in Lead I, and a deep, negative T-wave is commonly present, as in the following cases:

HR = 186 bpm, QRS = 80 ms, R = 2.125 mV, QT = 205 ms, 90>MEA>0 (normal)  | view the full report

HR = 140-146 bpm, P = 60-65 ms, 0.3-0.35 mV, PR = 90-95 ms, QRS = 85 ms, R = 2.45 mV, QT = 200-205 ms | view the full report

HR = 72 bpm, P = 65ms, 0.22-0.25 mv, PR = 140 ms, QRS = 80 ms, R = 1.7 mV, QT = 230 ms | view the full report

When a right-BBB (an R-BBB) is present, the dominant QRS wave is a late, deep S-wave (negative by definition) in Lead I with a wide and positive T-wave.

Although, due to the intraventricular conduction delay, the QRS-complex is wider than normal, its extra duration often comes at the expense of the ST-segment but does not interfere with the offset of the T-wave. In other words, there is typically no change in the QT-interval duration.