This ECG is a good example of sinus rhythm with aberrantly-conducted PACs. The tracing was donated to the ECG Guru several years ago by Dr. Ahmed from Sanjiban Hospital in India. We have no patient data for this tracing.
The underlying rhythm here is normal sinus rhythm. Most of the parameters – rate, PR interval, and QRS duration – are normal. The QTc interval, which is the QT interval corrected to a rate of 60 bpm, is prolonged at 568 ms. We do not know the patient’s clinical condition or medications, so we cannot guess at the reason. However, a prolonged QTc is associated with an increased risk of Torsades de pointes, a type of polymorphic ventricular tachycardia.
The first three beats appear the same (Leads I, II, and III). However, the first R-to-R interval is shorter than the second one. This could be due to rate variation, a concealed sinus block, or a premature atrial contraction (PAC). The P wave of the “early” beat, marked #1, looks slightly different from the other P waves in Lead II, but, because of the slow rate, there is no way to be sure without a longer rhythm strip. After the possible PAC, the rhythm becomes coupled, probably atrial bigeminy, where every other beat is a PAC. There are several mechanisms that cause grouped beating, but atrial ectopic bigeminy is the most common. Normally, PACs have different-looking P waves compared to the sinus beats. In this ECG, the P waves are often buried in the preceding T waves, and are hard to evaluate.
Interestingly, the PACs in this strip are conducted with varying amounts of aberrancy. Because they are premature, they find the ventricles in varying stages of repolarization. When the ventricles are not completely repolarized, the electrical impulse may conduct down the more recovered conduction pathways, while going around the less recovered pathways. A common form of aberrant conduction is right bundle branch block. In RBBB aberrant conduction, the impulse finds the right bundle branch temporarily unable to conduct, so it follows the left bundle branch down into the ventricles. The left ventricle is depolarlized slightly ahead of the right, and the common ECG pattern of a wide QRS with rSR’ pattern in V1 and Rs in Leads I and V6 will appear.
The PAC marked “2” probably has a small degree of aberrancy. The PACs marked “3” and “4” appear almost like the normal beats. PACs “5” and “6” have clearly taken on the form of right bundle branch block, including the T wave changes commonly seen with RBBB.
The clinical significance of the grouped beating, and of the prolonged QT interval, would have to be evaluated before treatment is decided. But, this ECG provides students with a good opportunity to see variations on aberrant conduction.