TEACHING POINTS
■ It is important to recognize rate-dependent left bundle branch block and to differentiate it from ventricular tachycardia.
■ Exercise-induced LBBB is often benign.
■ Further evaluation is warranted to rule out conditions such as cardiomyopathy, valvular heart disease, left ventricular hypertrophy, significant conduction disease, and occlusive coronary artery disease.
■ Cardiac workup should include, but is not limited to, a stress test, an echocardiogram, and Holter monitoring.
CASE
A 55-year-old female was referred to our cardiology office for chest pain that occurred during exertion, especially when climbing up hills. Her history was significant for hypertension, dyslipidema, hypothyroidism, and smoking. A review of systems was unremarkable. Physical examination revealed a 2/6 systolic murmur over the left lower sternal border and mild obesity. The patient said her father may have had coronary artery disease (CAD). Vital signs were as follows: BP 142/90 mm Hg, pulse 72 beats per minute, temperature 97.2°F, and weight 227.5 lb. The patient was scheduled for an exercise nuclear stress test, which showed normal ECG findings at rest (Figure 1). At stage I of the Bruce protocol, the patient's BP was 145/92 mm Hg and the heart rate was 132 beats per minute. A second ECG was obtained (Figure 2). A third ECG was taken during the recovery period after the patient had received medication, about 7 minutes into
the test (Figure 3).
The nuclear images on the stress test did not reveal any evidence of ischemia. Left ventricular systolic function was preserved, as shown by the gated images as well as an echocardiogram. A Holter monitor test was also performed and did not reveal any significant electrical or conduction abnormality.
During the exercise treadmill nuclear stress test, the patient developed rate-related wide-complex tachycardia. This can be mistaken for ventricular tachycardia, a much more serious condition. However, she was completely asymptomatic, and there was complete resolution of this rhythm when her heart rate slowed down. The nuclear part of the stress test was negative for ischemia. ECG failed to show any of the ventricular tachycardia criteria, which include atrioventricular dissociation; QRS axis between -90 degrees and ±180 degrees; positive QRS concordance (positive QRS V1 -V6); QRS duration of 140 ms or more with right bundle branch block (RBBB) pattern; and 160 ms or more with left bundle branch block (LBBB) pattern. Based on the previous criteria, the patient was diagnosed with rate-related LBBB.
DISCUSSION
The cardiac electrical impulse, which originates in the sinus node in the upper right atrium, spreads across both atria and travels through the atrioventricular (AV) node. Leaving the AV node, the electrical impulse penetrates into the ventricles via the bundle of His. From the bundle of His, the electrical impulse enters both the right and left bundle branches. These two bundle branches send the electrical impulse to the right and left ventricles and, when functioning normally, both ventricles contract nearly simultaneously.
Left bundle branch block occurs when transmission of the cardiac electrical impulse is delayed or fails to be conducted along the rapid conduction fibers of the main left bundle branch or in both the left anterior and posterior fascicles.1 The electrical impulse conducts through the right bundle branch and then spreads to the left ventricle, altering the synchronized contraction between the two ventricles and causing the characteristic ECG pattern of widened QRS.