Left anterior fascicular block

ECG: Sinus activity; normal PR, narrow QRS; left anterior fascicular block: left axis, rS pattern in the inferior leads, qR pattern in leads I, aVL, earlier R wave in aVR than in aVL, q wave in V1-V2;

Comments: This patient presents a typical electrocardiographic pattern of left anterior fascicular block. The left ventricular electrical activation wave is normally split into an anterior fascicle and a posterior fascicle, even though it is sometimes difficult to differentiate the two anatomical branches and there may be an additional third short, septal branch. Whereas left posterior fascicular block is very rare, left anterior fascicular block is the most frequently observed conduction disorder. Indeed, the anterior bundle, responsible for the activation of the upper portions of the septum and the anterolateral aspects of the left ventricle, is very narrow and vulnerable (by compression or stretching at the level of the outflow tract). In contrast, the posterior fascicular block is the recipient of dual blood supply and is exposed to lower pressures and less turbulence. A left anterior fascicular bloc may be idiopathic in a patient with a healthy heart, or observed in the setting of heart disease (ischemic, congenital, valvular, hypertensive).

Since electrical conduction is blocked in the left anterior fascicle, primary left ventricular activation is provided by the Purkinje network of the inferoposterior wall (initial portion of the QRS complex). Left ventricular activation subsequently reaches the anterior aspect by muscular conduction (middle and terminal portion of the QRS complex) and hence turns counterclockwise. This type of left ventricular activation is associated with characteristic modifications of the initial electrical vector and of the main electrical vector:

• The initial vector takes an abnormal orientation toward the right, forward and downward;
• The main vector takes an abnormal orientation upward, to the left and backward;

The electrocardiogram reveals:

1. A normal or slightly prolonged QRS duration (around 100 ms but always less than 120 ms); if the duration is longer, an associated abnormality must be investigated;
2. A QRS axis deviated to the left (between -40 and -90 degrees); the abnormal orientation of the main vector to the left, upward and forward explains this left axis deviation;
3. A qR pattern in leads I and aVL; the observed small q wave is related to the right, forward and downward deviation of the initial vectors;
4. A rS pattern in inferior leads (II, III and aVF); the initial r wave is explained by the deviation of the initial vectors to the right, forward and downward, the deep S wave by the deviation of the main vector to the left, backward and upward;
5. There is occasionally an increase in QRS voltages in the frontal leads without any associated ventricular hypertrophy;
6. The peak of the R wave often appears earlier in the aVL lead (delay in intrinsicoid deflection in aVL > 45 ms) than in aVR in conjunction with the counterclockwise loop of ventricular depolarization in the frontal plane;
7. In the right-sided precordial leads, a normal rS-like pattern or qrS pattern can be observed with an abnormal q wave due to the altered direction of the septal activation vectors; the presence of these q waves and a somewhat poor R wave progression must not lead to the erroneous diagnosis of infarction sequela;
8. In the left-sided precordial leads, the pattern may be normal (qR or qRs); a relatively wide or even predominant S wave can be observed (RS or rS patterns) with R/S pattern < 1; the displacement of the transition zone to the left is related to the higher direction of the main QRS vectors resulting in a reduction in R wave amplitude in the left precordial leads;
9. The delayed recording of the intrinsicoid deflection is normal in both the right and left precordial leads;
10. The direction and pattern of the repolarization are generally normal (possibility of negative T wave in aVL);

Take-home message: A left axis deviation on a narrow QRS in the absence of left ventricular hypertrophy should evoke the presence of an inferior infarction sequela (q wave in inferior leads) or a left anterior fascicular block (rS pattern in the inferior leads).