Fascicular Blocks

Generalities

There are several ways of proving that a given QRS pattern is due to a specific type of conduction abnormality.15-57'61 First is extrapolation from animal experiments.1561 Second is ECG-pathologic correlation.15,61 Third is an analysis of QRS changes produced by the inadvertent section of the conduction fascicles during open heart surgery or catheter-induced trauma.62 Fourth is a comparison of tracings obtained before, during, and after the appearance or disappearance of conduction disturbances that are either persistent or (spontaneously or iatrogenically) intermittent. Under such circumstances, the QRS changes produced by fascicular block occur side by side with the control morphologies.815616263 The various criteria proposed for diagnosis of fascicular blocks, though empirical, have been accepted for a very pragmatic reason: the need to interpret clinical ECGs. In reality, the sensitivity and specificity of these criteria require independent confirmation.6164 One can speculate that the latter may be provided by newer methods of intraoperative and body surface mapping and refinements in the technique of phase imaging or even perhaps Carto mapping, since few centers in the United States are currently performing histopathologic studies of the distal intraventricular conduction system.

Left Anterior Fascicular Block

In left anterior fascicular block (LAFB), the posteroinferior regions of the LV endocardium are activated abnormally before the anterosuperior LV area.815 After emerging from the posteroinferior division of the left bundle branch, the impulse first propagates in an inferior, rightward, and usually anterior direction for a short period of time, producing q waves in leads I and aVL and r waves in leads II, III, and aVF (see Fig. 1112). Thereafter, the general direction of the activation process (which determines the direction of the EA) occurs in a superior and leftward direction. Consequently, from the ECG viewpoint, the fascicles of the left branch behave more as if they were "superior" and "inferior" rather than "anterior" and "posterior" (Figs. 1112 and 11-13). For this reason, the most significant abnormalities produced by LAFB, in the absence of complete right bundle branch block (RBBB), occur in the standard and unipolar extremity leads rather than in the precordial leads8,15 (see Figs. 11-12 and 11-13). S waves frequently are recorded V5 and V6 because the depolarization wave first moves towards them and later, because of their relatively low position, away, in a more superior direction. The degree of left-axis deviation required for the diagnosis of complete LAFB has been a subject of debate and speculation.8 It should be remembered that LAFB is but one of the causes of left-axis (superior and leftward) deviation (Table 11-2). Criteria for the diagnosis of pure LAFB are presented in Table 11-3,864-68 and illustrative examples are shown in Figs. 11-12 and 11-13. When LAFB coexists with certain congenital types of right ventricular enlargement and extensive anterolateral MI, the EA can be shifted to the "undeterminate" (right superior) quadrant. Thus the constant feature of the axis deviation produced by LAFB is its superior orientation, not its superior and leftward orientation (abnormal left-axis deviation).61 Because of the multiple interconnections between the fascicles of the left bundle branch system, the appearance of LAFB does not increase QRS duration by more than 0.025 s.8 Therefore, a LAFB pattern with a wider QRS complex generally indicates the presence of additional conduction disturbances such as RBBB (Fig. 11-14, top), MI, or intraventricular conduction delays due to free wall fibrosis. Masquerading RBBB is said to be present when (with the classic findings in lead V1) lead I shows what seems to be a left bundle branch block (LBBB) due to the absence of q and S waves (see Fig. 11-14, bottom). This pattern has been attributed to a terminal delay perpendicular to lead I associated with diffuse intramyocardial fibrosis.15

Diffuse Artery

Figure 11-12: LAFB in a patient with primary conduction system disease. QRS duration: 0.10 s. At normal paper speeds (25 mm/s), the relationship between the peaks of the R waves (vertical lines) in simultaneously recorded leads II and III and aVL and aVR cannot be determined with the desired accuracy (see Fig. 11-13).

Figure 11-12: LAFB in a patient with primary conduction system disease. QRS duration: 0.10 s. At normal paper speeds (25 mm/s), the relationship between the peaks of the R waves (vertical lines) in simultaneously recorded leads II and III and aVL and aVR cannot be determined with the desired accuracy (see Fig. 11-13).

Frontal Artery

Figure 11-13: Derivation of electrocardiographic leads from a frontal plane QRS loop showing LAFB. Due to the counterclockwise rotation of the left superior loop, the peak of the R in aVL preceded the peak of this deflection in aVR (lower right). Furthermore, because the initial portion of the loop was inscribed on the positive half of the axis of lead III before it was inscribed on the positive half of the axis of lead II, the peak of the R in the former lead occurred before that in the latter lead. (From Castellanos A, Pina L, Zaman L, et al. Recent advances in the diagnosis of fascicular blocks. Cardiol Clin 1987; 5:469-488. Reproduced with permission from the publisher and authors.)

Lafb With Rbbb

Figure 11-14: LAFB with wide QRS complexes. Whereas panel A shows LAFB with RBBB, these conduction disturbances coexist with diffuse septal and inferoposterior fibrosis in panel B. Consequently, the expected small q wave and the wide S wave in lead I are not present. This pattern has been called "masquerading" bundle branch block because the standard leads suggest LBBB, while the chest leads are diagnostic of RBBB.

Figure 11-14: LAFB with wide QRS complexes. Whereas panel A shows LAFB with RBBB, these conduction disturbances coexist with diffuse septal and inferoposterior fibrosis in panel B. Consequently, the expected small q wave and the wide S wave in lead I are not present. This pattern has been called "masquerading" bundle branch block because the standard leads suggest LBBB, while the chest leads are diagnostic of RBBB.

Table 11-2: Causes of Abnormal (-30° to -90°) Left-Axis Deviation

Cause

Characteristic Features

1. Left anterior fascicular block

1. rS complexes in lead II with positive T waves

2. Extensive inferior wall (AC5)MI

2. Qr complexes in lead II with ST-segment

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