General ECG signs of conduction disorders
The recognition of bundle branch blocks is based on identifying several basic changes on the cardiogram. Since the impulse cannot travel via the standard pathway, ventricular activation occurs through alternative pathways, which takes more time and alters the depolarization vector.
Key general markers:
- Increased QRS complex width. Normally it is narrow, but in block it increases in duration ranging from 0.12 to 0.20 seconds.
- QRS deformation. The ventricular complex loses its usual pointed shape, becoming notched or slurred.
- Secondary repolarization changes. Discordance is observed — the opposite direction of the main QRS complex wave and the T wave (if QRS is directed upward, T will be negative, and vice versa).
Differential visual diagnosis
To determine precisely which branch is blocked, it is necessary to evaluate the morphology of the complexes in the chest leads ($V_1–V_6$).
Right bundle branch lesion (RBBB) In this case, the most pronounced changes are recorded in the right chest lead $V_1$. The QRS complex here is not only widened but also specifically notched. The classic picture is described as an rSR' type, where the second R' wave is usually taller than the first.
Left bundle branch lesion (LBBB) With this pathology, the activation vector changes differently. In the left chest leads ($V_5, V_6$), wide and deformed complexes are recorded, on which notches can often be noticed. Mirror-image in the right leads ($V_1, V_2$), a very deep S wave is formed (or the complex takes the QS form).
Complex and combined blocks
The left bundle of His has fascicles, and conduction disorders can affect both the main trunk and its individual parts. This creates prerequisites for the formation of combined pathologies.
- Bifascicular blocks. These represent a combined lesion: the right bundle branch and one of the left bundle fascicles are blocked. On the cardiogram, this manifests as a combination of typical RBBB signs and a marked deviation of the electrical axis of the heart (EAH), the direction of which depends on which specific left fascicle is affected.
- Trifascicular block. This is a severe condition in which prolongation of the P-Q interval (corresponding to first-degree AV block) is added to the bifascicular block. Pathophysiologically, this means damage to all three fascicles of the ventricular conduction system. This condition is associated with a high risk of developing complete (atrioventricular) heart block.
Clinical significance and management
The block itself is diagnosed only instrumentally, but the time of its appearance plays a decisive role in clinical practice.
If a bundle branch block occurs acutely (first detected on a fresh ECG), this is a formidable signal. As a rule, such a finding indicates an exacerbation of underlying heart disease (for example, acute ischemia or an inflammatory process). In such situations, the clinician's therapeutic strategy should be immediately directed at treating this background cardiac pathology.
Briefly: answers to topical questions
What is the normal duration of the QRS complex in seconds?
The normal duration of the QRS complex is 0.06 to 0.1 seconds.
Additional normative characteristics of the complex:
- Time parameters — width is 0.06–0.1 sec.
- Morphology — waves must be clean, without splitting or notches (the exception is minor allowable notches in lead III and aVF).
- Voltage — the average indicator is 5–20 mm. Calculated as the sum of the absolute values of the positive R wave and the largest negative wave.
If the complex amplitude does not exceed 5 mm in any of the leads, the concept of a low-voltage ECG is established.
For RBBB, look for the rSR' notch in the right lead V1. For LBBB, look for deformation in the left leads V5-V6 and a deep deflection (S or QS) in the right leads.
Class and exam questions
Is it possible to diagnose bundle branch block via auscultation or questioning?
No, this intraventricular conduction disorder is detected and confirmed exclusively via electrocardiography data.
What does the term "discordance" mean when describing blocks?
This is a secondary repolarization disorder in which the main wave of the widened QRS complex and the T wave are directed in opposite directions relative to the isoelectric line.
Which elements constitute a trifascicular block on an ECG?
It includes signs of bifascicular block (RBBB plus left bundle fascicular block) combined with prolongation of the P-Q interval (first-degree AV block).
What else to study on this topic
- Features of isolated conduction disorders in the left bundle fascicles
- Specific changes in the electrical axis of the heart in bifascicular blocks
- Mechanisms of transition of trifascicular block to complete heart block
- Differential diagnosis of underlying cardiac pathologies in acutely occurring blocks
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