Heart blocks

Classification of conduction disorders

According to the international classification, atrioventricular blocks and left bundle branch lesions are grouped under the common diagnostic code I44.

Atrioventricular conduction can be impaired at several levels of severity. The following variants are distinguished:

• First-degree atrioventricular block.
• Second-degree atrioventricular block.
• Complete atrioventricular block (corresponds to third degree).
• Other and unspecified forms.

Intraventricular conduction disorders include lesions of the left bundle branch (LBBB). Since it anatomically divides into fascicles, the diagnosis specifically distinguishes:

• Block of the anterior fascicle of the left bundle branch.
• Block of the posterior fascicle of the left bundle branch.
• Unspecified left bundle branch block, as well as other unspecified bundle lesions.

The role of ECG in heart failure

Recording a 12-lead electrocardiogram is a mandatory step for all patients suspected of having chronic heart failure (CHF). This approach has a high level of recommendation consensus (A) and evidence reliability (2).

When interpreting the cardiogram, the physician evaluates in detail:

1. The baseline heart rhythm and heart rate (HR).
2. The total duration and morphology of the ventricular QRS complex.
3. The presence of conduction disorders: both atrioventricular conduction and intraventricular conduction (presence of LBBB or right bundle branch block — RBBB) are assessed.
4. Signs of heart chamber hypertrophy and the presence of myocardial scar lesions.

There is a crucial clinical rule in cardiology: if a patient has an entirely normal ECG, the presence of chronic heart failure is considered highly unlikely.

Blocks and cardiac resynchronization therapy (CRT)

The detection of specific blocks directly influences the decision to prescribe cardiac resynchronization therapy. The main goal of this advanced technique is to reduce overall morbidity and mortality, as well as significantly alleviate clinical symptoms in severe patients.

A strict set of criteria is used to select patients for CRT:

• Clinical status: presence of symptomatic CHF that persists despite optimal medical therapy (OMT).
• Heart rhythm: mandatory presence of sinus rhythm.
• Echocardiographic parameters: pronounced decrease in contractility, where the left ventricular ejection fraction (LVEF) drops to 35% or lower.
• ECG criteria: a key condition is a significant widening of the QRS complex (duration $\ge$ 150 ms), and its shape must strictly correspond to the morphology of a left bundle branch block.

Conduction disorders in pulmonary embolism

When pulmonary embolism (PE) is suspected, the cardiogram is purposefully checked for signs reflecting acute overload and altered state of the right heart chambers. One such prominent marker is the appearance of a right bundle branch block (RBBB).

In addition to right bundle branch conduction disturbances, traditional ECG criteria for PE include:

• S1Q3T3 pattern: formation of a deep S wave in lead I, appearance of a pathologic Q wave and a negative T wave in lead III.
• Changes in chest leads: recording of a QR configuration in lead V1, as well as inversion (negative values) of T waves in leads V1 through V4.
• Rhythm disorders: various supraventricular arrhythmias frequently occur, among which atrial fibrillation is considered the most typical.

Important note: in many patients, these ECG phenomena may be completely absent. A normal or non-specific cardiogram is particularly common in non-massive PE.

Class and exam questions

What else to study on this topic

• QRS complex morphology in various types of intraventricular blocks
• Method for assessing left ventricular ejection fraction (LVEF) using echocardiography
• Principles of selecting optimal medical therapy (OMT) for heart failure
• Electrophysiological mechanisms behind the formation of the S1Q3T3 pattern
• The relationship between supraventricular arrhythmias, atrial fibrillation, and right heart overload

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