Sinoatrial block on ECG

Mechanism of development and core pathology

Sinoatrial (SA) block belongs to the broad group of sinus node dysfunctions (SND). The main problem in this condition is not the absence of an electrical impulse, but the inability of the impulse to exit correctly. The generated signal is blocked from conducting from the sinoatrial node directly to the atrial myocardium.

It is extremely critical to distinguish between rhythm disorders and conduction function disorders. Conditions such as sinus tachycardia, extrasystole, or atrial fibrillation are classified as arrhythmias (problems of impulse formation). SA block, however, represents precisely a signal conduction defect, although these processes are often analyzed together in diagnostics.

ECG presentation in sinoatrial block

The key diagnostic criterion for this block on an electrocardiogram is a sudden pause registered against the background of a completely regular sinus rhythm.

• Complete cycle dropout: during the resulting pause on the ECG, both the P wave and the QRST ventricular complex are completely absent.
• Mathematical sign: to confirm the diagnosis, it is necessary to measure the baseline distance between normal contractions. The duration of the pause in SA block is often strictly a multiple of the normal P-P interval (for example, it is exactly equal to two or more ordinary cardiac cycles).
• Replacement phenomena: during a prolonged pause, the heart may engage rescue mechanisms. On the ECG, this is manifested by the appearance of replacement beats—for example, an atrioventricular extrasystole.

Clinical manifestations and safety criteria

Clinically, the dropout of the cardiac cycle does not go unnoticed. Upon arterial palpation, the physician records a dropped pulse wave (resulting in a pulse deficit), and during chest auscultation, dropped heart sounds.

In addition, against the background of blocks, the patient may develop symptoms of bradycardia: severe fatigue, dizziness, and a subjective sensation of skipped beats or palpitations.

If conduction disorders are detected or myocardial injury is suspected, the patient requires monitoring. They can be discharged home only under strict compliance with the following safety conditions:

1. At least 6 hours have passed since the start of monitoring (or administration of the first dose of the drug).
2. The heart rate stably exceeds the minimum value recorded over these 6 hours.
3. Clinical symptoms of bradycardia are completely absent.
4. There is no dangerous prolongation of the QT interval on the ECG (it must be less than 500 ms).
5. There are no signs of newly diagnosed second-degree AV block or higher.

Classification of conduction disorders

Depending on the level of the conduction system lesion, SA block must be differentiated from other disorders localized lower down.

1. Atrioventricular (AV) blocks — delay at the AV junction level:

• First-degree: simple slowing of conduction (all impulses reach the ventricles).
• Second-degree (Type I / Wenckebach): gradual progressive prolongation of the P-Q interval before a dropped complex (Samojloff-Wenckebach periodicity).
• Second-degree (Type II / Mobitz II): sudden blocking of the impulse without preceding deterioration.
• Third-degree (complete): complete dissociation of atrial and ventricular rhythms.

2. Intraventricular blocks — impaired conduction along the bundle branches (below the AV node):

• Fascicular blocks: complete or incomplete right bundle branch block (RBBB) or left bundle branch block (LBBB), as well as individual left fascicles.
• Bifascicular block: a combination of RBBB with a block of one of the left fascicles (determined by a specific deviation of the electrical axis of the heart).
• Trifascicular block: a combination of bifascicular block with P-Q prolongation (first-degree AV block). This is damage to all three bundles with a high risk of progression to complete AV block.

During intracardiac EPS, the delay is also classified relative to the bundle of His: proximal (AH interval prolongation), medial/intra-Hisian (wide spike), and distal/infra-Hisian (HV interval prolongation).

Causes of blocks: myocardial injury

Conduction disorders can occur with infectious and toxic cardiac injuries. A telling example is ECG diagnostics in suspected severe botulism.

An ECG is ordered to detect concomitant myocarditis or cardiomyopathy. Typical early manifestations include sinus tachycardia, signs of myocardial hypoxia, and non-specific changes in the ST segment and T wave. In severe cases, formidable conduction disorders develop (up to complete AV block), electrical instability (up to ventricular fibrillation), and reduced contractility leading to systemic heart failure. Additionally, such patients undergo abdominal ultrasound to evaluate the digestive system.

Class and exam questions

What else to study on this topic

• Diagnostic criteria for first-, second-, and third-degree AV blocks
• Samojloff-Wenckebach periodicity (Mobitz I)
• ECG signs of bifascicular and trifascicular blocks
• Toxic myocardial injury in infections (using botulism as an example)
• Intracardiac EPS parameters: AH and HV intervals

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.