Sinoatrial blockade

Sinoatrial blockade or sinoatrial node blockade, the sinus atrial node of the heart where the initial action impulse is formed, is a disruption in the generation of this impulse or its passage to the atrial myocardium (intra-atrial conduction), causing heart rhythm failure.

Epidemiology

Pauses in the work of the sinoatrial node are quite common in healthy adults - usually during sleep and during periods of increased vagus nerve tone (during physical exertion, hypothermia, etc.).

According to foreign cardiologists, problems with the conduction system of the heart are detected in 12-17% of patients over 65 years of age.

Sinus atrial node dysfunction occurs in half of the cases as a side effect of the drug, as well as due to electrolyte imbalance or acute myocardial infarction. In cases of sinus node weakness syndrome, three to four patients out of ten develop sinoatrial block.

Causes of the sinoatrial blockade

In the conducting system of the heart, which ensures its automatic operation, the main driver of the heart rhythm or pacesmaker (from English pace - pace and make - make, make) is the sinus atrial, sinus or sinoatrial node (by duѕ sinuatriаlіѕ). It is a small area of specialized (pacing) cells located in the wall of the right atrium (atrium dextrum), which continuously generate initial (sinus) electrical impulses (action potential).

Sinus atrial node blockade is one of the serious rhythm and conduction disorders of the heart. The most common causes of its blockade are due to:

• symptomatic dysfunction by dus ѕinuatriаlіѕ - sinus node weakness syndrome (inability to produce physiologically adequate heart rate);
• coronary heart disease;
• right ventricular myocardial infarction - followed by postinfarction cardiosclerosis and fibrotic lesion of the pacing cell zone;
• atherosclerotic lesion or thrombosis of the artery (arteria nodorum sinoatrial) that supplies oxygen to the tissues of the sinus node;
• increased tone of the vagus nerve (efferent branches of which innervate the sinus node);
• Hyperkalemia of various etiologies - increased levels of potassium in the body, which leads to a violation of electrolyte balance;
• long-term use of cardiac glycosides (foxglove preparations containing digoxin glycoside), drugs of beta-adrenoblocker group (Bisoprolol, Bisoprol, etc.), calcium channel blockers, acetylcholinesterase inhibitors (psychotropic and neuroleptic drugs), tricyclic antidepressants.

As cardiologic practice shows, in most cases sinoatrial blockades in children are a consequence of congenital heart disease (fibrosis of the interventricular septum or aortic valve, holosystolic mitral valve prolapse), infectious diseases and epilepsy, and in adolescents - hypotonic type of vegeto-vascular dystonia.

For more information, see:

• Causes of sinus node weakness syndrome
• Features of vegeto-vascular dystonia in children

Incidentally, sinoatrial and sinoauricular blockade may be considered synonymous, but the term "sinoauricular" is recognized as obsolete and anatomically incorrect, since auriculae cordis means the auricle of the atrium (a muscular bulge or protrusion on its wall).

Risk factors

Sinus node dysfunction may be genetic or secondary to cardiovascular or systemic disease, and risk factors for the development of sinoatrial blockade include:

• Older age (with often detected idiopathic degeneration of this node and a decrease in the number of its cells);
• congestive heart failure;
• coronary atherosclerosis;
• myocarditis and rheumatic heart disease;
• sarcoidosis of the heart;
• kidney failure with oliguria (decreased urine output);
• Hyperinsulinemia and insulin resistance - type 2 diabetes;
• Adrenal damage with the development of hypoaldosteronism;
• parathyroid gland pathology - hyperparathyroidism;
• myxedema;
• autonomic nervous system disorders.

Pathogenesis

The impulse generated by the sinus atrial node (SA node) travels throughout the heart, establishing a normal heart rhythm. Its pacing cells initiate each heartbeat with spontaneous membrane depolarization driven by ion channels - pathways that conduct ions across the cell membrane of the muscle cell (sarcolemma). The electrical impulse is transmitted by the transitional cells to the right atrium and then through the rest of the cardiac conduction system. This ultimately leads to myocardial contraction.

Various mechanisms of sinoatrial blockade have been identified on the basis of CA-node electrograms: unidirectional blockade of impulse output from the node, bidirectional blockade of input and output, and impulse formation disorder (with absence of registered ECG of the node).

The pathogenesis of sinoatrial blockade as a manifestation of sinus node dysfunction is due to the fact that there is no membrane depolarization and the electrical impulse is delayed or blocked on its way to the atria, resulting in delayed atrial contraction. On the ECG, this is manifested by loss of P teeth (loss of atrial activation) and hence loss of QRS complexes (ventricular depolarization).

Repolarization in cardiomyocytes of the sinoatrial node and duration of the action potential is regulated by the current of potassium ions (K+) through cell membranes, the work of the pacemaker depends on changes in the concentration of potassium ions in the blood serum. And its increased level in hyperkalemia can cause changes in the frequency of excitation of this node and even stop it.

As for digoxin, this glycoside inhibits the membrane enzyme Na+/K+-ATPase (sodium-potassium adenosine triphosphatase), resulting in cellular depolarization and changes in ionic conductance.

Symptoms of the sinoatrial blockade

In sinoatrial blockade, the first signs may manifest themselves in the form of dizziness, the appearance of cold sweat, general weakness and rapid fatigue with a decrease in mental and physical performance.

And all of these symptoms are characteristic of sinus bradycardia - a decrease in heart rate of less than 60 beats/min.

Some people may experience fainting and altered mental status (due to decreased cerebral perfusion), shortness of breath, chest discomfort and chest pain with marked sinus arrhythmia.

In cardiology, three degrees of sinoatrial node blockade are distinguished.

Grade 1 sinoatrial block consists of a delay between the generation of an impulse and its transmission to the atrium. This rhythm is not recognized on surface ECG, and this condition is asymptomatic (with a slight decrease in HR).

There are two types of sinoatrial block of the 2nd degree. Type I - Wenckebach's blockade with gradual prolongation of the time of conduction of the electrical impulse from the CA-node to the atria, as a result of which the rhythm of heart contractions becomes irregular and slows down. In type II there is a loss of contraction of all heart departments without periodic slowing of CA-node impulse advancement; on ECG it is fixed by loss of P teeth during sinus rhythm.

Sinoatrial and atrioventricular block (AV blockade) with its types, Mobitz 1 and Mobitz 2, may occur simultaneously.

When none of the sinus impulses are conducted to the right atrium, grade 3 sinoatrial block or complete sinoatrial block is defined as the absence of atrial or ventricular activity due to failure to generate impulses and sinus node arrest, which most often results from severe cellular hypoxia associated with ischemia. In complete block, atrial asystole, and there may be pacemaker arrest.

It is not uncommon for sinus node block to be intermittent, and this is transient or transient sinoatrial blockade, in which normal sinus rhythm may persist for days or weeks between episodes. Sinus pause or arrest is defined as a temporary absence of sinus P waveforms on the ECG lasting from a few seconds to a few minutes.

Also read:

• Heart rhythm and conduction disorder: symptoms and diagnosis
• Symptoms and diagnosis of sinus node weakness syndrome

Complications and consequences

Major complications and consequences of sinus atrial node block include additional rhythm disturbances, including AV block, supraventricular or supraventricular tachycardia, bradysystolic atrial flutter (atrial fibrillation).

Severe 2 degree II blockages can develop a dangerous complication associated with dramatically impaired hemodynamics - Morgagni-Adams-Stokes syndrome.

Bradycardia - low heart rate, especially below 40 bpm - can lead to cardiac arrest.

Diagnostics of the sinoatrial blockade

When diagnosing any rhythm and conduction disorders of the heart, pulse measurement and auscultation of the heart are performed.

Laboratory tests include: general and biochemical blood tests, blood potassium levels, hemoglobin, creatinine, cholesterol and LDL; clinical urinalysis.

For a complete heart study you need instrumental diagnostics: electrocardiography (ECG in 12 leads), echocardiography (cardiac ultrasound), chest X-ray, Holter cardiovascular monitoring (ECG recording of heart rhythm during 24-48 hours).

A differential diagnosis is mandatory, in particular, with atrioventricular block, carotid sinus syndrome (with sinus bradycardia), hyperventilation syndrome, etc.

Treatment of the sinoatrial blockade

Standard treatment for patients with sinoatrial node block begins by treating the disease that caused it and medically managing the symptoms of heart rhythm disturbance, using drugs to prevent and correct heart failure, as well as arrhythmia medications.

Read more in the publication - Treatment of sinus node weakness syndrome

Emergency treatment consists of intravenous atropine sulfate (which increases HR) or external (percutaneous) cardiac stimulation.

Isoprenaline hydrochloride (Isoproterenol, Izadrin) and other beta-adrenomimetics are also administered by IV drip.

Restoring normal sinus rhythm may require surgery to place a pacemaker - a medical device that generates electrical impulses.

Prevention

There are no specific measures to prevent sinoatrial blockade, and, in addition to leading a healthy lifestyle, doctors recommend timely treatment of cardiovascular and systemic diseases.

Forecast

In sinus atrial node dysfunction, the prognosis is equivocal; without treatment, the mortality rate is about 2% per year.

Sinoatrial blockage and the army. The question of unsuitability for military service is decided by specialists of the military medical commission after examination. Asymptomatic blockade of the 1st degree is not an obstacle to military service.

Literature

• Shlyakhto, E. V. Cardiology: national guide / edited by E. V. Shlyakhto. - 2nd ed., revision and addendum - Moscow: GEOTAR-Media, 2021.
• Cardiology according to Hurst. Volumes 1, 2, 3. GEOTAR-Media, 2023.