Overview
Sinus atrial block, referred to as sinoatrial block, is caused by lesions in the tissues surrounding the sinus node, which prolongs or prevents the transmission of excitation from the sinus node to reach the atria, resulting in atrial-ventricular arrest. Sinus block can be temporary, persistent or recurrent. Patients with sinus block are often asymptomatic, but may have mild palpitations, a feeling of weakness, and “missed beats,” and cardiac auscultation may reveal arrhythmias, bradycardia, and “missed beats” (long intervals).
Questions you may be concerned about
Difference between sinus block and atrioventricular block
The difference between sinus block and atrioventricular block consists mainly of the difference in the site of the lesion and the difference in the electrocardiogram.
Normal electrocardiographic activity is initiated by the sinus node, via the inter-nodal bundle → interatrial bundle → atrioventricular node → Hippocampal bundle → left and right bundle branches → Purkinje fibers. The difference between sinoatrial block and atrioventricular block mainly includes the following aspects:
1. Lesion site: the main lesion site of sinus-atrial block is between the sinus node and the interjunctional bundle, and the electrocardiographic conduction to the atrioventricular node and the following parts of the delay or block; the main lesion site of atrioventricular block is in the atrioventricular node and the following parts of the atrioventricular node.
2. Electrocardiogram: Electrocardiogram reflects the electrical activity generated by myocardial contraction, but it cannot reflect the electrical activity of sinus node. Sinus atrioventricular block is usually characterized by prolongation or disappearance of P wave; atrioventricular block is mainly characterized by prolongation of the time between P wave and QRS wave, or disappearance of QRS wave after P wave, and severe atrioventricular block will have atrioventricular separation in the electrocardiogram.
When an ECG abnormality is found to exist, one should immediately seek medical attention in a hospital to assess the severity of the condition and follow the doctor’s instructions for treatment or follow-up, as severe abnormalities in cardiac electrical activity can lead to sudden death at any time, resulting in death.
Causes
1. Mostly seen in patients with organic heart disease, coronary artery disease is the most common cause, accounting for about 40%, due to myocardial ischemia leading to organic damage around the sinus node. Among them, the incidence of sinus block in acute inferior posterior wall myocardial infarction is 3.5%, which is much less than sinus bradycardia, and its pathogenesis can be secondary to increased vagal tone, but sinus node ischemia or infarction is also common. In addition, it is also seen in hypertensive heart damage, rheumatic heart disease, cardiomyopathy, congenital heart disease, chronic inflammation or ischemia caused by sinus node and its surrounding tissue lesions.
2. Hyperkalemia, hypercapnia, diphtheria, influenza, etc.
3. Degenerative sclerosis, fibrosis, steatosis or amyloidosis in the area around the sinus node.
4. Drug intoxication and high-dose propafenone can also cause, but mostly temporary, such as digitalis, quinidine, verapamil, propylamine, amiodarone, β-blockers, etc.
5. In healthy people with increased vagal tone or carotid sinus hypersensitivity, atropine test can be used to confirm.
6. A few causes are unknown, and individuals can be familial.
7. Rarely, it is caused by intravenous magnesium sulfate (caused by too fast injection), and it can also occur in hypokalemia <2.6mmol/L.
8. In a few cases, atrioventricular block may occur simultaneously, with progressive exacerbation, known as double junction syndrome.
Symptoms
Sinus atrioventricular block may be temporary, persistent or recurrent. Patients with sinus atrial block are often asymptomatic, but may have mild palpitations, a feeling of weakness, and “missed beats,” and cardiac auscultation may reveal arrhythmias, bradycardia, and “missed beats” (long intervals). If there are repeated episodes or prolonged block, continuous missed beats can occur, and there are no escape beats (the phenomenon that when the high cardiac pacing point delays or stops emitting impulses, the low pacing point emits impulses instead and agitates the heart), then dizziness, fainting, coma, and A.S. Syndrome can occur. In addition, there are clinical manifestations of the original disease.
Examination
An electrocardiogram is performed.
Diagnosis
Diagnosis is mainly based on the electrocardiogram. Sinus block can be categorized into first, second, high and third degree sinus block according to the electrocardiographic features.
First-degree sinus block is characterized by prolonged sinus conduction time, which is difficult to diagnose on the surface ECG; second-degree sinus block can be diagnosed based on the history, symptoms and ECG; third-degree sinus block is characterized by the disappearance of sinus P-wave, which is difficult to distinguish from sinus arrest.
Differential diagnosis
1. Differential diagnosis between second-degree type I sinus block and sinus arrhythmia
Since the PP interval of variant Wen’s type sinus atrial block varies in length, it is sometimes difficult to distinguish it from sinus arrhythmia. It can be differentiated according to the following points.
(1) It must be the sinus excitation cycle calculated by the Wen’s cycle, and the results of the trapezoidal diagram drawn by this cycle on the PP intervals similar to the Wen’s cycle in each lead of the electrocardiogram are roughly in line with the diagnosis, in order to diagnose this type of sinus atrioventricular block.
(2) Venn’s cycle The cycle repeats itself.
(3) The PP interval in sinus arrhythmia is associated with respiration and is characterized by a gradual shortening and lengthening of the PP interval. In this type of block, the PP interval changes in a certain pattern, gradually shortening, and finally a long interval of nearly twice the short PP interval occurs.
2. Identification of second-degree type II sinus block and 3:2 second-degree type I sinus block
Both may alternate between short PP intervals and long PP intervals, but the long PP interval in second-degree type I 3:2 sinus block is less than twice as long as the short PP interval; whereas the long PP interval in second-degree type II 3:2 sinus block is an integral multiple of the short PP interval.
3. Identification of second-degree type II sinus block and sinus pre-systolic dysthymia
The long PP interval in sinus pre-systolic dystonic rhythm is not twice the short PP interval. In contrast, the long PP interval in 3:2 sinus block type II is exactly twice as long as the sinus PP interval.
4. Identification of second-degree type III sinus block and sinus arrhythmia
The difference is that the PP interval of second-degree type III sinus block is suddenly shortened and lengthened, independent of the respiratory cycle. In sinus arrhythmia, the PP interval is gradually shortened and lengthened, and it is related to the respiratory cycle, short during inspiration and long during expiration.
5. Identification of high sinus block and sinus arrest
Sinus arrest generally has no obvious pattern, and there is no integer multiple relationship between long and short PP intervals, and it is rare to see sinus arrests with equal intervals in one ECG. In high sinus block, the long PP interval is always an integral multiple of the short PP interval, regardless of the degree of block. And, long PP intervals of equal length may recur. In sinus arrest, on the other hand, often the low rhythm point is also suppressed, and escape beats are not usually easy to occur. However, in high sinus atrioventricular block, when the heart stops for too long, atrioventricular junctional escape beats and escape rhythms or ventricular escape beats and ventricular escape rhythms often occur.
6. Identification of third-degree sinus atrioventricular block and prolonged sinus pause
Third-degree sinus atrioventricular block sometimes has an atrial escape rhythm or escape beats; sinus arrest is mostly without atrial escape beats or escape rhythms, due to pathological factors that inhibit the autonomic properties of the sinus node, and inhibit the atrial ectopic pacing points at the same time. However, those with atrial escape rhythms do not necessarily have sinus block. The identification of atrial block is difficult because it does not necessarily result in an atrial escape rhythm. In the dynamic electrocardiogram or electrocardiographic monitoring, if there has been a short or long sinus pause before a long period of time without a P wave, then it can be diagnosed as sinus pause; if there has been a first or second degree of sinus block, then it can be diagnosed as a third degree of sinus block.
7. Differences between third-degree sinus block and sinus-ventricular conduction
There are the following points:
(1) Sinus block may have an atrial escape rhythm, while the latter does not.
(2) Sinus atrioventricular block is characterized by atrioventricular junctional rhythm as the basic rhythm, so the QRS wave group is mostly supraventricular, while the latter is more broad and aberrant.
(3) The latter is often accompanied by hyperacute T waves due to hyperkalemia, while the former is not.
(4) If there is an increase in potassium, or if there is a clinically detectable disease causing hyperkalemia, diffuse complete intra-atrial block causing sinusoventricular conduction often develops, with less effect on the sinus node.
Complications
Complications such as syncope, hypotension, and A. S. syndrome may occur if sinus-atrial block is recurrent or of long duration.
Treatment
1. When treating sinus atrial block, the main treatment is to treat the primary disease.
2. For those who are temporarily asymptomatic, close observation can be carried out, no special treatment is needed, and the patients can mostly return to normal.
3. For those with frequent, recurrent, persistent attacks or obvious symptoms, atropine can be given orally or intravenously or subcutaneously. In addition, ephedrine or isoprenaline (wheezing) can be given orally.
4. In severe cases, isoprenaline can be added to 5% dextrose for slow intravenous infusion.
5. Implantation of an artificial pacemaker should be prompt in cases of syncope, A.S. syndrome and ineffective drug treatment.
Prognosis
If sinus block is occasional, it is mostly due to functional vagal tone increase, while frequent or long-lasting episodes are mostly due to organic causes. If the ventricular rate is greater than 50 beats/minute, short duration, no syncope, no A.S. syndrome occurs, the general prognosis is good. If frequent or prolonged sinus block occurs in the elderly or in patients with advanced cardiac disease, and if there is no fugitive rhythm, then A. S. syndrome may occur, and its prognosis is poor.
Prevention
1. Active treatment of primary diseases, timely control and elimination of triggers is the key to prevent the occurrence of this disease.
2. Reasonable use of digitalis preparations, quinidine and other anti-arrhythmic drugs.
3. Live and work regularly, have a proper diet, keep a good mood, and have proper physical exercise.