Overview
High AV block is an AV block with an AV conduction ratio of more than 2:1, manifesting as 3:1, 4:1, 5:1, etc. It is often a precursor to third-degree AV block and has similar severity and clinical significance to third-degree AV block. High AV block is often a precursor to third-degree AV block and is similar in severity and clinical significance to third-degree AV block.
Etiology
Many factors can affect the atrioventricular conduction system, the most common being organic cardiac lesions, such as ischemic heart disease, myocardial inflammatory lesions, and injury to the cardiac conduction system, and a few are seen in vagal hyperfunction, electrolyte disorders, and drug effects.
Symptoms
Most patients may be asymptomatic at rest, or have palpitations. During physical activity, there may be palpitations, dizziness, fatigue, chest tightness and shortness of breath. If the ventricular rate is too slow, especially if the heart has significant ischemia or other pathologies at the same time, or if it is complicated by acute extensive anterior wall myocardial infarction or acute severe myocarditis, the symptoms are severe, and heart failure or shock may occur, or unresponsiveness or confusion occurs due to the insufficient supply of blood to the brain, which can then develop into syncope (incidence of which can reach 60%), and A. S. syndrome. Widening of the pulse pressure difference and mild to moderate cardiac enlargement may occur as a result of the increase in diastolic ventricular filling volume and stroke volume.
Examination
1. Characteristics of atrioventricular conduction ratio
(1) Various atrioventricular (AV) conduction ratios are possible, generally >2:1, with even ratios (e.g., 4:1, 6:1, 8:1) more common than odd ratios (e.g., 3:1, 5:1).
(2) In the presence of arrhythmia, the atrioventricular ratio for the diagnosis of high atrioventricular block should be as follows: (1) in sinus rhythm, the atrioventricular ratio should be >2:1; (2) in atrial tachycardia, the atrioventricular ratio should be more than 4:1; (3) in atrial flutter, the atrioventricular ratio should be more than 5:1.
(3) The atrioventricular (AV) ratio may be fixed or variable. Fixed ratios of 6:1 or more are rare.
(4) The ratio of atrioventricular conduction is variable In 2:1 atrioventricular conduction or 3:2 ventricular block, such as the emergence of occult conduction, it can be in the form of 3:1 high degree of atrioventricular block. It is indistinguishable on the surface ECG from true 3:1 high-grade AV block due to blocking conduction disruption.
2. The P-R interval of downward conduction
The P-R interval can be normal or prolonged, but it is mostly fixed or unfixed, because the P wave occurs at different stages of the relative phase of inactivity (the R-P interval is of different lengths), and the degree of conduction delay varies, which may cause the P-R interval to be unfixed; it may also be unfixed in the presence of insidious conduction or hypernormal conduction. In addition, there may be transverse P-wave conduction, and even gradual prolongation of the P-R intervals of several neighboring downstream transmissions, similar to the phenomenon of Vinzel’s phenomenon.
3. Without or with fugitive beats, fugitive rhythms
(1) Without escape beats, the number of P waves is a multiple of the number of QRS waves, usually 3 or 4 times.
（Fugitive beats are usually atrioventricular. Ventricular escape beats are rare. In the case of a continuous escape rhythm, the P wave is not related to the escape beat, forming an incomplete atrioventricular deregulation, and ventricular capture or ventricular fusion waves may occur.
4. R-R interval
It is almost always irregular because junctional or ventricular escape beats often occur in addition to individual downbeat conduction. The R-R interval can be unexpectedly irregular when insidious and/or unexpected conduction (gap phenomenon, Weginsky’s phenomenon, and overt conduction) is involved. The R-R interval is regular only if the AV conduction ratio is constant and no escape beats occur. If different AV conduction ratios alternate (e.g., 2:1 versus 4:1), paired beats or pseudoduplex rhythms occur. In addition, ventricular pre-systole also causes ventricular arrhythmias.
5. Types of high AV block
It can be divided into two types according to the site of block: (1) Type I. Most of the block occurs at the level of the AV node, and a few block in the proximal part of the Hirschsprung’s bundle. Type II block occurs in the distal part of Hirschsprung’s bundle and in the bundle branches.
Diagnosis
1. Based on clinical history, symptoms and signs.
2. Electrocardiographic diagnostic criteria
(1) Scattered occurrence of 2 or more consecutive P waves that are not transmitted down to the ventricle because of the block.
(2) Atrioventricular block greater than 2:1 The electrocardiogram of atrioventricular block should analyze the P waves one by one and observe the time phase of the P waves. If more than half of the P waves occur before the peak of the ST segment or the T wave and are not transmitted to the ventricle, it cannot be diagnosed as a high degree of atrioventricular block, and if the ventricular rate is greater than 60 beats/minute, it is not necessarily a high degree of atrioventricular block, even though almost all of the P waves cannot be transmitted to the ventricle. because there are often confounding factors at play. Only when more than half of the P waves do not travel down to the ventricles during the response period of the cardiac cycle is high atrioventricular block diagnosed.
Treatment
1. Active treatment of the primary disease, timely control, elimination of causes and triggers is the key to the treatment and prevention of this disease.
2. Atropine and isoprenaline can be used if symptoms are obvious. Atropine usage: 0.5-1mg every 4-6 hours, intramuscular or oral; isoproterenol usage: 5-10mg sublingually every 4 hours. for those with A-s syndrome, 0.5mg/dl continuous intravenous drip can be used to maintain the ventricular rate at 60-70 beats/minute.
3. Complicated by acute myocarditis, acute myocardial infarction or cardiac surgery after injury, need to place a temporary pacemaker treatment.