The normal heart rhythm originates in the sinus node at a frequency of 60 to 100 beats/min. Sinus node impulses sequentially excite the atria and ventricles via the atrioventricular conduction system. An abnormality in either the origin of the rhythm or in the conduction of the impulses is called an arrhythmia. Arrhythmias are divided into tachyarrhythmias and bradyarrhythmias. According to their pathogenesis, slow arrhythmias can be divided into: pathological sinus node syndrome, atrioventricular block and intraventricular block.
I. Clinical manifestations
The clinical manifestations of bradyarrhythmias depend on the degree of bradycardia and the rate of progression. If the heart rate is not lower than 50bpm, or the disease progresses slowly, the patient may have no obvious clinical symptoms, and the slow arrhythmia is only discovered by chance when recording ECG. With further slowing of the heart rate, such as a heart rate below 50 bpm or a long interval of more than 3 seconds, the patient may develop symptoms associated with “symptomatic bradycardia”.
Symptomatic bradycardia” refers to a series of symptoms directly caused by the slow heart rate or severe conduction block, resulting in decreased cardiac output and insufficient blood supply to vital organs and tissues, especially the brain, which can be manifested as dizziness, haziness, near syncope, fainting and other symptoms of insufficient blood supply to the brain. Long-term bradycardia can also cause systemic symptoms, such as weakness, palpitations, chest tightness, shortness of breath, sinking of both lower limbs or decreased activity endurance, etc.
Auxiliary examination
(A) Routine examination
(1) Routine blood tests, electrolytes, liver and kidney functions, markers of myocardial injury, endocrine system (such as thyroid function), and autoimmune system tests.
(B) ECG examination and dynamic ECG examination (most important).
1.Sick sinus node syndrome (SSS), referred to as sick sinus syndrome, includes a series of arrhythmias: sinus bradycardia, sinus arrest, sinus block, and slow fast syndrome.
The main electrocardiographic manifestations are.
(i) severe sinus bradycardia with a heart rate <50 beats/min.
(ii) Sinus arrest and/or sinus block.
(iii) Slow-fast syndrome, manifested by alternating paroxysmal tachycardia and bradycardia, with tachycardia as supraventricular tachycardia, atrial tachycardia, atrial flutter or atrial fibrillation.
Sinus bradycardia, when the heart rate in sinus rhythm is <60 beats per minute is called sinus bradycardia. It is common in healthy adults (especially athletes, elderly) and during sleep and the effect of some drugs such as beta-blockers, calcium channel blockers (thiodiazepines, isoptin), digitalis, etc.
Sinus arrest, sinus arrest is the inability of the sinus node to issue impulses resulting in a period of no impulse production, no depolarization of the atria and no pulsation of the ventricles. The ECG shows that P waves are not seen for a period of time that is significantly longer than the usual P-P interval, or that neither P waves nor QRS waves are present, and there is no metric relationship between the long P-P interval and the underlying sinus P-P interval.
Sinus block, sinus block is a transient block of the sinus node impulse, i.e., the impulse generated by the sinus node, partially or completely fails to reach the atria, causing atrial and ventricular arrest. Only second-degree sinus block can be demonstrated on the ECG. The ECG shows a long interval between P waves. It is a multiple of the basic P-P interval.
In the slow-fast syndrome, bradycardia alternates with tachycardia. Bradycardia is sinus bradycardia, sinus block, and sinus arrest. Tachycardia is supraventricular tachycardia, mainly atrial tachycardia, atrial flutter and atrial fibrillation.
2.Atrioventricular block (AVB)
Atrioventricular block refers to the blockage of impulses in the atrioventricular conduction process. According to the severity of the block, it is divided into: first degree AV block (atrial excitation is completely transmitted down to the ventricle, but conduction is delayed), second degree AV block (atrial excitation is partially transmitted down to the ventricle) and third degree AV block (atrial excitation is completely transmitted down to the ventricle, also called complete AV block).
First-degree AV block: Each P wave is followed by a corresponding QRS wave, but the P-R interval is prolonged. The ECG shows a P-R interval > 0.20 seconds and each P wave is followed by a QRS wave cluster.
Second-degree AV block: Some atrial excitation cannot be transmitted to the ventricles, and the ECG shows a portion of P waves without corresponding QRS waves, and the ratio of AV conduction may be 2:1; 3:2; 4:3. Second-degree AV block can be divided into two types. Type I, also known as Man’s phenomenon, or Mohs type I, has a progressive prolongation of the P-R interval until the P wave is blocked and the ventricle is dehiscent; the R-R interval is progressively shortened until the P wave is blocked; type II, also known as Mohs type II, is the same as type I in that part of the P wave is not transmitted to the ventricle and no QRS wave can be generated, but the difference is that type II block occurs without a progressive prolongation of the P -R interval is progressively prolonged.
Type II atrioventricular block. Also, there is no change in the first cardiac cycle after the block. The electrocardiogram shows a fixed P-R interval, which can be normal or prolonged. the QRS wave group has intermittent decompensation, and the degree of block can change frequently, which can be 1:1; 2:1; 3:1; 3:2; 4:3, etc. The level of type II block is below the atrioventricular node, i.e., within and below the bundle of Hirschsprung. The pacing site of fugitive rhythm is low, so the rhythm is unstable and the risk of sudden death is high. The slower the frequency of the escape rhythm and the wider and more distorted the QRS, the lower the escape site and the greater the risk. Permanent pacing therapy is required.
In 2:1 AV block, in this particular type, there is a QRS wave only every other P wave, so it becomes impossible to determine whether there is a prolongation of the P-R interval after an undescended P wave. Therefore, 2:1 block cannot be diagnosed as II degree I or II degree II block. In addition to the width of the QRS wave, it is possible to determine which type of 2:1 block is present by observing whether there is an electrocardiographic manifestation of degree II type I or II type II on the Holter record, based on Holter examination.
Third degree atrioventricular block is also called complete atrioventricular block: the atrial impulse is completely blocked from reaching the ventricles, and the ECG shows complete atrioventricular separation, with P waves and QRS wave groups unrelated to each other; the atrial rate is faster than the ventricular rate; the ventricular rhythm is maintained by the junctional zone or the ventricular autonomic pacing point. The level of block can be either in the atrioventricular or subatrial node. The width and frequency of ectopic QRS waves can help determine the site of block. If the block is located above the branch of the Hirschsprung bundle, the escape pacing point mostly originates from the atrioventricular junctional area immediately below the branch, and the QRS wave group is not widened.
3.Intraventricular block
Intraventricular block: It refers to the conduction block below the branch of the bundle of Hirschsprung, and is generally divided into left and right bundle branch block and left anterior branch and left posterior branch block. Bundle branch block, branch block and non-specific intraventricular block are usually asymptomatic and do not require direct treatment, but often have poor prognostic significance. Especially in heart failure combined with intraventricular block, the mortality rate is increased and is an indication for cardiac resynchronization therapy.
Right bundle branch block: seen in healthy individuals, but the presence of RBBB in anterior wall myocardial infarction suggests substantial damage. In nodal disease, the new appearance of RBBB may indicate progressive cardiac damage. Transient RBBB may occur after pulmonary infarction.
ECG manifestations are manifested by abnormal QRS wave groups.
(i) rsR/type in lead V1 with narrow r waves and high and wide R’ waves.
(ii) V5 and V6 leads showed qRs or Rs type with wide S waves.
③ Ⅰ leads have significantly widened S waves, avR leads have wide R waves.
④T waves are in opposite direction to the main wave of QRS wave group.
Complete and incomplete right bundle-branch conduction block is manifested in the QRS time frame, with the former having a QRS time frame ≥0.12 seconds and the latter <0.12 seconds.
Left bundle branch block.
QRS wave group abnormalities, ECG manifested as
(1) Widened R waves in leads V5 and V6 with flattened, blurred or tangential tips (M-shaped R waves) and no Q waves in front of them.
②V1 leads mostly show rS or QS type, with wide S waves.
③ Ⅰlead R wave is wide or with a tangent.
④T waves are in the opposite direction to the main wave of the QRS wave group.
QRS time limit ≥ 0.12 seconds is complete block, QRS < 0.12 seconds is incomplete block. Left bundle branch block interferes with other ECG diagnoses, such as acute anterior wall myocardial infarction.
Double bundle branch block: Double bundle branch block is an intraventricular conduction block caused by conduction disturbance in the main part of the left and right bundle branches.
(C) Imaging tests
(1) Echocardiography: It can understand the heart structure, function and the presence of superfluous organisms, etc., and should be used as a routine examination.
(2) Coronary angiography and CT angiography
Patients considering bradycardia related to ischemia should undergo coronary angiography or CT angiography to understand the condition of coronary arteries and also to help decide whether to perform interventional or bypass treatment.
(iv) Atropine test
To determine whether bradycardia or heart block is caused by increased vagal tone, atropine test is feasible. Atropine 1.5-2.0 mg (0.03 mg/kg) was injected intravenously, and ECG was recorded before, immediately, 1, 2, 3, 5, 10, 15 and 20 minutes after injection.
The presence of bradycardia due to vagal hyperfunction was indicated if the heart rate could be ≥90 beats/min after the injection. Use with caution in patients with glaucoma and prostatic hypertrophy.
(v) Esophageal pacing and invasive cardiac electrophysiology
The sinus node and atrioventricular node function can be evaluated.
Diagnosis (a) Diagnostic criteria of sick sinus syndrome
(i) Sinus bradycardia <40 beats/min, lasting >1 minute.
(ii) Second-degree type II sinus block (Figure 4-26).
(iii) sinus arrest >3.0 seconds.
④ sinus bradycardia with short-onset atrial fibrillation, atrial flutter and supraventricular tachycardia, with sinus pulsation recovery time >2 seconds at the cessation of the attack.
(v) Esophageal pacing electrophysiology with sinus node recovery time > 2,000 ms and corrected sinus node recovery time > 550 ms.
(vi) 24-hour ambulatory electrocardiogram with a mean heart rate < 55 beats/min and a total beat count < 80,000.
(vii) Sinus heart rate <90 beats/min or heart rate increase <30 beats/min on atropine test.
(viii) During exercise test, the heart rate cannot exceed the basal heart rate of 30 beats/min or the maximum heart rate of <90 beats/min.
(B) Diagnostic criteria for atrioventricular block
1, one degree of atrioventricular block: atrial waves (P waves) can all travel down to the ventricle to cause QRS waves, but conduction is delayed, manifested as prolonged P-R interval.
Electrocardiographic features.
(1) Each sinus P wave can travel down to the ventricle and generate QRS-T wave clusters.
(2) P-R interval >0.20s (adults) and P-R interval ≥0.18s (pediatric patients under 14 years old).
(2) Second degree atrioventricular block: some atrial waves (P waves) cannot travel down to the ventricles.
According to whether the PR interval is fixed or not, it is divided into second degree I atrioventricular block (P-R interval is gradually prolonged until the QRS waves fall off after the P waves, also called Wen’s phenomenon or Mohs type I); second degree II (P-R interval is fixed, QRS wave group has intermittent dehiscence, also called Mohs type II)
Second degree type I atrioventricular block (called Man’s phenomenon or Mohs type I) ECG features.
(1) Sinus P waves with regular P-P spacing.
(2) The P-R interval is gradually prolonged until the P wave cannot be transmitted downward and the QRS wave group is dislodged, after which the P-R interval is again the shortest
(3) The P-R interval before shedding is gradually shortened.
(4) The QRS waveform pattern is normal.
Second degree type II atrioventricular block (Mohs type II) electrocardiographic features.
(1) The P-R interval is fixed and may be normal or prolonged.
(2) The QRS wave group has intermittent dehiscence, and the degree of block may vary frequently, and may be 1:1; 2:1; 3:1; 3:2; 4:3, etc. The QRS wave group of the downward transmission mostly shows a bundle branch conduction block pattern.
(3) In second degree type II AV block, the block site is mostly below the Hitchcock bundle, and the QRS wave group is often widened at this time.
(3) Third degree AV block, also called complete AV block: the atrial wave (P wave) cannot be transmitted down to the excited ventricle.
Electrocardiographic features:
(1) Atrial and ventricular excitation, complete atrioventricular disconnection, equal P-P and R-R intervals, irregular P-R interval.
(2) The ventricular rate is slower than the atrial rate.
(3) The QRS wave group can be normal and wide or deformed. If the pacing point is located in the junctional area or the Hitchcock bundle, the QRS wave group morphology is basically normal; if the pacing point is located below the Hitchcock bundle, the QRS wave group is wide and deformed.
(C) Diagnostic criteria of intraventricular block
1, complete right bundle branch conduction block electrocardiographic features.
(1) QRS wave group time prolongation ≥0.12 s. The QRS wave group in V1 (V2) leads is partially normal at the beginning and widens to rsR’ wave or broad R wave with tangent at the end.
(2) V5, I, aVL, II, III, and aVF leads have qRs or Rs waves with widened S waves (terminal waves).
(3) Secondary ST-T changes (T wave in opposite direction to the main wave of QRS, ST segment shift).
2, Complete left bundle branch conduction block ECG features.
(1) QRS wave group time prolongation, ≥0.12 seconds
(2) V1, V2 present broad and deep QS waves presenting rS waves, I, aVL, V5, V6 generally no q waves and S waves. r waves are broad, coarse and blunt with tangents
(3) QRS electric axis left deviation secondary to ST and T wave changes.
3.What are the electrocardiograms that cause the widening of QRS wave group?
QRS wave group time >0.12s indicates intraventricular conduction disorder. It is commonly seen in intraventricular conduction block, ventricular hypertrophy, preexcitation syndrome, etc.
III. Treatment
(A) Treatment of the cause or primary disease
First of all, the cause of bradycardia should be clarified as much as possible, and the reversible factors that cause bradycardia should be treated, such as correcting electrolyte disorders, improving thyroid function and improving myocardial ischemia.
(ii) Drug treatment
If the patient develops symptoms related to bradycardia and the bradycardia is expected to be corrected in a short period of time, temporary measures can be taken to increase the ventricular rate, including pharmacotherapy and temporary pacemaker implantation.
1.Drug therapy
(1) Isoproterenol
(1) Mechanism of action: As a β-receptor stimulant in the treatment of sinus bradycardia, it has a significant excitatory effect on the sinus node and can accelerate conduction, so it can accelerate the heart rate.
②Use and dose: Isoproterenol 1 mg is added to 500 ml of 10% glucose solution slowly, and the drip rate is adjusted according to the heart rate and the patient’s response.
③Caution: It can cause panic, tachyarrhythmia, such as premature beats, tachycardia, etc. It should be ordered slowly and the heart rate should not exceed 70 beats/min when isoproterenol is ordered quietly.
2.Atropine
Mechanism of action: Atropine competitively counteracts the excitation of acetylcholine on M-cholinergic receptors, relieves the inhibition of the heart by the vagus nerve, and accelerates the heart rate.
②Use and dosage: Atropine, 0.5-2 mg intravenous injection.
(3) Precautions: May cause urinary difficulties, elevated intraocular pressure, taste, abdominal distension, etc. Use with caution or discontinue in patients with glaucoma and prostatic hyperplasia.
(2) Temporary cardiac pacing
Temporary cardiac pacing should be considered when there is severe bradycardia, when the effect of drugs is not obvious, or when bradycardia caused by ischemia is worried that the above drugs will aggravate myocardial ischemia.
3. Permanent pacemaker implantation: generally indicated for symptomatic bradyarrhythmias. A permanent pacemaker should be implanted in the following cases (due to space limitations, only indications for which a pacemaker should be implanted and is recommended are listed here)
(1) Sinus bradycardia
① Class I indications (a pacemaker should be placed)
A. Symptomatic bradycardia, including sinus arrest leading to frequent symptoms (Level of evidence: C).
B. Symptomatic variable temporal insufficiency (Level of Evidence: C).
C. Symptomatic sinus bradycardia triggered by medications that must be applied (Level of Evidence: C).
② Class IIa indications (pacemaker placement recommended)
A. Abnormal sinus node function at a heart rate <40 beats/min without clear evidence of a definite correlation between symptomatic episodes and bradycardia (Level of Evidence: C).
B. Syncope of unknown origin with clinically confirmed or electrophysiologic findings of significant sinus node dysfunction (Level of Evidence: C).
(2) Atrioventricular block: indications for acquired atrioventricular block pacemaker placement.
(1) Class I indications (pacemaker should be placed)
A. Third-degree or high degree AV block with associated symptoms (including heart failure) or ventricular arrhythmias (Level of Evidence: C).
B. Patients with drug-induced third-degree or high atrioventricular block, but for whom a user is necessary to treat other conditions (Level of Evidence: C)
C. Patients with third-degree or high atrioventricular block with pause during sinus rhythm for ≥3 seconds or escape heart rate <40 beats/min or escape originating below the AV node (Level of Evidence: C).
D. Patients with third-degree or high atrioventricular block with atrial fibrillation with pacing arrest for ≥5 seconds (Level of Evidence: C).
E. Patients with third-degree or high atrioventricular block caused by ablation of the atrioventricular junction zone (Level of Evidence: C).
F. Patients with irreversible third-degree or high atrioventricular block occurring postoperatively (Level of Evidence: C).
G. Patients with combined neuromuscular pathology, such as ankylosing muscular dystrophy, Série’s syndrome, etc. with third degree or high atrioventricular block (Level of Evidence: B).
H. Symptomatic second-degree AV block, regardless of the type and site of block (Level of Evidence: B).
I. Patients with asymptomatic sustained third-degree AV block, although with a ventricular rate of ≥40 beats/min, combined with cardiac enlargement or left heart dysfunction, or with a site of block below the AV node (Level of Evidence: B)
J. Second- or third-degree AV block during exercise in patients without myocardial ischemia (Level of Evidence: C).
② Class IIa indications (pacemaker placement recommended)
A. Persistent third-degree AV block with escape rhythm ≥ 40 beats/min, even if the patient is asymptomatic and has an enlarged heart (Level of Evidence: C)
B. asymptomatic second-degree AV block with electrophysiological examination confirming that the site of block is within or below the bundle of Hirschsprung (Level of Evidence: B)
C. Patients with symptomatic or hemodynamically abnormal first- or second-degree AV block (Level of Evidence: B).
D. In patients with second-degree type II AV block with narrow QRS wave clusters, pacemaker implantation when wide QRS wave clusters are present is a Class I indication (Level of Evidence: B).
(3) Intraventricular block
①Class I indications (pacemaker should be placed)
A. High atrioventricular block or intermittent third-degree atrioventricular block (Level of evidence: B)
B. Second-degree type II AV block (Level of Evidence: B).
C. alternating bundle branch block (Level of Evidence: C).
②Class IIa indications (pacemaker placement recommended)
A. syncope and atrioventricular block cannot be confirmed to be related, but other causes can be excluded, especially ventricular tachycardia (Level of evidence: B)
B. Incidental finding of HV interval ≥100 ms during electrophysiological examination in an asymptomatic patient (Level of evidence: B)
C. pacing-induced sub-Hitchcock block on electrophysiological examination, although nonphysiological (Level of Evidence: B).