Blood-pressure and glucose control may be effective in preventing heart block, a common form of arrhythmia, and the subsequent need for a pacemaker, according to a study by researchers at UC San Francisco.
In an analysis of more than 6,000 Finnish patients, appearing online May 24, 2019, in JAMA Network Open, UCSF researchers found that more than half of the cases of heart block resulted from high blood pressure or elevated blood sugar.
Atrioventricular (AV) block occurs when electrical conduction is impaired between the heart’s four chambers, most often by fibrosis or sclerosis.
It is often felt as the heart skipping a beat.
Atrioventricular (AV) block is partial or complete interruption of impulse transmission from the atria to the ventricles.
The most common cause is idiopathic fibrosis and sclerosis of the conduction system.
Diagnosis is by ECG; symptoms and treatment depend on degree of block, but treatment, when necessary, usually involves pacing.
The most common causes of AV block are
- Idiopathic fibrosis and sclerosis of the conduction system (about 50% of patients)
- Ischemic heart disease (40%)
The remaining cases of AV block are caused by
- Drugs (eg, beta-blockers, calcium channel blockers, digoxin, amiodarone)
- Increased vagal tone
- Congenital heart, genetic, or other disorders
AV block may be partial or complete. First-degree and second-degree blocks are partial. Third degree blocks are complete.
First-degree AV block
All normal P waves are followed by QRS complexes, but the PR interval is longer than normal (> 0.20 sec—see Figure: Atrioventricular block.).
For 1st-degree block, conduction is slowed without skipped beats. All normal P waves are followed by QRS complexes, but the PR interval is longer than normal (> 0.2 sec). For 3rd-degree block, there is no relationship between P waves and QRS complexes, and the P wave rate is greater than the QRS rate.
First-degree AV block may be physiologic in younger patients with high vagal tone and in well-trained athletes.
First-degree AV block is rarely symptomatic and no treatment is required, but further investigation may be indicated when it accompanies another heart disorder or appears to be caused by drugs.
Second-degree AV block
Some normal P waves are followed by QRS complexes, but some are not. Two types exist.
In Mobitz type I 2nd-degree AV block, the PR interval progressively lengthens with each beat until the atrial impulse is not conducted and the QRS complex is dropped (Wenckebach phenomenon); AV nodal conduction resumes with the next beat, and the sequence is repeated (see Figure: Mobitz type I 2nd-degree atrioventricular block.).
Mobitz type I 2nd-degree atrioventricular block.
The PR interval progressively lengthens with each beat until the atrial impulse is not conducted and the QRS complex is dropped (Wenckebach phenomenon); AV nodal conduction resumes with the next beat, and the sequence is repeated.
4:3 Mobitz Type I 2nd-Degree Atrioventricular Block
IMAGE COURTESY OF L. BRENT MITCHELL, MD.
Mobitz type I 2nd-degree AV block may be physiologic in younger and more athletic patients.
The block occurs at the AV node in about 75% of patients with a narrow QRS complex and at infranodal sites (His bundle, bundle branches, or fascicles) in the rest.
If the block becomes complete, a reliable junctional escape rhythm typically develops.
Treatment is therefore unnecessary unless the block causes symptomatic bradycardia and transient or reversible causes have been excluded.
Treatment is pacemaker insertion, which may also benefit asymptomatic patients with Mobitz type I 2nd-degree AV block at infranodal sites detected by electrophysiologic studies done for other reasons.
In Mobitz type II 2nd-degree AV block, the PR interval remains constant. Beats are intermittently nonconducted and QRS complexes dropped, usually in a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave (see Figure: Mobitz type II 2nd-degree atrioventricular block.).
Mobitz type II 2nd-degree atrioventricular block.
The PR interval remains constant. Beats are intermittently nonconducted, and QRS complexes dropped, usually in a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave.
Mobitz type II 2nd-degree AV block is always pathologic; the block occurs at the His bundle in 20% of patients and in the bundle branches in the rest.
Patients may be asymptomatic or experience light-headedness, presyncope, and syncope, depending on the ratio of conducted to blocked beats.
Patients are at risk of developing symptomatic high-grade or complete AV block, in which the escape rhythm is likely to be ventricular and thus too slow and unreliable to maintain systemic perfusion; therefore, a pacemaker is indicated.
Second-degree atrioventricular block (high grade).
The distinction between Mobitz type I and Mobitz type II block is difficult to make because 2 P waves are never conducted in a row.
Risk of complete AV block is difficult to predict, and a pacemaker is indicated.
Patients with any form of 2nd-degree AV block and a structural heart disorder should be considered candidates for permanent pacing unless there is a transient or reversible cause.
Third-degree AV block
Heart block is complete (see Figure: Third-degree atrioventricular block.).
Third-degree atrioventricular block.
There is no electrical communication between the atria and ventricles and no relationship between P waves and QRS complexes (AV dissociation).
Cardiac function is maintained by an escape junctional or ventricular pacemaker.
Escape rhythms originating above the bifurcation of the His bundle produce narrow QRS complexes, relatively rapid (> 40 beats/min) and reliable heart rates, and mild symptoms (eg, fatigue, postural light-headedness, effort intolerance).
Escape rhythms originating below the bifurcation produce wider QRS complexes, slower and unreliable heart rates, and more severe symptoms (eg, presyncope, syncope, heart failure).
Signs include those of AV dissociation, such as cannon a waves, BP fluctuations, and changes in loudness of the 1st heart sound (S1). Risk of asystole-related syncope and sudden death is greater if low escape rhythms are present.Overview of Atrioventricular Block
Most patients require a pacemaker (see Table: Examples of Pacemaker Codes).
If the block is caused by antiarrhythmic drugs, stopping the drug may be effective, although temporary pacing may be needed.
A block caused by acute inferior MI usually reflects AV nodal dysfunction and may respond to atropine or resolve spontaneously over several days.
A block caused by anterior MI usually reflects extensive myocardial necrosis involving the His-Purkinje system and requires immediate transvenous pacemaker insertion with interim external pacing as necessary.
Spontaneous resolution may occur but warrants evaluation of AV nodal and infranodal conduction (eg, electrophysiologic study, exercise testing, 24-h ECG).
Most patients with congenital 3rd-degree AV block have a junctional escape rhythm that maintains a reasonable rate, but they require a permanent pacemaker before they reach middle age.
Less commonly, patients with congenital AV block have a slow escape rhythm and require a permanent pacemaker at a young age, perhaps even during infancy.
An estimated 3 million people worldwide have pacemakers, and 600,000 pacemakers are implanted annually.
But while a common treatment and low-risk procedure, it can result in serious complications.
Generator charges also carry a high risk of infection in and around the heart.
However, there has been limited research on whether behavioral modifications can prevent heart block and which ethnicities are most at risk.
“It is perhaps precisely because pacemakers so successfully and immediately address cases of heart block that we have previously failed to devote more attention to prevention of this important disease,” said senior author Gregory Marcus, MD, MAS, a UCSF Health cardiologist and associate chief of cardiology for research in the UCSF Division of Cardiology.
“In addition to the prevention and treatment of myocardial infarction and heart failure, effective treatment of hypertension and maintenance of normal blood sugars may be useful prevention strategies.”
This is the first community-based study to evaluate the possible association between common modifiable cardiovascular risks and heart block occurrence requiring pacemaker implantation.
In the JAMA Network Open study, Marcus and his colleagues used the Mini-Finland Health Survey, which was designed to represent the country’s population aged 30 and over, and consisted of an in-home interview and clinic examination on various health subjects.
They studied 6,146 Caucasian patients enrolled from 1978 to 1980, then reviewed the patients’ hospital records from 1987 to 2011 to determine heart block incidents.
Over an average follow up of 25 years, 58 patients developed AV block.
The researchers found that older age, being male, higher systolic blood pressure, higher fasting glucose, history of myocardial infarction and history of congestive heart failure independently increased the likelihood of occurrence.
Of those factors, two directly modifiable risk factors were identified: every 10 millimeter increase in systolic blood pressure resulted in 22 percent greater risk, and every millimeter increase in fasting glucose resulted in 19 percent greater risk.
Taking into account the prevalence of these modifiable risk factors in the population and assuming causal relationships, they estimated that 47 percent of AV blocks in the 58 patients would have been avoided with ideal blood pressure and 11 percent with normal fasting glucose.
The authors note that the study occurred in a solely Caucasian population and advised caution in applying findings to other populations.
“Given the prevalence of heart block in the adult male population, as well as the multiple risks associated with pacemakers, it would be worthwhile to pursue further research on this connection,” said Marcus, who holds the Endowed Professorship of Atrial Fibrillation Research in the UCSF School of Medicine.
“This new information also may help persuade hypertensive individuals to receive and continue their prescribed treatments.”
Journal information: JAMA Network Open
Provided by University of California, San Francisco