heart rhythm guide
heart rhythm guide

Slow Heart Rates


    Abnormally heart rhythms causing excessively slow heart rates are much more common in elderly patients. Frequently, the onset of a slow heart rate in an elderly patient is often gradual and insidious and may not recognized by the patient’s family. The elderly patient may exhibit loss of energy and stamina and may become listless and inactive. Some elderly patients with markedly reduced heart rates develop episodes of confusion, forgetfulness, or impaired ability to walk.

    Excessively slow heart rates frequently cause symptoms of fatigue, lethargy, or dizziness. Heart rates less than 35/min may cause fainting or near fainting episodes. When the heart rate is less than 20/min patients generally cannot stand upright.

    Alternatively, the onset of slow heart rates in elderly patients may be sudden and can occur without any warning. Some disorders of the heart rhythm cause sudden profound slowing of the heart rate or prolonged pauses, which may cause sudden loss of consciousness. Patients who are standing when they experience a sudden fall in heart rate are much more likely to faint than are patients who are lying supine. However, when the heart stops beating for more than ten seconds, patients almost always lose consciousness, whether they are standing upright or supine. Abrupt falls in elderly patients may result in fractures or concussion due to head trauma.

    Until the introduction of permanent pacemakers 50 years ago, there was no effective treatment for patients with symptoms caused by slow heart rhythms. Modern implantable dual chamber pacemakers have revolutionized the care of elderly patients with symptomatic bradyarrhythmias.

Sinus Bradycardia

    The most common slow heart rate is sinus bradycardia, which is characterized by a P-wave followed by a QRS, at regular slow intervals with a rate less than 60/min. Sinus bradycardia is common in normal persons particularly while asleep at night and is common in athletes. Asymptomatic individuals with sinus bradycardia never require specific treatment.

Sinus Bradycardia

Sinus Bradycardia

Each P-wave is followed by a QRS-complex and the heart rate is less than 60 beats/min. In this example the heart rate is 25 beats/min. Permanent pacemaker insertion is indicated in patients with severe sympomatic sinus bradycardia.

    Elderly patients occasionally develop a heart rhythm disorder referred to as the bradycardia-tachycardia or sick sinus syndrome. In this syndrome, episodes of marked sinus bradycardia alternate with episodes of atrial tachycardia, atrial flutter, or atrial fibrillation with a rapid ventricular rate. These patients are often highly symptomatic, experiencing weakness, dizziness and fatigue during severe bradycardia and experiencing palpitations, fluttering of the heart and dizziness during episodes of rapid tachycardia. This bradycardia-tachycardia syndrome is the most common indication for pacemaker insertion in the United States. Patients with the bradycardia-tachycardia syndrome feel much improved following insertion of a dual chamber pacemaker because bradyarrhythmias are effectively eliminated, and because atrial tachycardia, such as paroxysmal atrial fibrillation, are often reduced in frequency. Furthermore, once a dual chamber pacemaker is inserted, antiarrhythmic medications, which may cause bradyarrhythmias, can be safely given to suppress atrial tachycardias. For example, administering a beta-blocker to a patient with bradycardia-tachycardia syndrome may suppress tachyarrhythmias but also worsen bradycardia; pacing eliminates any beta-blocker induced bradyarrhythmias.

    The second cause of bradyarrhymias in elderly patients is conduction disease usually due to idiopathic fibrosis of the conduction system. Interruption of atrio-ventricular conduction through the AV node results in different degrees of conduction disturbance referred to as first, second, and third degree heart block. In first degree heart block, conduction through the AV-node is prolonged but all atrial impulses are conducted through to the ventricles, resulting in a regular rhythm with every P-wave followed by a QRS-complex but the PR-interval exceeds 0.20 seconds. First degree heart block never requires insertion of a pacemaker.

    Second degree heart block results from impaired AV conduction resulting in intermittent dropped QRS complexes, either at regular or irregular intervals. Second degree heart block is sub-divided into Wenckebach block characterized by progressive PR prolongation followed by a non-conducted P-wave causing a dropped QRS and Type II second degree block characterized by droped QRS complexes at regular intervals with fixed PR intervals. Wenckebach second degree block never requires pacemaker implantation except if severe symptomatic bradycardia is present. Type II second degree block often progresses to higher degrees of AV-block resulting in more severe symptomatic bradyarrhythmias and frequently requires pacemaker implantation.

Wenckebach second Degree AV Block

Wenckebach second Degree AV Block

Wenckebach AV-block is characterized by progressively prolonged P-R intervals followed by a dropped QRS-complex. Wenckebach block does not progress to higher degrees of AV-block and does not require pacing.

    Third degree or complete AV-block occurs when none of the atrial impulses are conducted through the AV-node to the ventricules. The atrial and ventricular rhythms are completely independent. The atrial rhythm may be sinus or atrial tachycardia, flutter or fibrillation. The ventricular rhythm is usually a slow idioventricular rhythm with a wide QRA and a ventricular rate usually less than 40 beats/min. Patients with complete heart block frequently experience lightheadedness, generalized weakness, fainting spells, and symptoms of palpitations or dropped beats. Implantation of a permanent dual chamber pacemaker is virtually always indicated in patients with complete heart block and extends patients’ lives considerably. In the era before permanent pacemakers became available, patients with complete heart block lived on an average of approximately six months.

Complete Heart Block

Complete Heart Block

Third degree or complete AV block occurs when no atrial impulses are conducted to the ventricles. The atria and ventricles beat independently. Typically, a slow junctional or ventricular escape rhythm develops. Insertion of a permanent pacemaker is indicated.

Next Section: Pacemakers