Bradycardias

Bradycardias are slow heart rates or rhythms. By definition, bradycardia means the heart at rest is less than 60 beats per minute. In some extremely healthy adults the resting hear rates might be down to 35 and 40 BPM. Normally these individuals are completely asymptomatic of the slow heart rate and there is no concern. However, an abnormally slow heart rate in an older person or in someone in not so good physical condition can be of concern especially when there are symptoms suggestive of inadequate blood flow. When your heart does not pump enough blood because the heart rate is too slow to meet your body's needs you may feel tired, weak, sluggish, dizzy or light-headed, and short of breath. A common cause of bradycardia in older patients is a condition is called sick sinus syndrome. Sometimes bradycardias are caused by medications that can be replaced or stopped, such as beta-blockers, some calcium blockers, digoxin, and others. If medications are not the problem, slow heart rhythms are easily treated with implantation of a pacemaker, and people can live for decades after having a pacemaker implanted.

Slow rates may be normal. Trained athletes often exhibit rhythms in the low thirties without exhibiting adverse effects; their heart rate readily increases with physical exertion. Abnormally slow rhythms may be ascribed to two general mechanisms: - failure of the SA node to generate impulses (Sinus Bradycardia) and failure of the impulses to conduct normally to the ventricles (AV Block).

SA node disease is the most common cause of bradyarrhythmia. However, sinus bradycardia does not always indicate disease since the SA node is sensitive to the autonomic nervous system. Usually the cause (etiology) of the SA node disease is unknown. It appears to be degenerate with fibrous or fatty infiltration decreasing the number of functional SA node cells. Advancing age is associated with fewer functional cells.

Sinus bradycardia and sinus pauses can occur as an intermittent or sustained arrhythmia. Bradycardia interspersed with bouts of Atrial tachyarrhythmia is called bradycardia-tachycardia syndrome (Brady-Tachy syndrome).

First Degree Block is a conduction delay only, not a block, which occurs at the level of the AV node or His Bundle. This slow rhythm produces no symptoms since all atrial impulses are transmitted to the ventricles and normal heart rate regulation is maintained.

Second Degree AV Block means that impulse conduction to the ventricles is intermittent with some impulses being conducted and others being blocked. Second-degree block is often divided into two types dependent upon whether the block is preceded by a beat to beat slowing of conduction (Mobitz I) or occurs abruptly without first slowing (Mobitz II). The latter is usually below the AV node.

Third Degree AV Block means that the block is complete and no atrial impulses are conducted to ventricles. Then ventricular rhythm is totally dependent upon subsidiary pacemakers. When complete block is due to a defect in the AV node effective subsidiary pacemakers can usually sustain life. Blocks below the AV node can be life threatening.

The tolerance to slow heart rate depends upon a persons overall cardiovascular health. In young people with congenital heart bloc, slow rates may be well tolerated at rest and even during exercise. In contrast, a drop in heart rate may critically curtail cardiac output in patients with mild heart impairment. Acute symptoms may consist of lightheadedness, threatened loss of consciousness, or syncope. Less dramatic but vitally important is the gradual development of overt signs of heart failure e.g. shortness of breath, fluid retention. Pacemaker therapy in such patients may act as a heart failure treatment.

Determining the site of the block provides useful therapeutic and prognostic information. Blocks in the AV node (proximal block) usually have a favorable outcome, whereas blocks in the His-Purkinje system (distal blocks) maybe lethal. AV nodal block is well tolerated because subsidiary (rescue) pacemakers within the node or immediately downstream of it can maintain a stable heart rate of 40-50 beats per minute. In contrast, blocks below the AV node and His bundle may result in dangerously slow (below 30 beats/min) and erratic rhythms.