In normal conditions heartbeat impulses are generated in the sinoatrial node[?].
If the impulse is generated in other parts of the heart it is regarded as a
pathological phenomenon.
The most simple and effective diagnostic test for assessment of heart rhythm is
electrocardiogram (abbreviated ECG or EKG). Holter monitors are 24 hour ECGs.
The mechanism responsible for clinically important arrhythmias can occasionally be further characterized by electrophysiologial studies.
Mode of initiation
Automaticity
Enhanced or abnormal
Spontaneous onset of tachycardia
No premature beats leading to the arrhythmia
Gradual increase in the rate of the arrhythmia over the first 5-10 beats ("warm up")
EKG appearance of the first tachycardia's beat is identical to the rest
Reentry
Initiation is with a premature beat followed by a slight pause
This is followed by the arrhythmia (corresponding to premature beat, unidirectional block, slow conduction)
"Warm up" is unusual
EKG appearance of the first tachycardia beat need not be identical to the rest
Mode of termination in response to overdrive pacing (pacing the heart at a rate faster than the tachycardia rate). Sometimes the application of electrical pacing from outside the heart itself is useful diagnostically. This is most commonly done by placing an electrode into the cardiac chambers and delivering electrical current across the endocardium
Automaticity
Often shows "overdrive suppression"
The arrhythmia seems to be terminated by pacing only to return after several seconds with a gradual resumption of the pre-pacing rate
This is related to increased activity of the Na+ - K+ pump with Na+ loading. This causes the cell to have a more negative resting membrane potential and takes longer to reach threshold.
Reentry
Often terminates in response to overdrive pacing
Without subsequent arrhythmia resumption
Tachycardia stops because paced impulses have entered circuit in both limbs causing bi-directional block
The precise role of triggered activity as a mechanism on human arrhythmias has not been studied in adequate detail to characterize modes of initiation or response to pacing.
Other means exist to differentiate more clearly among arrhythmias at the time of invasive electrophysiologic study.
SADS, or sudden arrythmia death syndrome[?], is a rare condition blamed for some otherwise unexplained deaths. It is related to long QT syndrome, QT being the time taken by the heart to recover after each beat. Patients with long QT can be given beta-blocker drugs to slow the heartbeat, and are advised to refrain from competitive sports.
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