Arrhythmia

Health Sciences > Medicine > Cardiology > Cardiac Arrhythmias

Cardiac arrhytmias is a term applied to disturbances of :

• frequency of heart beats
• regularity of heart beats
• origin of heart beats impulse

Frequency

Increased frequency is described as tachycardia (more than 100 beats/min)
Decreased frequency is described as bradycardia (less than 60 beats/min)

Regularity

Irregularity may be absolute - for example atrial fibrillation or with a pattern - for example bigeminia or trigeminia.

Origin

In normal conditions heart beat impulses are generated in sinoatrial node. If the impulse is generated in other parts of the heart it is regarded as a pathological phenomenon.

Diagnosis

The most simple and effective diagnostic test for assesment of heart rhythm is electrocardiogram ( abbreviated ECG or EKG. Holtor monitors are 24 hour ECG's.

   *  The mechanism responsible for clinically important arrhythmias can occasionally be discerned from characteristic features
   * 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 oh human arrhythmias has not be studied in adequate detail to characterize nodes of initiation or response to pacing
   * Other means exist to differentiate more clearly among arrhythmias at the time of invasive electrophysiologic study