Arrhythmia

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Cardiac arrhythmias are disturbances of any of the following:

  • frequency of heartbeat
  • regularity of heartbeats
  • origin of heartbeat impulse

Several forms of cardiac arrhythmia are life-threatening and a medical emergency.

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 bigeminy or trigeminy).

Origin

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.

Diagnosis

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

    SADS, or sudden arrhythmia death syndrome, is a term used to describe sudden death due to cardiac arrest brought on by an arrhythmic episode. Victims of SADS are typically in adolescence or their early 20s, and were previously not diagnosed with any type of cardiac disease.

    The most common causes of SADS are long QT syndrome, Brugada syndrome, and hypertrophic cardiomyopathy.