The term «arrhythmia» refers to alterations in the heart rate or disturbances of regularity of the heart rhythm, implying changes in the cycle duration on the ECG records or changes in the duration and form of waves within a cycle.
The main classes of arrhythmia include:
• disorders of impulse initiation;
• disorders of impulse conduction;
• simultaneous abnormalities of impulse generation and conduction.
DISORDERS OF IMPULSE INITIATION
Disorders of impulse initiation include:
• changes in the normal automaticity of the sinoatrial node;
• abnormal automaticity;
• triggered activity.
Changes in normal automaticity of the sinoatrial node
Changes in the normal automaticity of the sinoatrial node are manifested in tachycardia, bradycardia, and sinus arrhythmia.
Tachycardia is a heart rate in excess of 100 beats per min.
Bradycardia is a heart rate that is less than 60 beats per min.
Sinus arrhythmia is characterized by abnormal variations in the R-Rinterval seen in the ECG recordings.
Sinus tachycardia
It represents a physiological response to a variety of stresses, such as fever, volume depletion, anxiety, exercise, thyrotoxicosis, hypoxemia, hypotension, or congestive heart failure.
Sinus bradycardia
It results from the activation of the vagal influences, or a decrease in sympathetic tone. Some substances can directly inhibit the function of the sinoatrial node, such as drugs (digitalis, quinine, opiates), or metabolites (uncojugated bilirubine, bile acids).
Sinus node dysfunction is commonly manifested ted as paroxysmal dizziness, presyncope, or syncope. These symptoms usually result from abrupt prolonged sinus pauses caused by failure of sinus impulse formation (sinus arrest) or block of the conduction of sinus impulses to the surrounding atrial tissue (sinus exit block).