cardiovascular disorders
Sinus Node Dysfunction
Introduction
Sinus node dysfunction refers to a number of conditions causing physiologically inappropriate atrial rates. Symptoms may be minimal or include weakness, effort intolerance, palpitations, and syncope. Diagnosis is by electrocardiography. Symptomatic patients require a pacemaker. (See also Overview of Arrhythmias .) Sinus node dysfunction includes Inappropriate sinus bradycardia Alternating bradycardia and atrial tachyarrhythmias (bradycardia-tachycardia syndrome) Sinus pause or arrest Sinoatrial (SA) exit block Sinus node dysfunction affects mainly older patients, especially those with another cardiac disorder or Diabetes Mellitus (DM) . Sinus pauseis temporary cessation of sinus node activity, seen on electrocardiography (ECG) as disappearance of P waves for seconds to minutes. The pause usually triggers escape activity in lower pacemakers (eg, atrial or junctional), preserving heart rate and function, but long pauses cause dizziness and syncope. InSA exit block,the SA node depolarizes, but conduction of impulses to atrial tissue (exit conduction) is impaired. In first-degree SA block, the SA node impulse exit conduction is merely slowed, and ECG is normal. In type I second-degree SA (SA Wenckebach) block, impulse exit conduction slows before blocking, seen on the ECG as a P-P interval that decreases progressively until the P wave drops altogether, creating a pause and the appearance of grouped beats; the duration of the pause is less than 2 P-P cycles. In type II second-degree SA block, impulse exit conduction is blocked without slowing beforehand, producing a pause that is a multiple (usually twice) of the P-P interval and the appearance of grouped beats. In third-degree SA block, impulse exit conduction is blocked; normal P waves are absent, giving the appearance of sinus arrest. 