The sinoatrial (SA) node represents the integrated activity of pacemaker cells, sometimes called P cells, in a compact region at the junction of the high right atrium and the superior vena cava. Perinodal cells, sometimes called transitional or (T) cells, transmit the electrical impulse from the SA node to the right atrium. Each of these cell types has distinct expression profiles of ion channels and gap junctions.
Given the architecture of the SA node, SA nodal dysfunction typically results from either abnormalities in impulse generation by the P cells or abnormalities in conduction across the T cells. SA nodal dysfunction is more commonly an acquired condition, but in some patients it can be inherited, with gene mutations having been described in some forms of inherited SA nodal dysfunction . Patients with SA nodal dysfunction may be asymptomatic or highly symptomatic as in cases of sick sinus syndrome.
Sinoatrial nodal pauses, arrest, and exit block will be discussed here. Additional details regarding the anatomy and electrophysiology of the SA node, as well as a discussion of the sick sinus syndrome, are presented separately. (See "Manifestations and causes of the sick sinus syndrome".)
Sinus pause, arrest, and exit block may arise from ischemic, inflammatory, or infiltrative or fibrotic disease of the SA node, excessive vagal tone, sleep apnea, digitalis, and some antiarrhythmic and other drugs. The causes of sinus node dysfunction are discussed in detail elsewhere. (See "Manifestations and causes of the sick sinus syndrome", section on 'Etiology' and "Obstructive sleep apnea and cardiovascular disease", section on 'Other arrhythmias' and "Cardiac arrhythmias due to digoxin toxicity", section on 'Sinus bradycardia, tachycardia, block, and arrest'.)
TYPES OF SA NODAL DYSFUNCTION
Sinus pause or arrest — A sinus pause or arrest is defined as the transient absence of sinus P waves on the electrocardiogram (ECG) that may last from two seconds to several minutes (waveform 1). This abnormality is an alteration in discharge by the SA pacemaker; as a result, the duration of the pause has no arithmetical relationship to the basic sinus rate (ie, the cycle length of the pause is not a multiple of the basic sinus cycle length as would occur with 2:1 or 3:1 SA nodal block). The pause or arrest often allows escape beats or rhythms to occur, but lower pacemakers may be sluggish or even absent in the sick sinus syndrome.