Atrial Dysrhythmias

Premature Atrial Complex (PAC)

• An ectopic focus in the atria causes an early PQRST complex followed by a non-compensatory pause.
• On ECG, the P wave will be abnormal, inverted if the pacemaker is low in the atria, and the PR interval may be shorter than usual.
• The QRS complex will be narrow and sharp with a 1:1 conduction ratio.
• The PP and RR intervals will be early due to the premature complex.
• PACs can be caused by increased sympathetic tone, autonomic nervous system infections, sympathetic emetic drugs, caffeine, nicotine, hypoxia, cardiovascular disease, atrial hypertrophy, valve problems, increased automaticity, and re-entry.
• Clinical significance depends on the underlying cause.
• For example, if the PAC is a result of a pathological process in the atria it could be clinically significant.

Wandering Atrial Pacemaker

• The impulse occurs at various different points in the atria.
• The rhythm is associated with multiple orderly pacemaker sites from the SA node to the AV junction.
• It is generally associated with increased automaticity in conduction cells.
• On ECG, the rate is typically 60-100/min and variable, the rhythm can be slightly irregular, and the P waves will be inconsistent as they are coming from different pacemakers.
• The PR interval will be normal but can be shorter if the ectopic focus is closer to the AV node.
• The QRS complex is normal.
• The origin of the wandering pacemaker is variable and depends on where it is firing from.
• While not typically significant in the immediate term, the wandering atrial pacemaker can be caused by aging and medications.

Atrial Tachycardia

• A single ectopic focus produces the rhythm, which can be triggered by a PAC.
• On ECG, the rate is 160-240/min, the rhythm is typically regular, the P wave will be abnormal, identical, and inconsistent, and the PR interval will be fairly regular.
• The QRS complex will be narrow and sharply pointed with a normal time and can be dropped (2:1 ratio).
• The PP and RR interval should be regular.
• Causes of atrial tachycardia include electrolyte imbalances, hypoxia, chronic lung disease, coronary heart disease, acute coronary syndromes, rheumatic heart disease, and acute alcohol toxicity.
• Clinical significance depends on the underlying cause.
• For example, if the atrial tachycardia is associated with acute coronary syndrome, it could be clinically significant.
• Rapid depolarization transferred to the ventricles can lead to poor ventricular filling, decreased cardiac output, and increased oxygen demand.

Multifocal Atrial Tachycardia (MAT)

• Multiple foci depolarizing (more than three) create the rhythm.
• On ECG, the rate is 100-150/min, the rhythm is typically regular, the P waves are abnormal and variable, and the PR interval will be variable depending on the ectopic focus location relative to the AV node.
• The QRS complex is narrow and sharply pointed.
• The PP and RR interval can potentially vary.
• The causes of MAT are similar to those of atrial tachycardia.
• Clinical significance depends on the underlying cause. For example, MAT associated with acute coronary syndrome is clinically significant.
• Similar to atrial tachycardia, MAT leads to rapid depolarization to the ventricles, poor ventricular filling, decreased cardiac output, and increased oxygen demand.

Atrial Flutter

• Increased atrial automaticity or re-entry circuit causes atrial ectopic pacemaker to create flutter waves (f waves).
• On ECG, the atrial rate is 240-360/min and the ventricular rate is half the atrial rate.
• The rhythm is typically regular but may be irregular if the conduction ratio changes.
• The P wave is called an F wave and exhibits a sawtooth pattern.
• The PR interval (now the FR interval) is difficult to measure.
• The QRS complexes should be regular, but consistently irregular if the number of F waves changes.
• Causes of atrial flutter can be either paroxysmal (occurs transiently) or chronic.
• Clinical significance depends on the ventricular rate, with higher rates increasing the risk of new or exacerbating preexisting conditions.

Atrial Fibrillation

• Increased automaticity or re-entry in the atria causes chaotic f waves with an irregular ventricular response.
• On ECG, the atrial rate is 350-600/min. The ventricular rate can be under 60/min.
• The rhythm is irregularly irregular.
• The P wave is an f wave, and the PR interval is essentially absent.
• The QRS complexes are normal, but the conduction ratio is random.
• Common causes include mitral valve disease, coronary heart disease, hypertension, hyperthyroidism, congestive cardiac failure, acute myocarditis and pericarditis, lung disease, excessive alcohol and caffeine consumption, emotional distress and certain medications.
• Clinically significant due to the risk of clot formation, which can lead to embolism (a clot blockage of a blood vessel).
• An embolism in a cerebral artery can result in stroke.