4.2 Atrial Dysrhythmias (PACs, Atrial Fibrillation, Atrial Flutter, SVT)

Atrial Dysrhythmias

Atrial dysrhythmias arise when an ectopic focus within the atrial myocardium (outside the sinoatrial node) initiates electrical impulses, or when abnormal reentrant circuits develop within atrial tissue. Because depolarization spreads through the atria via non-physiological pathways, the resulting P waves differ in morphology from normal sinus P waves. However, because the electrical impulse still travels to the ventricles through the normal AV node, bundle of His, and Purkinje network, the QRS complex typically remains narrow (< 0.12 seconds), unless intraventricular conduction delays (such as bundle branch blocks or aberrancy) co-exist. Atrial dysrhythmias encompass a spectrum of clinical conditions ranging from benign premature beats to hemodynamically compromising tachyarrhythmias and high-risk thromboembolic states.

---

1. Premature Atrial Complexes (PACs)

A Premature Atrial Complex (PAC) is a single, early cardiac depolarization originating from an irritable ectopic site in either the right or left atrium. PACs interrupt the underlying cardiac rhythm and are extremely common in both healthy individuals and patients with organic heart disease.

ECG Diagnostic Criteria

• Rate & Regularity: The underlying rhythm is generally regular but is disrupted by the premature beat.
• P Wave Morphology: The P wave of the PAC occurs earlier than the expected sinus P wave. Because it originates outside the SA node, its morphology differs from sinus P waves: it may be tall and peaked, notched, inverted, flattened, or biphasic.
• PR Interval: May be normal (0.12–0.20 seconds), prolonged, or shortened depending on how close the ectopic focus is to the AV node.
• QRS Complex: Usually narrow (< 0.12 seconds) and identical in appearance to the patient's normal QRS complexes.
• Pause Type: PACs are typically followed by a non-compensatory pause. The ectopic atrial impulse depolarizes and resets the SA node before it would have naturally discharged. Consequently, the time interval between the beat preceding the PAC and the beat following the PAC is less than two normal R-R intervals.

Critical Exam Trap: Non-Conducted (Blocked) PACs

A non-conducted PAC occurs when an ectopic atrial impulse fires extremely early during the cardiac cycle, reaching the AV node while it is still in its absolute refractory period.

• ECG Appearance: An early ectopic P wave is visible, but it is not followed by a QRS complex. This early P wave often hides within the T wave of the preceding beat, distorting its height or shape (producing a notched or unusually tall T wave).
• Clinical Trap: Non-conducted PACs produce sudden pauses on the ECG baseline. Unwary technicians frequently misinterpret these pauses as Sinus Arrest, SA Exit Block, or Second-Degree AV Block. Always closely inspect the preceding T wave for a hidden, premature P wave whenever an unexpected pause appears on the tracing!

---

2. Atrial Fibrillation (Afib)

Atrial Fibrillation is the most frequent sustained cardiac dysrhythmia encountered in clinical practice. It is characterized by chaotic, uncoordinated electrical activity originating from multiple micro-reentrant circuits and rapid ectopic foci (most commonly located near the pulmonary vein ostia in the left atrium).

ECG Diagnostic Criteria

• Atrial Rate: Immeasurable, typically ranging from 350 to 600 bpm.
• Ventricular Rate: Variable. Categorized as:
  • Controlled Ventricular Response: Ventricular rate < 100 bpm.
  • Rapid Ventricular Response (RVR): Ventricular rate > 100 bpm (often 110–180 bpm).
• Regularity: Irregularly irregular. The R-R intervals show no repeating pattern or mathematical consistency.
• P Waves: Absent. Replaced by irregular, chaotic baseline fluctuations termed fibrillatory ('f') waves.
• PR Interval: Unmeasurable.
• QRS Duration: Usually narrow (< 0.12 seconds).

Hemodynamic Consequences: Loss of Atrial Kick

In a normal cardiac cycle, atrial contraction (termed the atrial kick) contributes 20% to 30% of total ventricular filling volume (end-diastolic volume). In Afib, the atria quiver incoherently at 350–600 times per minute rather than contracting forcefully. This loss of atrial kick causes an immediate 20–30% drop in cardiac output. In patients with impaired diastolic function, heart failure, or aortic stenosis, this acute drop in ventricular filling can precipitate severe hypotension, acute pulmonary edema, and cardiogenic shock.

Thromboembolic Risk & Stroke Stratification (CHA2DS2-VASc)

Because the atria fail to contract, blood stasis occurs within the atrial chambers, particularly inside the left atrial appendage (LAA). Stagnant blood leads to mural thrombus formation. If a clot breaks loose, it enters systemic circulation, posing a catastrophic risk for embolic stroke or systemic arterial embolism.

Clinicians use the CHA2DS2-VASc risk scoring system to stratify stroke risk and determine the necessity for long-term oral anticoagulation (such as DOACs like apixaban/rivaroxaban or warfarin):

Clinical Standard: A CHA2DS2-VASc score of ≥ 2 in men or ≥ 3 in women indicates high stroke risk and mandates chronic oral anticoagulation.

---

3. Atrial Flutter

Atrial Flutter is a macro-reentrant atrial dysrhythmia, most commonly involving a single continuous reentrant loop around the tricuspid valve annulus in the right atrium (the cavotricuspid isthmus).

ECG Diagnostic Criteria

• Atrial Rate: Regular, typically 250 to 350 bpm (classically 300 bpm).
• Ventricular Rate: Depends on the AV conduction ratio (e.g., at 2:1 conduction, ventricular rate is ~150 bpm; at 4:1 conduction, ventricular rate is ~75 bpm).
• Regularity: Atrial rhythm is perfectly regular. Ventricular rhythm is regular if the AV conduction ratio is fixed, or irregular if the AV block ratio varies.
• P Waves: Absent. Replaced by uniform, prominent "sawtooth" or "picket fence" flutter ('F') waves, best appreciated in inferior leads (II, III, aVF) and V1.
• PR Interval: Unmeasurable.

AV Conduction Ratios

The AV node cannot conduct electrical impulses at 300 bpm due to its physiological refractory period. Consequently, it acts as a protective gatekeeper, blocking a portion of the flutter impulses:

• 2:1 Conduction Ratio: 2 flutter waves for every 1 QRS complex. Atrial rate 300 bpm → Ventricular rate 150 bpm. (A narrow-complex regular tachycardia at exactly 150 bpm should always raise high suspicion for 2:1 Atrial Flutter!).
• 3:1 Conduction Ratio: 3 flutter waves per QRS complex. Atrial rate 300 bpm → Ventricular rate 100 bpm.
• 4:1 Conduction Ratio: 4 flutter waves per QRS complex. Atrial rate 300 bpm → Ventricular rate 75 bpm.
• Variable Block: The AV conduction ratio shifts between 2:1, 3:1, and 4:1, producing an irregular ventricular response that can mimic Afib.

---

4. Multifocal Atrial Tachycardia (MAT) & Wandering Atrial Pacemaker (WAP)

Multifocal Atrial Tachycardia (MAT) occurs when multiple distinct ectopic foci in the atria fire impulses sequentially.

ECG Diagnostic Criteria for MAT

• Heart Rate: > 100 bpm (typically 101–150 bpm). If the rate is < 100 bpm with identical P wave criteria, the rhythm is termed Wandering Atrial Pacemaker (WAP).
• Regularity: Irregularly irregular R-R intervals.
• P Waves: At least 3 distinctly different P wave morphologies must be identified in the exact same ECG lead.
• PR Intervals: Variable PR intervals as the distance from each ectopic focus to the AV node differs.
• Isoelectric Baseline: An isoelectric baseline is present between P waves (differentiating MAT from the continuous chaotic baseline of Afib).

Clinical Association: Pulmonary Disease

MAT is classically and strongly associated with severe underlying pulmonary disease, particularly Chronic Obstructive Pulmonary Disease (COPD), acute exacerbations of severe asthma, pulmonary hypertension, hypoxia, and hypercapnia. The mechanism involves atrial stretch, hypoxemia, increased circulating catecholamines, and electrolyte disturbances (such as hypomagnesemia or hypokalemia).

Exam Trap: Do not treat MAT with beta-blockers in patients with severe COPD/bronchospasm! Treatment prioritizes correcting hypoxia, treating the underlying pulmonary condition, and administering non-dihydropyridine calcium channel blockers (like verapamil or diltiazem) if rate control is needed.

---

5. Paroxysmal Supraventricular Tachycardia (PSVT) & AVNRT

Supraventricular Tachycardia (SVT) is a broad diagnostic category including any fast rhythm originating above the ventricles. However, clinically, the term Paroxysmal Supraventricular Tachycardia (PSVT) refers to sudden-onset, rapid, regular tachycardias, most commonly caused by AV Nodal Reentrant Tachycardia (AVNRT) or Atrioventricular Reentrant Tachycardia (AVRT).

Mechanism of AVNRT

AVNRT involves a dual electrical pathway within the AV node itself: a fast pathway (with a longer refractory period) and a slow pathway (with a shorter refractory period). A premature atrial beat can enter the slow pathway while the fast pathway is refractory, setting up a self-sustaining reentrant loop spinning inside the AV node. The loop sends impulses downward to the ventricles and upward to the atria simultaneously.

ECG Diagnostic Criteria

• Rate: 150 to 250 bpm (typically 170–220 bpm).
• Regularity: Extremely regular R-R intervals.
• P Waves: Frequently buried within the QRS complex due to simultaneous atrial and ventricular depolarization. If visible, they appear as retrograde P waves (inverted in leads II, III, aVF) appearing immediately after the QRS complex, often forming a pseudo-R' wave in lead V1 or a pseudo-S wave in lead II.
• QRS Duration: Narrow (< 0.12 seconds).

Stepwise Emergency Management Protocol

1. Vagal Maneuvers: First-line intervention for hemodynamically stable patients. Techniques such as the Modified Valsalva Maneuver (bearing down against a closed glottis for 15 seconds, followed by passive leg raise) or carotid sinus massage stimulate the vagus nerve. Parasympathetic outflow slows conduction through the AV node, which can break the reentrant circuit and restore sinus rhythm.
2. Adenosine Therapy: If vagal maneuvers fail, Adenosine is the drug of choice. Adenosine is a potent AV nodal blocking agent that transiently slows or completely blocks conduction through the AV node.
   • Administration: Must be administered as a rapid IV push (over 1–2 seconds) through a large proximal IV line (e.g., antecubital fossa), immediately followed by a rapid 20 mL normal saline flush. This rapid delivery is essential because Adenosine has an extraordinarily short half-life of less than 10 seconds in blood.
   • Dosing: Initial dose is 6 mg IV push. If the rhythm does not convert within 1 to 2 minutes, a second dose of 12 mg IV push is administered.
   • Expected Finding: Adenosine causes a brief period of AV block or asystole (often 2–6 seconds) on the monitor before the SA node resumes dominance. Warn the patient they may experience flushing, chest tightness, and a sense of impending doom during administration.
3. Synchronized Cardioversion: If the patient is unstable (exhibiting hypotension, altered mental status, acute heart failure, or ischemic chest pain), skip vagal maneuvers and adenosine, and perform immediate synchronized electrical cardioversion starting at 50–100 Joules.