Atrial Flutter and Arrhythmia Review

A reentrant circuit is composed of two separate conduction pathways that are eventually joined together by common tissue.
Conduction Velocities: The two pathways within the circuit possess different conduction velocities:
- Pathway B: Conducts electrical impulses faster.
- Pathway A: Conductive impulses slower.
Refractory Periods: The refractory period is defined as the time required by the cardiac tissue to recover from an electrical impulse before it is capable of conducting another one. The two pathways have distinct refractory characteristics:
- Pathway B: Exhibits a slower recovery time, resulting in a longer refractory period.
- Pathway A: Exhibits a faster recovery time, resulting in a shorter refractory period.

Entry of Premature Beat: The circuit is initiated when a critically timed premature beat enters the pathways.
Initial Blockage: Because Pathway B has a longer refractory period, it may still be refractory when the premature beat arrives; consequently, the impulse is blocked in Pathway B.
Path of Conduction: Because Pathway A has a shorter refractory pe riod, it is not refractory and can accept the impulse, conducting it down the circuit.
Circuit Completion: Due to the slower conduction speed of Pathway A, by the time the impulse reaches the end of the circuit, Pathway B has had sufficient time to recover. This allows the impulse to travel retrograde (back up) the circuit via Pathway B.
Sustained Tachycardia: The impulse then reenters Pathway A and continues to travel in the same repetitive loop, establishing a reentry tachycardia.

Rhythm: Generally regular, though it can occasionally be irregular.
Rate: In a typical recording, the atrial rate may be approximately $210$ , resulting in a ventricular rate of $110\text{ bpm}$ .
P Waves: Characterized by the presence of flutter waves, or "F waves."
Sawtooth Pattern: Atrial flutter typically displays a classic "sawtooth" pattern on the ECG.
P-R Interval: F waves appear consistent. A typical ratio is $2$ F waves for every $1$ QRS complex ( $2:1$ ), though $3:1$ conduction is also common.
QRS Duration: The typical duration is $0.12\text{ sec}$ . The normal recognized range is $0.04-0.12\text{ sec}$ .

Definition: A Supraventricular Tachycardia (SVT) of the right atrium (RA).
Circuit Location: It originates from a reentrant circuit orbiting the tricuspid annulus (TVA).
Anatomical Structures: The circuit utilizes specific anatomical structures to form the loop, including:
- Superior Vena Cava (SVC).
- Inferior Vena Cava (IVC).
- Coronary Sinus (CS).
- Crista Terminalis.
Direction of Flow:
- Counter-clockwise (Most Common): Impulse flows counter-clockwise around the TVA. This produces negative flutter waves in Leads II, III, and aVF, and positive flutter waves in Lead V1.
- Clockwise: Impulse flows clockwise around the TVA. This produces positive flutter waves in Leads II, III, and aVF, and negative flutter waves in Lead V1.
CavoTricuspid Isthmus (CTI): This is a critical strip of tissue located between the IVC and the Tricuspid Valve. The CTI is considered a vital part of the circuit and is the gold standard location for therapeutic ablation.

Definition: Any atrial flutter that originates from a location outside of the typical CTI-dependent pathway.
Etiology: Typically caused by scarring in the atria resulting from prior cardiac procedures (such as surgery or ablation) or underlying heart diseases including:
- Heart Failure.
- Coronary Artery Disease (CAD).
- Cardiomyopathy.
- Rheumatic Heart Disease.
- Pericarditis.
Common Right Atrial (RA) Locations:
- Upper loop flutter (where the CTI is not involved in the circuit).
- Flutter circulating around a surgical scar.
- Flutter circulating around an atrial septal defect (ASD) repair patch.
Common Left Atrial (LA) Locations:
- Circulating around the pulmonary veins.
- Circulating around the mitral valve.
- Over the roof of the Left Atrium.

Coronary Sinus (CS) Activation: The activation pattern observed in the CS during flutter is used to narrow down the site of origin.
Distal to Proximal Activation: Indicates an origin in the Left Atrium.
Proximal to Distal Activation: Usually indicates a Right Atrial origin, though in certain cases, it may still be Left Atrial.

Focal AT Definition: Characterized by a rapid, regular rhythm originating from a discrete area in the atria.
Mapping Objective: Identify the P wave (atrial depolarization) of the specific beat of interest.
Marking Guidelines:
- Select the depolarization phase of the atrial beat(s).
- Mark the start and end times at inflection points on the ECG voltage trace.
- If mapping multiple beats, ensure inflection points are consistent across leads.
- Use the lead(s) with the earliest and latest depolarization times as the reference for markings. Scrolling vertically through leads helps identify the best reference.
- Atrial Tachycardia segments typically range between $80-150\text{ ms}$ in duration.
Exclusion Criteria (Do Not Select):
- Normal QRS complexes.
- T waves.
- Pacing spikes.
- Quiescent periods.
- Fused beats.
- Periods with contaminating noise (e.g., high-frequency noise or baseline wander).
- Segments where the P wave and T wave are not clearly distinct.

AF Definition: The most common cardiac arrhythmia, associated with disorganized electrical activity and irregular ventricular contractions. It results from rapid triggers or functional reentry, leading to extremely rapid atrial rates.
ECG Presentation: P waves are typically absent due to the rapid rate. Key features include an irregular RR interval and the lack of a clear isoelectric baseline.
Marking Guidelines:
- Select atrial activity starting at the offset of a T wave and ending at the onset of the following Q wave (TQ segments).
- Ensure lead voltages at the start and end points of the interval are as close to the isoelectric baseline ( $0\text{ voltage}$ ) as possible.
- vMap automatically subdivides selected segments into individual atrial beats found between a T wave and the subsequent QRS complex.
- Mapping is more effective for ECGs with long RR intervals.
Exclusion Criteria (Do Not Select):
- Start or end times occurring in the middle of a QT interval.
- T waves.
- Periods with contaminating noise (high-frequency noise or baseline wander).

Typical Flutter: Can be mapped utilizing either the AFib Library or the AT Library in the vMap software.
Atypical Flutter: Similarly, atypical cases can be analyzed using either the AFib Library or the AT Library to determine the site of origin.
3D Map Correlation: vMap data correlates with 3D anatomical maps (e.g., showing a cycle length of approximately $410\text{ ms}$ and specific voltage markings at $-114\text{ mA}$ to $110\text{ mA}$ ) to pinpoint the reentrant circuit or focal origin.