Atrial Flutter and Atrial Fibrillation

Atrial flutter (AFl) shares the same risk factors as atrial fibrillation (AF). In remodelled atria, most commonly in the right atrium, a re-entry circuit develops along which the electrical impulse can circulate, most commonly at a rate of 240–300/min.

For the impulse to start circulating within a re-entry circuit, a trigger is required; the most common triggers are:

• atrial premature beat
• AF (short episode)

20 % of patients with AF also have AFl (typical or atypical).

In 50 % of patients with AFl, AF develops in the future.

The thromboembolic risk in AFl is the same as in AF:

• therefore, anticoagulation therapy in AFl is initiated as in AF, according to the CHA₂DS₂-VA score.

AFl is classified according to the location of the re-entry circuit and the direction of impulse rotation into:

• Typical AFl
• CCW typical AFl (Counter-Clockwise AFl)
• CW typical AFl (Clockwise AFl)
• Atypical AFl

AFl is present in 1 % of the population, of which:

• CCW typical AFl (80 %)
• CW typical AFl (10 %)
• Atypical AFl (10 %)

Typical AFl means that the re-entry circuit is located in the right atrium and proceeds as follows:

• Down the free wall of the right atrium (anterior to the crista terminalis)
• Through the cavotricuspid isthmus (CTI)
• Up through the atrial septum
• Then across the roof of the right atrium back to the free wall of the right atrium.
• The diameter of the re-entry is approximately 3 cm, corresponding to a re-entry path length of approximately 9 cm.
• Depending on the direction of impulse rotation within the re-entry circuit, typical AFl is divided into:
• CCW AFl (Counter-Clockwise)
• the impulse rotates counter-clockwise
• CW AFl (Clockwise)
• the impulse rotates clockwise

Atypical AFl means that the re-entry circuit differs from that of typical AFl.

• In atypical AFl, re-entry may occur through any anatomical region in the left or right atrium.
• Atypical AFl is not classified according to the direction of impulse rotation.
• The diameter of re-entry in atypical AFl is at least 1 cm, corresponding to a re-entry path length of approximately 3 cm.

Unblocked AFl means that the AV node begins to conduct AFl impulses to the ventricles with 1:1 conduction (without block).

• The resulting ventricular rate is 240–300/min (the patient is hemodynamically unstable).
• Unblocked AFl may occur
• during treatment of AF with class IC antiarrhythmic drugs (Propafenone, Flecainide).
• Therefore, class IC antiarrhythmics are administered together with drugs that slow AV nodal conduction (beta-blockers, Verapamil, Diltiazem).
• The risk of unblocked AFl is 2–6 %
• in patients with AF or AFl receiving class IC antiarrhythmics without AV nodal blocking drugs,
• 20 % of patients with AF also have AFl, which may not be documented on ECG.

Mechanism of unblocked AFl during class IC antiarrhythmic therapy in patients with AFl:

• Class IC antiarrhythmics slow conduction within the re-entry circuit.
• An impulse circulating at approximately 300/min is slowed to 200–240/min.
• The impulse thus circulates more slowly within the re-entry circuit and with each “turn” begins to conduct to the ventricles through the AV node,
• because impulses reach the AV node at a rate at which 1:1 conduction becomes possible.
• At higher rates, impulses encounter the effective refractory period of the AV node,
• and the AV node blocks conduction, for example with 2:1 or 3:1 conduction.

When treating AF or AFl with class IC antiarrhythmics, AV nodal blocking drugs must always be co-administered:

• Beta-blockers, Verapamil, or Diltiazem,
• AV nodal blocking drugs prevent the occurrence of unblocked 1:1 AFl.

For treatment of AFl, radiofrequency ablation is recommended because antiarrhythmic therapy has minimal effect.

• The principle of pharmacological treatment of AFl is to slow conduction of AFl through the AV node,
• so that AFl is conducted to the ventricles at a rate of <100/min.
• AV nodal blocking drugs are used: beta-blockers, Verapamil, Diltiazem.
• Pharmacological therapy usually cannot terminate the re-entry circuit.
• Class IC antiarrhythmics slow re-entry but do not terminate it; there is a risk of unblocked 1:1 AFl,
• therefore class IC antiarrhythmics are always administered together with AV nodal blocking drugs:
• beta-blockers, Verapamil, Diltiazem.