Explaining Flutter waves morphology

Answer: “Typical” Counterclockwise atrial flutter with predominantly 3:1 AV conduction. There is also likely a Left Anterior Fasicular Block.

• Atrial flutter is a re-entrant circuit and relies on the same properties of re-entry to sustain

1. Slow limb (usually diseased tissue or region of anatomical disruption such as cavotricuspid isthmus)
2. Faster limb with more rapid conduction velocity.
3. Ectopic of initiation

Atrial flutter is more common in people with a history of cardiac disease, scar burden, atrial dilation, valvular pathology etc as these conditions can place the atria under stress, promote fibrosis and are therefore more likely to possess the substrate necessary to initiate & sustain atrial flutter. Fibrosis is an excellent substrate for sustaining a re-entrant arrhythmia as it can promote slowing of conduction velocity as well as conduction block.

The re-entrant circuit of CCW CTI dependent flutter is shown below.

• The re-entry circuit is confined to RA & utilises the Cavotricuspid isthmus region as the “slow limb” of re-entry, accounting for roughly 33% of the total tachycardia cycle length.

• Also involves the interatrial septum & RA lateral wall & RA roof (fast limb) and accounts for 66% of the reentry time. Usually 250-350bpm (240-170ms).

Explaining the ECG morphology of Atrial flutter

There are 3 main features that are all usually present in the context of anticlockwise typical CTI dependent atrial flutter:

1. Negative sawtooth pattern in lead II
2. Positive Flutter waves in V1
3. Negative or isoelectric Flutter waves in V6.

• CTI dependent Atrial flutter waves rarely have a “resting baseline” in the inferior leads, because the majority of the flutter circuit is usually travelling either against, or with, the lead II vectors, producing continuously changing negative and positive deflections respectively.

Explaining the Flutter wave ECG

• The cranial-Caudal activation occurs relatively quickly, as the Crista terminalis conducts much faster in a downward direction (compared to an upward direction). This makes some sense, as sinus rhythm would conduct in a downward fashion in most circumstances. This leads to a sharp positive upstroke in the inferior leads (Red)

• The CTI codnuction velocity is slow, due to fibre disruption and other factors including gap junction expression (lets not go there today…). (Teal)

• Cranial to Caudal activation up the septum and Left Atrium goes against the inferior lead vectors, producing a prolonged downward deflection in the inferior leads (purple)

My two cents:

• Counterclockwise CTI dependent atrial flutter accounts for almost 90% of all atrial flutters encountered in the wild.

• Risk factors include any which promote atrial remodelling & slowing of conduction velocity –  valvular disease, hypertension, diabetes, ischaemic heart disease etc.

• The anatomy of the Right Atrium dictates the conduction velocity of the atrial flutter circuit, which in turn dictates the morphology of atrial flutter waves on the ECG, producing the classic “saw tooth” pattern.

• Anticlockwise atrial flutter is characterised by:
  1. Negative sawtooth pattern in lead II
  2. Positive Flutter waves in V1
  3. Negative or isoelectric Flutter waves in V6.

Thanks for tuning in :)
Cheers
Mitch & CPP Team

For more on Atrial Flutter, see our ECG course here, or our EP course here for more detail.