Complex Atrial Flutter: Beyond the Standard Circuit

When flutter is not the straightforward circuit it appears to be

In my previous article on atrial flutter, I described the most common form — typical cavotricuspid isthmus-dependent flutter — and its highly predictable response to CTI ablation. For the majority of patients, this is the complete picture. The circuit is well understood, the ablation target is clearly defined, and outcomes are excellent.

But a significant minority of patients with flutter have what we call atypical or complex flutter — arrhythmias that involve circuits other than the standard right atrial isthmus. These are more challenging to diagnose, more demanding to ablate, and require the kind of detailed three-dimensional electroanatomical mapping and electrophysiological expertise that is the hallmark of a high-volume specialist centre. This is an area in which I have both a research track record and extensive clinical experience.

What Makes Flutter ‘Complex’?

Complex flutter is broadly defined as any macroreentrant atrial arrhythmia whose circuit does not depend on the cavotricuspid isthmus. The main types include:

• Perimitral flutter: A re-entrant circuit that travels around the mitral valve annulus in the left atrium. This is the most common form of left atrial flutter and is frequently seen in patients who have previously undergone
• AF ablation, as the ablation lesion lines create new anatomical barriers that can form the boundaries of a perimitral circuit.
• Roof-dependent flutter: A circuit involving the roof of the left atrium, often bounded by ablation lesion lines from prior PVI.
• Scar-related flutter: Circuits anchored to areas of atrial fibrosis, whether from prior ablation, structural heart disease, or idiopathic scarring. These are the most challenging to characterise and ablate.
• Cristal flutter: Circuits involving the crista terminalis in the right atrium — a muscular ridge that can form an unexpected conduction barrier.

The Diagnostic Challenge

The characteristic ‘sawtooth’ ECG pattern that reliably identifies typical CTI flutter is not present in atypical flutter. The ECG morphology is variable and often does not immediately suggest a re-entrant mechanism. Patients frequently receive a diagnosis of ‘atrial tachycardia’ or even ‘fast AF’ before the re-entrant nature of the rhythm is recognised. This diagnostic imprecision matters, because the ablation approach is completely different.

Definitive diagnosis and mapping of complex flutter requires invasive electrophysiology study with high-density, high-resolution three-dimensional mapping. At OneWelbeck Heart Health, I use both the CARTO and Precision systems, which allow me to create detailed activation maps of the atria during tachycardia — coloured voltage and timing maps that make the circuit visible and the ablation target explicit.

I have published research on the characterisation and ablation of complex flutter circuits, including perimitral flutter (published 2017) and cavotricuspid isthmus-dependent flutter, as well as the 2022 RIPPLE mapping study for atrial tachycardias. These publications reflect a sustained research interest in the electrophysiology of complex macroreentrant circuits.

Ablation of Complex Flutter: Approach and Outcomes

Ablation of complex flutter follows the principle of identifying and creating a conduction block line across the isthmus of the re-entrant circuit — but in atypical flutter, that isthmus may be in the left atrium, may be bounded by areas of scar, or may be narrow and technically demanding to cross completely.

For perimitral flutter, the standard ablation approach is a mitral isthmus line — a line of lesions connecting the left inferior pulmonary vein to the mitral annulus. Achieving complete bidirectional block across this line requires meticulous technique, as the mitral isthmus region is often thick-walled and contains the epicardial vein of Marshall, which requires additional strategies to ablate completely.

Success rates for complex flutter ablation are lower than for typical CTI flutter, reflecting the greater technical complexity — but in experienced hands, durable termination of the arrhythmia and freedom from recurrence are achievable for the majority of patients.

Who Should Perform Complex Flutter Ablation?

The honest answer is: a specialist electrophysiologist with substantial experience in complex atrial arrhythmia mapping and ablation, working in a centre equipped with high-density three-dimensional mapping systems. Complex flutter is not a procedure for the generalist cardiologist or the occasional electrophysiologist.

If you have been told that your flutter is ‘difficult’ or ‘unusual’, or if you have previously had an ablation for flutter that did not work, I would encourage you to seek a second opinion at a specialist centre. With the right expertise and technology, most complex flutter circuits can be successfully characterised and treated.

To discuss SVT or palpitations with Dr Boon Lim, book a consultation at OneWelbeck Heart Health — drboonlim.co.uk or call 0208 187 9888