Type I (Counterclockwise, Common) Atrial Flutter (after a sketch for Dr. Gregory K. Feld, University of California - San Diego, CA)
Role of Crista Terminalis in Atrial Electrical Compartmentalization(based on work by Dr. Michel Haissaguerre, Bordeaux, France; and Dr. M. Lesh, Arrhythmia Service, UCSF, San Francisco, CA)
Atypical Atrial Flutters(coming soon)

Atrial flutter is a stable monomorphic intra-atrial macroreentrant tachycardia which has been described in all ages but most commonly occurs in patients with ischemia or pressure/volume overload. In a substantial majority of cases the substrate for generalization to atrial fibrillation is already present and hence a majority of atrial flutters eventually transition to fibrillation. As a rule atrial flutters depend on a protected isthmus to provide critical spatial and temporal delay in maintaining a large, stable circuit. Ablation of this isthmus destroys the circuit. The most common circuit in humans is a counterclockwise loop down the trabeculated side of the crista terminalis and up the septum through an isthmus between IVC, TA, and CS (type I flutter). Cartoon of tricuspid annulus shown as hatched band.(Figure after Greg K. Feld MD, UCSD Electrophysiology, San Diego, CA.)

Left. This open right atrium (from a fluoro spot shot) shows the relative positions of specialized mapping catheters and a 10-mHz intracardiac ultrasound catheter used to define the anatomic landmarks and barriers to type 1 atrial flutter.
Along the crista terminalis (displaced with the open flap of RA free wall) lies a special 20-pole fitted catheter to map conduction along and across this thick muscular ridge. Against the septum is another catheter to characterize wavefronts moving up and down the septum.

In the most common form of flutter, ablation at the narrow channel between IVC and TV (sharply flexed catheter in foreground on left) is relatively simple and usually curative. The fibrous valve rings and smooth muscle ostia are absolute barriers to conduction; a small band of slowly-conducting transitional myocardium threads through this gap.

Right. Another specialized 14-pole atrial mapping catheter used to record simultaneously from the septum (proximal four to six electrodes) and the RA free wall (distal eight electrodes) to determine conduction delay across the crista terminalis. When lone AF or atrial flutter is present in a young person, automatic foci with very high intrinsic rates may sometimes be found in the fossa ovalis region in this manner. These foci drive the atria so rapidly that wave fragmentation and reentry occur, causing AF. Ablating them largely terminates the AF. Populations of myocytes in and around the fossa ovalis and atrial septum have been shown to be capable of such automaticity in the normal heart in dogs and in humans.

(Diagram adapted from fluoro images courtesy of Dr. Michael Lesh, Arrhythmia Services, University of California - San Francisco; and Dr. Michel Haissaguerre, Hopital Cardiologique du Haut-Leveque, Bordeaux, France.

Atrial flutter circuits may encompass any part of the two atria, including left atrial structures, so long as conditions for reentry are met. As the wave of depolarization meanders around electrical barriers (scars, vessels, valves, suture lines) through narrow areas with impaired conduction, it eventually returns to its region of origin which has since repolarized, giving rise to the endless loop. Thus right or left atrial incisions, including atrial septum repairs, transplant anastomoses, and valve repairs, can give rise to reentrant atrial tachycardias and flutters of bizarre morphology requiring detailed, simultaneous 3-D mapping of both atria to identify appropriate ablation sites. Coming soon: An example of atypical atrial flutter in the setting of old atrial incisions.