Peri-mitral atrial flutter is a common postoperative arrhythmia that often occurs after isolation of the pulmonary veins (PVs), especially when the posterior wall of the left atrium (LA) is also encompassed to create a so-called “box lesion” around all four PVs ( Fig. 48.1 ). Unlike classic right atrial flutter, which occurs spontaneously in the clinical setting, peri-mitral atrial flutter does not occur clinically except after catheter or surgical ablation. Indeed, peri-mitral atrial flutter is a classic example of an iatrogenic arrhythmia defined as “an adverse condition that results from treatment by a physician, nurse, or allied health professional.”
The box lesions in both the Maze-III procedure (A) and the Maze-IV procedure (B) encompass the orifices of all four pulmonary veins (PVs) and the intervening posterior left atrial wall between the right and left PVs. From an electrophysiological standpoint, they are identical. However, they are created by different ablation techniques in the two iterations of the Maze procedure. Ironically, the creation of a perfect box lesion in the left atrium appears to be the essential component necessary for the development of iatrogenic postoperative peri-mitral flutter.
Peri-mitral atrial flutter was called “atypical left atrial flutter” from the first time it was ever diagnosed following the second Maze procedure in 1988 until catheter ablation for atrial fibrillation (AF) was introduced some 10 years later. There was no apparent reason for changing the name of the arrhythmia, but historically, it has been a common practice for electrophysiologists (EPs) to change the names of electrophysiological events for no particular reason. Thus, readers should be neither confused nor surprised by the fact that “peri-mitral flutter,” “atypical flutter,” “atypical atrial flutter,” and “atypical left atrial flutter” all refer to the same iatrogenic arrhythmia. However, as readers will learn in this chapter, the moniker “atypical left atrial flutter” is a more accurate description of the arrhythmia than “peri-mitral flutter” because of the multiple anatomic routes the macro-reentrant circuit of this arrhythmia can take to complete one cycle. Much like the three potential macro-reentrant drivers of classic right atrial flutter (see Chapter 12 , Fig. 12.3 ), the macro-reentrant driver of atypical left atrial flutter can use different anatomic structures that have nothing to do with the mitral valve (MV). These structures include the isthmus between the left atrial appendage (LAA) orifice and the left superior PV (LSPV; commonly called the “coumadin ridge”) the isthmus between the LAA orifice and the MV annulus, the coronary sinus, and the myocardium of the posteroinferior portion of the left atrial isthmus between the lower PVs and the MV annulus. Nevertheless, because “peri-mitral flutter” has been adopted by our electrophysiology colleagues and is perhaps easier to remember, we will use this designation for the remainder of this chapter.
Electrophysiology of Peri-mitral Atrial Flutter
The macro‐reentrant circuit that drives peri-mitral flutter following a simple box lesion in the LA uses the atrial septum, anterior left atrial wall, lateral left atrial wall, and posterior left atrial isthmus ( Fig. 48.2 ). In the left atrial isthmus, the peri-mitral flutter wave can propagate across either the posteroinferior left atrial myocardium or through the coronary sinus. The macro-reentrant circuit is physically larger than that of classic atrial flutter in the right atrium (RA), and as a result, it takes longer for electrical activity to complete one complete cycle around the circuit. Thus peri-mitral flutter is said to have a longer “cycle length” than that of classic right atrial flutter.
The macro-reentrant circuit of peri-mitral atrial flutter after a simple box lesion in the left atrium (LA) (not shown) uses all sides of the LA. Posteriorly, it can also use the coronary sinus. (See text for further discussion.)
The cycle length of classic right atrial flutter is 200 ms, meaning that electrical activity propagates around the circuit five times per second, or 300 times per minute ( Table 48.1 ). This results in a regular atrial rate of 300 beats/min, which is blocked 2:1 in the atrioventricular node, resulting in a regular ventricular rate of 150 beats/min. However, the cycle length of peri-mitral flutter is approximately 250 ms, meaning that electrical activity propagates around the peri-mitral circuit only four times per second, resulting in a regular atrial rate of 240 beats/min and a regular ventricular rate of 120 beats/min.
TABLE 48.1
Differences in the Cycle Length of the Macro-reentrant Driver of Classic Right Atrial Flutter and Postablation Peri-Mitral Flutter
| Electrophysiology | Classic RA Flutter | Peri-Mitral Flutter |
|---|---|---|
| Cycle length (ms) | 200 | 250 |
| Cycles/s | 5 | 4 |
| Atrial rate (beats/min) | 300 | 240 |
| Atrioventricular node block | 2:1 | 2:1 |
| Ventricular rate (beats/min) | 150 | 120 |
A common pitfall relating to peri-mitral flutter is that it can be easily mistaken for sinus tachycardia immediately after AF surgery or catheter ablation. As an example, imagine that you are performing an off-pump thoracoscopic procedure to ablate AF that has been present for the past 5 years. During the ablation procedure, the AF suddenly breaks into a stable, regular tachycardia of 120 beats/min. Chances are that you and your surgical team will be elated at the successful ablation because it appears that the patient is now in a sinus tachycardia that will be expected to slow down and stabilize with time. However, it is extremely rare for a patient to have sinus tachycardia immediately after surgical ablation of long-standing persistent AF because the sinoatrial node has been suppressed for so long that it takes quite some time to recover. Thus, sinus bradycardia is far more common during the early post-ablation period in patients who have had AF for long periods of time. Therefore, one should be aware that any time a stable heartbeat of around 120 beats/min is seen in the operating room immediately after AF ablation, it is likely that the patient has peri-mitral flutter, not sinus tachycardia.
Lesions of the Maze Procedure That Prevent Postoperative Peri-mitral Atrial Flutter
As with any reentrant circuit, the macro-reentrant circuit that drives peri-mitral flutter can be ablated by interrupting it in only one place. In the Maze procedure, we chose to preclude postoperative peri-mitral flutter by interrupting conduction across the left atrial isthmus because we believed this isthmus to be the narrowest and most accessible common anatomic pathway in all the different reentrant circuits that can cause the arrhythmia ( Fig. 48.3 ). However, we learned during the development of the left atrial isolation procedure nearly a decade before the first Maze procedure (see Chapter 9 , Fig. 9.14 ) that electrical conduction across the left atrial isthmus can occur through the posteroinferior left atrial myocardium or through the walls of the coronary sinus ( Fig. 48.4 ). Therefore, to block conduction across the left atrial isthmus, it is necessary to create lesions in both the atrial muscle (the endocardial “mitral line”) and in the coronary sinus with a circumferential cryolesion ( Fig. 48.5 ). It is critical to place the mitral line and the coronary sinus cryolesion in the same plane . Otherwise, the electrical activity can skirt around the coronary sinus cryolesion and continue to propagate across the left atrial isthmus ( Fig. 48.6 ).
Left atrial isthmus. The left atrial isthmus (yellow dashed rectangle) is the part of the left atrium between the inferior pulmonary veins and the posterior mitral valve annulus and includes the left atrioventricular groove that harbors the coronary sinus.
Both the left atrial myocardium and the coronary sinus in the left atrial isthmus are capable of conducting electrical activity, and they can both be a part of the macro-reentrant peri-mitral atrial flutter (see Fig. 48.2 ).
