The left atrial appendage (LAA) is a special structure of the heart, with important differences in anatomic structure and physiology from the left atrium (LA). Embryologically, the LAA has a different origin from the rest of the LA. On day 22 of gestation, the cardiac tube is formed and starts to fold. Its caudal part folds posteriorly, cranially, and to the left, creating the common atria. The septum primum and secundum create the interatrial septum between days 27 and 35. Beginning at day 35, there is intimate contact of the LA with developing pulmonary buds. From days 35 to 50, fusion of the pulmonary veins (PVs) in the LA evolves into what will become the posterior wall of the adult LA that is formed adjacent to the PV tissue. The primary remnant of the original common atria in the LA is the LAA. This separate origin explains the difference in anatomy in the LA and the LAA. It also explains the prominent trabeculations in the LAA and in the right atrial appendage that are not present in the main body of the LA, right atrium (RA), or interatrial septum. This anatomic difference is also seen at the cellular level and in the relative degree of innervation by the automatic nervous system. There is also an important difference between the functions of the body of the LA and its appendage (LAA), especially in atrial peptide secretion.

The LAA is an extension on the left side of the LA and normally represents approximately 6% to 7% of the total left atrial volume. The normal size of the LAA is around 44.4 ± 10.9 mm in length, 4.5 ± 2.3 mm in width and 6.9 ± 2.7 mL in internal volume. Externally, the LAA overlaps the left atrioventricular (AV) groove, and its tip usually reaches the right ventricular outflow track. In rare instances, the LAA tip may be oriented toward the transverse sinus in close contact with the left main coronary artery. Commonly, the LAA has small, thin fat pads on the outer surface. Its shape varies and may have one to three lobes. The most common configurations are either one lobe (51%) or two lobes (41%), with the three-lobe configuration being the least common (8%), though the three-lobe configuration is more often associated with thrombus formation independent of clinical risk and stasis. The LAA shape has been defined over the years in types, subtypes, or different shapes. It has been described as being as a horseshoe, cockscomb, paper fan, wing, mountain, hook, or swan. However, the most accepted morphologic classification has categorized the LAA into four anatomic groups: chicken wing, windsock, cauliflower, and cactus. There is a certain overlapping between these shapes, and on occasion the same appendage can be defined in a different shape depending on the perspective from which it is viewed ( Fig. 41.1 ). As an example, the four “anatomic types” of LAA shown in Fig. 41.1 are actually different imaging angles of the same LAA! Such apparent variations in anatomy based on imaging angles, rather than actual anatomic differences, calls into question the use of this classification system in deciding whether or not to close the LAA at the time of catheter ablation or cardiac surgery.

The four anatomic shapes of the left atrial appendage (LAA) based on common cardiac imaging techniques. Surprisingly, the apparent differences in the anatomic shapes displayed here were created from a silastic mold of the same LAA that was simply rotated for photographic purposes in four different planes of view. In other words, this is the same LAA. See text for further discussion.

(Courtesy Professor Andrew Cook, University College London, United Kingdom.)

The LAA has important muscular trabeculations that are largely responsible for the force of its brief active contraction ( Fig. 41.2 ). Indeed, large LAA pectinate muscles seen on echocardiograms are often difficult to differentiate from LAA thrombi. The orifice of the LAA varies in size, shape, and position. It is usually located between the orifices of the left PVs and the mitral valve at the level of P1. The LAA orifice can be above the left superior PV (LSPV) (type I, 9%), anterior to the LSPV (type II, 78%), or below the LSPV (type III; 13%). The shape of the orifice can be round, oval (69%), triangular, tear-drop shaped, or footlike.

Retroillumination of the human left atrium highlighting variations in atrial wall thickness. The numbers correspond to the (1) left superior pulmonary vein (PV), (2) left inferior PV, (3) pericardial recession, (4) coumadin ridge between the left superior PV and the left atrial appendage, (5) left atrial appendage, and (6) fat pad in the atrioventricular groove.

The area of the LAA orifice changes significantly during each cardiac cycle, becoming smaller during the atrial contractile (pump) phase. The primary reason is the specific anatomy of the distal end of Bachmann’s bundle. This important bundle of muscle fibers is located on the superior surface of the atrial muscle mass and extends from the top of the RA to the top of the LA. Although its primary purpose is to convey sinus impulses rapidly from the RA to the LA, its distal muscle fibers diverge at the base of the LAA, resulting in their surrounding the orifice of the LAA (see Chapter 3 , Fig. 3.12 ). One limb of these distal fibers of Bachmann’s bundle pass between the LSPV and the LAA, where the atrial wall is particularly thick. This thickened area of the LA between the LSPV and the LAA is commonly called the “coumadin ridge” ( Figs. 41.2 and 41.3 ). In Chapter 48 , which describes postoperative peri-mitral atrial flutter, this area is called the “LAA–LSPV isthmus” (see Chapter 48 , Figs. 48.7 and 48.10 ). Because of its thickness, it is quite difficult to create a transmural lesion across the coumadin ridge using unipolar radiofrequency (RF), whether it is delivered endocardially with a catheter or epicardially with a unipolar RF pen.

Internal view of the human left atrium. The numbers correspond to the (1) mitral vestibule (LAA-MV isthmus), (2) left atrial appendage, (3) coumadin ridge, (4) extrapectineal trabeculations that can be seen in 30% of patients, (5) mitral valve, (6) fossa ovalis, and (7) assorted other endocardial pouches.

The anterior limb of the distal Bachmann’s bundle lies between the LAA orifice and the mitral valve at the level of P1, an area called the “mitral vestibule” ( Fig. 41.4 ). In Chapter 48 describing postoperative peri-mitral atrial flutter, this mitral vestibule is called the “LAA–MV isthmus” (see Chapter 48 , Figs. 48.7 and 48.11 ). The mitral vestibule is a continuation of what was the embryologic LAA and harbors extrapectinate trabeculations with very thin atrial walls between the thick trabeculations in approximately 30% of patients. ^, The relative locations of the anterior and posterior limbs of Bachmann’s bundle result in the LAA orifice being surrounded by its distal muscle fibers. In patients with atrial fibrillation (AF), the orifice tends to dilatate and becomes more round with no variation during most of the cardiac cycle. It is important to note that two-dimensional transesophageal echocardiography (TEE) usually underestimates the diameter and circumference of the LAA orifice compared with three-dimensional TEE.

Two views of the same human heart with the blue line in the upper panel indicating the plane of the photo in the lower panel . The differences in thickness of the atrial wall can be appreciated. (1) Coumadin Ridge (“LAA-LSPV isthmus”), (2) left atrial appendage and mitral vestibule (“LAA-MV isthmus), (3) circumflex artery, (4) mitral valve at the level of P1.