Advanced Cardiac Anatomy: Application in Translational Research Tailored to Current and Future Technology

Preclinical animal studies have played a crucial role in device development from the era of surgical prosthetic valve development to today with percutaneous implantation of stents, stent-grafts, aortic grafts, and valves. Many animal models have been utilized, such as dogs, pigs, and sheep, but to date, none of them are identical to man especially when disease is present. Understanding the anatomic differences between human and animal models is of the utmost importance. The main purpose of preclinical animal studies is to confirm the safety of the devices prior to performing human studies. The competence of the implanter and the pathologist interpreting the results is critical.

Prior to the submission of preclinical studies to the U.S. Food and Drug Administration (FDA), the FDA encourages meetings before the performance of animal studies that are to be carried out under the guidlines of the Good Laboratory Practice (GLP). These are intended to assure the quality of the studies, the integrity of the safety data, type of studies, choice of animal model, and adequate numbers and duration of the studies needed.

The choice of the animal model depends on the device being evaluated. It is important to determine which animal model is the most appropriate that closely resembles man. Normal animals are the best for the evaluation as disease models likely introduce more variability; however, some devices are meant to treat iatrogenic diseases and may require implantation of a device prior to a new treatment. Each animal model has both merits and demerits, and there is a need to choose the most suitable model depending on the type of devices and the size of devices being evaluated. Similarities in the size, anatomic distribution, and time-dependent progression of coverage or intimal/endothelial growth over the device are important parameters to keep in mind when selecting the animal model (1).

The pig model has historically been used most frequently in the last 2 decades for vascular implants including coronary, peripheral, aortic, and recently, even for valves. Pig currently rivals canine and sheep as the most commonly used animal model in experimental cardiology for vascular work, but most of the valvular work in the past was done in sheep while canines are no longer used; in fact, the canine model has been banned in a large number of states in the United States. Smaller animal models (e.g., rabbit iliac artery) can provide complementary data because of less inflammation and delayed endothelialization as compared to the pig model. Also, optimal dose-finding studies and mechanism of action to understand the role of underlying atherosclerotic disease or the development of neoatherosclerosis following device implantation may be accomplished in the rabbit model while significantly reducing the cost of such studies.

The human heart is a trapezoidal shape, the apex of which is formed by both the right and left ventricles. The left anterior descending coronary artery runs in the interventricular sulcus, which is located anteriorly from the base of the heart to the apex (2). On the other hand, pig and sheep hearts are Valentine-shaped because of differences in the rib cage due to four-legged posture as compared to man with an erect posture who walks on two legs (1). In the pig and sheep, the proportion of the right ventricle in the anterior view is smaller than in human heart, and therefore, the apex is formed only by the left ventricle and the left anterior descending artery is more to the right and center than in the human heart (Figure 1). The posterior interventricular groove is located along the midpoint of the diaphragmatic surface and divides the posterior surface of the heart into one-third occupied by the atria and two-thirds by the ventricle. At the crux of the heart, the atrioventricular groove and the right side chambers (atrium and the ventricule) are separated from the left side chambers.

Figure 1. Anterior View of Pig, Sheep, and Human Hearts

In pigs, sheep, and canines, the right and left atrial structure is separated by the interatrial septum. The atria are located at the base of the heart, which includes the sinus venosus, crista terminalis, fossa ovalis, Eustachian valve (valve of the inferior vena cava), and Thebesian valve (valve of the coronary sinus) (3). All large mammalian atria also have an atrial appendage the size and shape of which varies among species (2,3). Compared to the human, the location of the fossa ovalis in the pig is deep-set and superior and closer to the orifice of the supra vena cava, while in the sheep, it is more posterior and caudal (Figures 2A and 2B). The length of the interatrial septum is longer in the pig as compared to humans. The fossa ovalis has a valve-like flap in the left atrium that prevents the blood flow from the left atrium into the right atrium after birth, while in utero, the blood flows from the right atrium into the left atrium bypassing the lung due to free communication between the two atrial chambers.

Figure 2. Fossa Ovalis Location

The right and left ventricles of pigs and sheep contain essentially the same components that are structurally very similar to the human heart, forming inlet, apical, and outlet regions. While left ventricular walls are more muscular than those of the right ventricles, the walls of both ventricles near the apex have inter-anastomosing muscular ridges and columns termed the trabeculae carinae that serve to strengthen the walls and increase the force exerted during contraction (1). The trabeculations in pig and sheep hearts are much coarser than human hearts (Figure 3). The right ventricle has three papillary muscles and the atrioventricular valve, i.e., tricuspid valve, has three leaflets whereas the left ventricle has two papillary muscles (anterolateral and posteromedial) and two atrioventricular leaflets of the mitral valve (anterior and posterior; also called the aortic and the mural leaflets with the latter having three scallops). The chordae tendineae of the mitral valve in the pig heart are shorter and coarser than in the human heart (Figure 3).

Figure 3. Three-chamber View of the Pig, Sheep, and Human Hearts 

The location of the papillary muscles is different in the human. The papillary muscles originates in the long-axis at the upper two-thirds and lower one-third of the ventricle in human hearts whereas in the porcine and ovine hearts, the papillary muscles originate close to the apex of the heart.  Also, the papillary muscles are more prominent in pigs and sheep models and are located anteriorly, close to one another, with the papillary heads closer to the atrioventricular annulus (Figure 4).  The trabeculations in the left ventricle are coarser in pigs and sheep as compared to humans. The left ventricular wall in pig and sheep hearts are much thicker than in the same size-matched human heart. Both the animal models have muscular bands that contain Purkinje fibers, which are unlike the human heart where these are less obvious and cannot be easily distinguished from the working myocardium.

Figure 4. Papillary Muscle Level Short-axis View of the Left Ventricle in Pig, Sheep, and Human Hearts

The moderator band is in direct continuity with the septomarginal trabeculations of the interventricular septum and is prominent in the right ventricle in the pig; it is located higher on the septal wall compared to the human heart. The moderator band is situated higher in sheep, pig, and dog hearts as compared to human hearts. In the pig heart, the moderator band is in direct continuity with the septomarginal trabeculation to the parietal wall of the right ventricle. On the other hand, in the human heart, the moderator band is less prominent and is in direct continuity with the anterior papillary muscle. The apical trabeculations are coarser in the apex of the right ventricle in sheep and pig hearts as compared to the human heart, which has finer trabeculae.

The subaortic outflow tract in both pig and sheep hearts, as in the human heart, have a fibrous continuity between the anterior mitral leaflet and the aortic valve; however, the extent to which the aortic valve is supported by ventricular musculature is different. In the pig, three-quarters of the circumference of the aortic valve is supported by the ventricular musculature, which results in the reduction of the membranous septum. Also, the fibrous trigone in pigs is more prominent than in humans. In this regard, two recent studies reported that the short “length” of the membraneous septum is a predictor of postoperative conduction damage in transcatheter aortic valve replacement, which is more frequent in the self-expandable valve (4,5) but is also observed in balloon-expandable valves. In sheep, the membraneous interventricular septum is absent. In a four-chamber view of the sheep heart, it is easy to appreciate that two-thirds of the heart is formed by the left chambers, while in the human, right and left chambers are of equal proportion with each occupying one half.

Pig, sheep, and canine hearts have epicardial coronary arteries similar to man and consist of two main arteries with ostia that are usually located just below the sinotubular junction in their respective right and left coronary sinuses. Dogs and sheep typically have a left dominant coronary artery that mainly supplies the blood to the left ventricular myocardium, while pigs have a balanced type of arterial distribution that supplies the left ventricular myocardium equally from both right and left coronary arteries (1,6). In human hearts, 90% have a right dominant system that supplies the posterior wall of the left ventricle, whereas in 10%, there is a left dominant system with the left circumflex supplying the posterior wall of the left ventricle. The coronary circulation of the pig heart has fewer collaterals between vascular branches, whereas the coronary circulation of the dog heart is extensively collateralized (7,8) and in man, it is variable. The venous system consists of three major venous pathways: coronary sinus; anterior cardiac veins; and Thebesian veins. All three are present in pig, sheep, dog, and human hearts. Although the structure of the venous system is similar in all, the main difference is that the left azygous vein drains the left thoracic cavity directly into the coronary sinus in pig and sheep but in man, the superior caval vein (oblique vein) is reduced or absent.

The pig heart is the favored model for preclinical cardiovascular research; however, it is not an ideal model for prosthetic valve evaluation. Young pigs grow rapidly within a short period of months and will result in enlargement of the valve annulus, therefore making it difficult for prosthetic valves research (9). Trabeculations in the right and left ventricle are coarse in pig, and therefore the ventricular and atrial cavities are smaller and do not allow the large devices that are the appropriate size for humans. Juvenile animals have a rapid calcium turnover and therefore are more likely to get accelerated calcification of prosthetic valves (10). Also, the pig does not tolerate a heart-lung machine as does the sheep model (11). On the other hand, sheep are widely used for preclinical studies for bioprosthetic valve research; the weight of the sheep at 12 months is approximately 50 lbs and the growth rate decreases after 12 months (12), whereas the weight of domestic pig at 3 months is approximately 55 lbs and increases rapidly to 220 lbs by 6 months (13). Mini swine are significantly smaller and at 6 months weigh approximately 45 lbs; however, they continue to grow rapidly, weighing close to 90 lbs at 1 year of life. Therefore the sheep is a better model for valve safety studies (14). Although all sizes cannot be used in pigs and sheep as their hearts are smaller than human diseased hearts, smaller implant sizes are acceptable for prosthetic valve work (14).

When considering the safety of a new medical device, we perform animal studies to confirm that safety. Both sheep and pigs are currently being used for coronary and peripheral arteries, although pig is the favored model and sheep is more frequently used for percutaneous valve placement. Most of the surgical valve replacement work was done in sheep, but today, pigs are being used more and more for percutaneous valve replacement. Percutaneous implantation of valves is difficult in both sheep and pigs because of the differences in the ascending and arch of the aorta as compared to the human and also the arch vessel branching is different in these animals compared to man. Nevertheless, sheep and pig hearts are feasible models for aortic and mitral valve work; however, we cannot emphasize enough that for sizing, human autopsy hearts may be significantly better for in vitro bench testing.