Functional Anatomy of the Sheep Heart as a Model for Testing Cardiovascular Devices.

The aim of the study is to assess the availability of this animal model for preclinical testing of implantable cardiovascular devices by studying numerical anatomical and functional characteristics of the ovine heart and to establish their differences from those of the human heart.

Materials and methods: The study was performed on 17 healthy crossbred Romanov sheep weighing 20-29 kg in group 1 (n=7) and 30-43 kg in group 2 (n=10). All animals underwent echocardiography examination on the Philips CX-50 apparatus (revision 3.1.2; Philips, Netherlands) with a sector-phased S5-1 sensor from the right parasternal projection (long and short axis) to determine the heart rate, right ventricular wall thickness in diastole, right and left ventricular end-diastolic dimensions (RV EDD and LV EDD), left ventricular end-systolic dimension (LV ESD), interventricular septum (IVS) thickness in systole and diastole, left ventricular posterior wall thickness in systole and diastole.Functional parameters of the left ventricle (left ventricle end-systolic and end-diastolic volumes (LV ESV and LV EDV), ejection fraction and shortening fraction) were calculated using the modified Simpson method built into the echocardiography software. The diameter of the mitral annulus and the characteristics of the aortic root were also measured: the diameters of the aortic valve, Valsalva sinuses, and sinotubular junction, as well as the height of the aortic root from the fibrous ring to the line of the sinotubular junction. Direct measurements of the ascending aorta and pulmonary artery diameters, intercommissural distances, and the height of the aortic valve leaflets were performed after autopsy.

Results: A number of anatomical and functional parameters of the sheep heart such as ejection fraction, myocardium thickness, LV EDD and LV ESD, aorta, and pulmonary artery diameters, have been established to be close to those of the human heart. At the same time, LV EDV and LV ESV of the sheep are significantly lower than in humans, even in relation to the body surface area, and the average mitral valve diameter is larger. Despite the same diameters of the aortic valve, Valsalva sinuses, and sinotubular junction, the structures of the ovine and human aortic roots are different: the sheep root features a smaller height and intercommissural distances of the cusps. In addition, some differences were found in the arrangement of the cusps in relation to the valve axis: the intercommissural distance of the right coronary leaflet was almost 2 times greater than the similar indicator of the left coronary leaflet.Most anatomical and functional parameters have not shown any correlation with the animals' body weight. Only in group 2, a significant positive correlation between body weight and the height of the aortic valve leaflets was found.

Conclusion: The anatomical and functional characteristics of the sheep heart are close but not identical to human hearts. The sheep is a valid experimental model for preclinical testing of implantable cardiovascular devices, but a successful experiment requires careful screening of animals with echocardiographic assessment of the target zone parameters and selection of the appropriate device size.