Ejection murmurs of subaortic and aortic stenosis occur commonly in the mammalian population. Boxer-dogs have a high prevalence of systolic ejection murmurs (50 to 80%). The origin of these murmurs is a subject of discussion, especially in those cases that anatomical lesions are not evident, as with many cases of mild aortic stenosis. It has been speculated that turbulence is one of the most likely genesis of these ejection murmurs. Boxer-dogs with soft murmurs provide useful model to evaluate physical factors of aortic flow that may produce murmur.
A total of 15 Boxer-dogs were evaluated with physical examination, electrocardiography, and a complete echocardiographic exam. For studies conducted in both the catheterization and NMR laboratories, 7 Boxers were induced to anesthesia for evaluation of the fluid mechanical parameters. Sounds were recorded from the ascending aorta and the torso surface in the cath-lab. S2 was used to calculate transfer function between oscillations in the heart and oscillations on the body surface.
The images to study aortic blood flow were acquired on a 1.5T Siemens MRI system for 5 locations of the ascending aorta, 3 physiological states, and 7 dogs.
Mean and peak velocity, area, Reynolds number, Womersly parameter, energy, flow, momentum, and vorticity were calculated from the velocity-encoded images using MRI. Reynolds numbers were above the critical values indicated in the literature. Re’s correlated well with momentum (r2 =0.54) and flow (r2 =0.86). Rotational velocity in CCW direction were greatest at the arch (p<0.05), clockwise vortices were greater at the root and Valsalva-sinus (p<0.05). Ten variables indicated that turbulence may have occurred at the proximal regions of the aorta. The murmur of mild aortic stenosis was originated in the proximal regions of the ascending aorta, demonstrated by the most “violent” fluid-mechanical activities in these regions. There was a mathematical relationship between intensity of oscillations within the heart generating sound and the intensity of oscillation detected on the thoracic wall as heart sound.