J. David Deck

Stress Sharing Between the Sinus and Leaflets of Canine Aortic Valve

A knowledge of the behavior of the aortic valve sinuses is necessary to the understanding of stress sharing between the sinuses and the leaflets. Radiopaque markers were placed on the sinuses and the leaflets of dogs during cardiopulmonary bypass, and the movement of the markers was studied using fluoroscopy. The center of the sinus moved radially during each cardiac cycle, but in an inconsistent manner. The sinus was under a dual influence: the passive influence of aortic pressure and the active influence of myocardial contraction. The longitudinal curvature of the sinus showed no dimensional change, whereas the radius of the circumferential curvature decreased by 15.7% from systole to diastole. In diastole, the stress in the sinus was 6.1 g/mm2 and was 24.3 g/mm2 circumferentially and 12.1 g/mm2 radially in the leaflet. Histologically, the main stress-bearing component of the leaflet was made up of thick, dense, collagenous fibers oriented circumferentially. These fibers curved into the sinus wall instead of inserting straight into the aortic wall, thereby suggesting that the high stress in the leaflet is shared with the sinus and that continuity of the circumferential stress exists between the leaflet and the sinus. The leaflet does not pull inwardly on the aortic wall. In diastole, the sinus adapts to the new stress conditions in the leaflet by reducing its radius of circumferential curvature. This stress sharing is important for the longevity of the aortic valve.

Stresses of Natural versus Prosthetic Aortic Valve Leaflets in Vivo

During normal function of the aortic valve, the aortic leaflets undergo not only cyclic loading and unloading but also cyclic reversal of their curvature. The stresses induced in the leaflet due to these variations have been computed using a new concept based on the structure of the leaflet. Membrane stresses have been related to the pressure difference across the leaflet and bending stresses to the leaflet curvature. Total stresses were obtained by adding the two stresses. Total stresses in bioprosthetic and synthetic leaflets also were computed using the same approach. In systole, the natural leaflet is subjected to much lower total stress than a bioprosthetic or a synthetic leaflet. The natural leaflet is not subjected to compressive stresses during the cardiac cycle, whereas bioprosthetic and synthetic leaflets must sustain compressive stresses during systole. The differences in stress patterns of these leaflets indicate that there is a difference in their longevity.

The cyclic changes and structure of the base of the aortic valve

Abstract The structure and behavior of the base of the aortic valve in the dogs were investigated. The structure was studied under light microscopy to determine the distribution of collagenous, elastic, and myocardial elements. The dimensional changes were studied in vivo, by attaching radio-paque markers to the base and observing their movement by x-ray studies. The base is partly composed of ventricular myocardium. Two of the three trigonal regions consist of myocardium, and the right and left coronary leaflets and sinuses are encompassed by ventricular myocardium. The collagenous tissue that lies internal to the myocardium is neither dense nor thick and does not form a complete ring. The cyclic dimensional changes in the base are similar to the cyclic changes in left ventricular geometry and volume. The base perimeter is maximal in early systole coincident with the “rounding” of the ventricular cavity during isovolumetric contraction. The base perimeter decreases during systole when the ventricular volume decreases during systolic ejection. The base perimeter increases during diastole as the ventricular volume increases due to diastolic filling. The amount of cyclic change in the base perimeter at normal systemic pressure was different for different dogs. In four dogs the amount of change varied from 5% to 28% over a wide range of systemic pressures. The importance of the behavior of the base in normal valvular function is discussed. It is speculated that the mismatch between the nonexpansile sewing ring of an aortic bioprosthesis and the normally expansile base of the valve could cause occasional periprosthetic leaks.

Intramural stress as a causative factor in atherosclerotic lesions of the aortic valve.

Topographic distribution of atherosclerotic lesions of the aortic valve was investigated in rabbits on a 2%-cholesterol-enriched diet and related to distribution of intramural stress in the valve. Initially the lesions appeared at the base of the leaflet on the aortic face and with time spread further out into the leaflet and up the wall of the aortic sinus. In the leaflet, the lesion occurred only in the pressure-bearing part and was primarily composed of a mass of foam cells. By 10 weeks primary fatty plaques were still confined to the aortic face but fibroblasts within the leaflet had also taken up fat. Even after 33 weeks, the atheromatous plaque had not spread beyond the pressure-bearing part of the leaflet. From silicone rubber casts of the valve it was observed that only part of the leaflet was under pressure and the remaining leaflet sustained no pressure gradient. The maximum intramural stress occurred during diastole on the pressure-bearing part. In systole, the blood flow produced shear stress on the entire leaflet. Hence, occurrence of atherosclerotic lesions only in the area of maximum intramural stress suggests that intramural stress and not shear stress plays an important role in accelerating the process of atherosclerosis.

RETARDATION OF THE NEWT LIMB REGENERATION WITH SEMICARBAZIDE, AN INHIBITOR OF HISTAMINE FORMATION.

It is widely known that the substance histamine is a noxious agent released from cells when they are injured. At the outset these experiments were intended to show whetherhistamine, persumably released by newt limb amputation, might also promote beginning steps in regeneration. In addition, according to a new view of the physiology of histamine, its formation metabolically by the decarboxylation of histidine is an attribute of the rapid growth of many tissues. These experiments were mainly designed to interfere with histamine production and thereby disrupt the rapid growth of the limb regenerate.