Relation Between Mural Force and Pressure in the Left Ventricle of the Dog

Abstract

• The study of strips of cardiac muscle has provided much valuable information regarding those factors which determine the force generated during contraction. Interrelationships between force, velocity of shortening, di&stolic stretch, duration of systole, and contractility have been delineated. "5 These findings are not, however, directly and siiiaply applicable to the physiology of the intact heart because the force generated by the myocardium and the corresponding pressure are by no means synonymous. In order to relate the physiology of muscle strips to that of the intact ventricle, the quantitative relation between force in the wall of the ventricle and pressure in the cavity must be known.The importance of such considerations in cardiac physiology has long been recognized. Burton, 10 and Linzbaeh. 11 More recently, Levine and Wagman have discussed the possible influence of the size and shape of the heart on myocardial oxygen consumption, emphasizing the important distinction between the pressure developed by the ventricle and the tension exerted by the fibers. Supported by American Heart Association grant. Work was done during Dr. Hefner's tenure as an Established Investigator of the American Heart Association and Dr. Sheffield's tenure of a TJ. S. Public Health Service Research Fellowship (HF-9775).Received for publication March 30, 1962. lows: Gravitational effects are ignored since they are unimportant in this situation. Consider a static left ventricle of any shape and size containing blood under a given pressure, as in figure 1. Now visualize an imaginary plane through this ventricle as shown in the figure. This imaginary plane divides the ventricle into two parts and passes through a rim of myocardium and a cross-sectional area of the cavity. From elementary hydrostatics it is known that the pressure of the blood in the cavity creates a force in a direction perpendicular to the imaginary plane exactly equal to the product of the pressure (which is force per unit area) times that cross-sectional area of the cavity included in the imaginary plane. The shape of this cross-sectional area of the cavity in our imaginary plane is immaterial, and the size and shape of the rest of the ventricle are also completely without effect on this relation. Since we began by postulating a static ventricle, Newton's laws of motion require that the force mentioned above tending to separate the two parts of the ventricle on each side of the plane be exactly balanced by an equal and opposite force. This equal and opposite force must exist in the rim of myocardium included in the imaginary plane, shown by the stippled area in figure 1. Note that this force exists in the rim of myocardium determined by the imaginary plane, its direction is perpendicular to the plane, and its magnitude is determined only by the pressure and cross-sectional area of the cavity and is independent of the thickness of the wall, the shape or cross-sectional area of the rim, the size or shape of the remainder of the ventricle, the distribution of f...

Hemodynamic Responses to Exercise in Clinically Normal Middle-aged Men and in Those with Angina Pectoris *

“…The load resisting shortening of the myocardial fibers in the heart at any instant is equal to the tension existing in those fibers at the same time. This tension also varies with the size of the ventricular cavity as has recently been demonstrated experimentally by Hefner, Sheffield, Cobbs, and Klip (22). In applying these known facts to the findings of the present experiments, we conclude that three factors may have contributed to the reduced rate of ejection that characterized the patients with angina pectoris.…”

The effects of posture and isoproterenol on the velocity of left ventricular contraction in man

“…It has been predicted that the ejection phase of contraction of the normal left ventricle is not strictly isotonic and that the force in the left ventricular wall should fall during ejection (37). Moreover, there is evidence in dogs demonstrating a peak force early during ejection followed by a decline which is slight relative to the peak force (38). A similar linear fall in ventricular wall tension was observed in the normal human heart by Gault, Ross, and Braunwald (39), though in that study the magnitude of force decrement was relatively large.…”

“…It has been shown that in the heart of the normal dog (38) and of man (39), ventricular wall tension during systole falls in a nearly-linear fashion after development of peak force early in the ejection phase. Therefore, ventricular wall force in this study is represented as a mean tension during ejection, MFE.…”

Left Ventricular Volume in Man Measured by Thermodilution*

“…Ventricular wall forces, stresses, and circumferential shortening will vary with curvature in a nonspherical chamber. The problems raised by the choice of the appropriate geometric form, however, can in part be mitigated as follows: 1) The force acting upon the wall of the intact canine left ventricle calculated directly from the intraventricular pressure and inner radius at the transverse equator of the chamber (7rr2P) correlated well with wall force directly recorded from a strain gauge (13). These strain gauges were oriented on the ventricular wall in both perpendicular and parallel fashion to the equator, corresponding to minimal and maximal lines of tensile force (14), without altering the correlation.…”