The physics of left ventricular ejection and its implications for muscle mechanics

The ejection stage of the left ventricle of the heart is analyzed using the proper form of Newton's Second Law of Motion and a simple cylindrical ventricular model. The Laplace relation is then used to calculate the dynamic relationship between force, velocity and acceleration in an average, circumferential, equitorial myocardial wall fiber, yielding: Fm=Fa + Cacf + Dvcf2 where Fm is the force in the fiber, Fa the fiber force that would be required to match aortic pressure, acf the acceleration and vcf the velocity of shortening of the fiber. C is a constant, and D is a geometrical parameter that varies smoothly with time. The significance of the above equation is discussed both in terms of muscle function (total muscular force) and pump function afterload Fa and the differential force, Fm - Fa): The afterload component Fa provides the pressure head needed to force the blood through the capillaries (Poiseuille flow) and varies with time in accordance with the impedance of the vascular system (auxotonic loading). The differential component force Fm - Fa provides the ventricular impulse needed to overcome the inertia of the system due to (1) the mass of the blood, (2) the geometrical constriction of the outflow tract as one moves downstream (Bernoulli effect), and (2) the moving ventricular walls. Analysis of this sort should help in attempting to separate the intrinsic properties of cardiac muscle and the pump function of the intact heart. The important role that isolated muscle experiments can play in this effort is discussed.