Page 11, Cardiac Performance, Dr. D. Penney


Inotropicity:

The major factor responsible for controlling stroke volume is inotropicity or contractility.

Inotropicity / Contractility - Defined by a shift of the Frank-Starling curve to the right or left (Figure 2.04), change in dP/dt, Vmax, ejection fraction, and shift to right or left of the isovolumic line of the pressure/volume loop. Thus, contractility / inotropicity have specific restricted definitions (above), while "strength of contraction" is a more general term and should not be confused with them.

Increased "strength of contraction" is gained by increased inotropicity / contractility, but in a different way (change of state in muscle, mobilization of Ca++, etc.) than by an increased preload, ie. Frank-Starling mechanism (more advantageous positioning of the thick and thin myofilaments in muscle).

In Figure 2.04, a shift from A -> B indicates increased end-diastolic volume (Frank-Starling mechanism). A shift from A -> C indicates a positive inotropic mechanism (increased available Ca++?). A shift from A -> D also indicates a positive inotropic mechanism.

Actually, a whole family of ventricular function curves are possible (ie. a whole spectrum of inotropic states) and undoubtedly exist in a given individual under different physiological conditions. Increased inotropicity produces greater performance at the same or smaller preload. There is of course an increased cost for this in terms of ATP, oxygen utilization, etc., and efficiency, in its turn, is decreased.



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