Contractility is another factor that influences cardiac output. Contractility of the myocardium represents the ability of the heart to contract independent of the influence from preload or afterload. Substances that cause an increase in intracellular calcium ions lead to an increase in contractility. Different concentrations of calcium ions in the cell lead to a different degree of binding between the actin (thin) and myosin (thick) filaments of the heart muscle.

Direct determination of cardiac contractility is not possible in the clinical setting. Therefore, surrogate parameters are used to evaluate or estimate the contractility.

Cardiac Function Index (CFI)

The Cardiac Function Index can be used to estimate cardiac contractility. It represents the relation of the flow (Cardiac Output) and the Preload Volume (GEDV). Thus, Cardiac Function Index is a preload related cardiac performance parameter.