Myocardial Contractility: is a term used in to describe the performance of cardiac muscle.

It is often defined as: the intrinsic ability of a cardiac muscle fibre to contract at a given fibre length.

The five determinants of myocardial performance are:

• Heart rate
• Conduction velocity
• Preload
• Afterload
• Contractility

If myocardial performance changes while preload, afterload, heart rate, and conduction velocity are all constant, then the change in performance must be due to the change in contractility.

Any chemicals that affects contractility is called inotropic agent. For example drugs such as catecholamines (norepinephrine and epinephrine) that enhance contractility are considered to have a positive inotropic effect.

For example:

• An increase in sympathetic stimulation to the heart increases contractility AND heart rate.
• An increase in contractility tends to increase stroke volume and thus a secondary increase in preload.

All factors that cause an increase in contractility work by causing an increase in intracellular Ca++ during contraction.