Contractility
Contractility is one of the factors which affect myocardial performance. It is often defined in the following roundabout way:
The determinants of myocardial performance are:
- Heart rate
- Preload
- Afterload
- Contractility
If myocardial performance changes while preload, afterload and heart rate are all constant, then the change in performance must be due to the change in contractility.
A might be thought that a better definition would be something like: Contractility is a property that represents the strength of myocardial contraction. However, this type of definition that not separate out contractility from the other factors that affect how strongly the myocardium contracts. In particular, consider this: An increase in preload results in an increased force of contraction - this is Starling's law of the heart - but this does not require a change in contractility.
This property of 'contractility' proved necessary to explain why some interventions (eg an adrenaline infusion) could cause an increase in myocardial performance even if, as could be shown in experiments, the preload, afterload and contractility were all held constant. Experimental work controlling the other factors affecting myocardial performance was necessary because a change in contractility will tend to directly of indirectly be associated with a change in the other factors. For example:
- An increase in sympathetic stimulation to the heart would increase contractility AND heart rate.
- An increase in contractility would tend to increase stroke volume and cause a secondary decrease in preload
All factors that cause an increase in contractility work by causing an increase in intracellular [Ca++] during contraction.