Contractility

Contractility refers to the inherent strength of the heart muscle – referred to as inotropy.

Factors affecting contractility

Factors that increase contractility:

• Increased Beta adrenergic stimulation
• Increased preload
• Reduced vagal tone
• Positive inotropic agents

Factors that reduce contractility:

• Reduced sympathetic stimulation
• Reduced preload
• Increased vagal tone
• Negative inotropic agents

Measures of contractility

Global or whole-heart contractility can be assessed by a variety of invasive or non-invasive ways. Invasive means involve measuring pressure changes directly in the ventricle. Non-invasive means are more commonly used clinically and include variables measured by cardiac ultrasound (echocardiography) or magnetic resonance imaging (MRI).

Effect of contractility on myocardial performance

An increase in contractility results in:

• An increase in stroke volume
• A reduction in preload
• An increase in MVO₂

A reduction in contractility will have the opposite effects

Manifestations of abnormal contractility

If contractility is too high:

• This condition will likely go undetected.

If contractility is too low:

• The resultant fall in BP will manifest as signs of hypotension
• Fluid accumulation in the organs that drain into the weak ventricle

Effects of the autonomic nervous system on contractility

The sympathetic nervous system increases contractility. The parasympathetic nervous system decreases contractility.