Concepts in Myocardial Performance

The determinants of myocardial performance are the factors that affect the ability of the heart to fill, eject, and effectively deliver circulation to the tissues, and they include:

• Heart rate
• Preload
• Afterload
• Contractility
• Distensibility
• Synergy of contraction

Involved in several of these factors is the concept of myocardial oxygen demand (MVO₂).

Myocardial oxygen demand

This refers to the amount of oxygen required or “demanded” of the heart to contract.

Factors affecting MVO₂

The following factors increase the myocardial oxygen requirement to contract:

• An increase in heart rate
• An increase in contractility
• An increase in afterload
• An increase in wall stress

Note the opposite changes in any of these factors reduces MVO₂

Wall Stress

Wall stress refers to the tension applied to a cross sectional area of muscle and the units are force per unit area. Laplace’s Law is used to describe wall stress:

Wall stress = (pressure x radius) divided by (2 x wall thickness)

Stroke Volume

SV is the volume of blood ejected from the heart with each contraction. Under normal conditions only about 50% of the volume of blood present in the heart is ejected with each heart beat.

Cardiac Output

CO is a commonly used measure of the performance of the heart.

CO = Heart Rate x Stroke Volume.

CO is sometimes indexed to body weight and is called cardiac index (CI = CO/Body Weight [kg])