Cardiovascular Physiology and Pathophysiology
PhysiologyStructure and Function4 Topics
Lymphatics and Edema Formation
Vascular Control3 Topics
The Cardiac Cycle
Compensation for Circulatory Failure
Determinants of Myocardial Performance7 Topics
Neuro-Control of Heart and Vasculature4 Topics
Electro-Mechanical Association4 Topics
Electrical Side of the Heart4 Topics
Causes of Heart Failure
PathophysiologyDefining Heart Failure
MVO2 and Heart Failure
Cardiac Output and Heart Failure7 Topics
Vascular Tone in Heart Failure
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
- 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 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)
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.
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])