The most common complaint of the cardiac patient is shortness of breath. For this reason the average physician uses the respiratory response to effort as a measure of the efficiency of the circulation. Nevertheless, the intimate relation existing among circulation, respiration and metabolism is not generally appreciated by members of the profession.
The basis of life lies in the chemical changes which we term metabolism. It is the energy liberated by the oxidation of carbon and hydrogen that maintains the body functions and supplies the forces required for external work. Indeed, the amount of oxygen which the body uses in a unit of time has been found to be the most accurate measure of the energy output.
The oxygen consumed by the tissues must be transported from the lungs to the working cells by the blood. Failure to supply oxygen in sufficient amount to maintain the well-being of the tissues