Cardiac oxygen requirement, determinants

B. Determinants of Cardiac Oxygen Requirement The pharmacologic treatment of coronary insufficiency is based on physiologic factors that control the myocardial oxygen requirement. A major determinant is myocardial fiber tension, ie, the higher the tension, the greater the oxygen requirement. 

The major determinants of cardiac oxygen consumption are heart rate, myocardial wall tension (directly related to ventricular cavity pressure and volume) and force and velocity of contraction. Under normal conditions, the heart extracts oxygen at a near-maximal rate and added requirements are met by a reduction in coronary vascular resistance and an increase in blood flow. In the atherosclerotic heart, coronary vessels in the ischemic region and particularly in the subendocarium are already dilated and their ability to increase flow by further dilatation is severely compromised. Thus, under these conditions, increasing oxygen demand may exceed supply, myocardial ischemia ensues and the patient manifests symptoms of angina pectoris. 2... 

Intensive care aims at the replacement or support of deficient central functions. Important functions are respiration and a sufficient cardiac and circulation function. Monitoring the gas exchange in intensive care especially of ventilated patients requires, in addition to gas analysis at several measuring sites in the body, information on the heart function, eg, heart rate, on the pressure of the circulation system, eg, arterial pressure, and on the actual utilization of the available amount of oxygen. With regard to the methods required to achieve information that is as complete as possible, the clinical task consists in the determination of gas concentrations or partial pressures in the gas phase of the lung, airways, and equipment hoses, in the transportation system of the blood, and in the tissue. In recent years there has been an increasing importance of non-invasive methods, as invasive procedures are combined with risk and additional stress for the patient. 

---