![]() ![]() Two phases can be defined in the Frank–Starling curve ( Fig. Left ventricle pressure could be used as a measure of preload, provided the relationship between pressure and ventricular volume is constant.Īccording to the Frank–Starling law, at a constant heart rate, cardiac output is directly proportional to preload, at least up to a certain point, beyond which CO would not increase despite further elevations in ventricle filling pressure, and could in fact decrease under certain conditions. In the clinical setting we can define preload as the ventricular dimension in telediastole (end-diastole). This is because stretching of the fiber involves slight elongation of the sarcomere, thereby increasing the number of sites of interaction between actin and myosin. According to the Frank–Starling law, there is a direct relationship between the degree of fiber elongation in diastole and posterior shortening of the myocardial fiber in systole. Preload is determined by myocardial fiber length before contraction. In turn, systolic volume depends on the following. If CO is associated to other values offering information on the determinants of cardiac output and on the balance between oxygen supply and consumption, we may obtain a more accurate idea of whether global cardiac function is adequate or not.Īs seen from the above formula, the determinants of cardiac output are systolic volume (SV) and heart rate (HR). Consequently, the cardiac output value alone is not enough to assess heart function and the hemodynamic condition of the patient. Critically ill patients generally have abnormal oxygen demands as a result of the underlying disease-triggering process. However, cardiac output, as the main determinant of oxygen transport to the different body regions, must be adapted to the needs of the body at all times therefore, as a single or isolated piece of information, a value within the “normal” range is unable to indicate whether cardiac function is optimum. In academic terms, the normal CO range in the healthy adult is about 4–6.5 l/min (2.5 l/min per m 2 body surface would be the corresponding cardiac index), under resting conditions. ![]()
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