الفهرس 
Basic Cardiovascular PhysiologyOnly 14 pages are availabe for public view
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Abstract
In normal human body 60% of body weight is composed of water in males and 50% in females. Body water is further divided into intracellular (66%) and extracellular (34%) compartments. Extracellular fluid is distributed in the intravascular compartment (25%) and in the interstitial compartment (75%).
There is 2 types of fluid can be used in resuscitation crystalloids and colloids. The choice of fluid for resuscitation in the critical care units was debated for many years. While crystalloids are rapidly available for resuscitation, they require large volumes to achieve the desired effect. On the other hand, colloids are expensive and rapidly expand the intravascular space.
The Frank Starling principle is based on the length-tension relationship within the ventricle. If ventricular end diastolic volume (preload) is increased it follows that the ventricular fiber length is also increased, resulting in an increased ‘tension’ of the muscle. In this way, cardiac output is directly related to venous return. When heart rate is constant, cardiac output is directly related to preload (up to a certain point.) An increase in preload will increase the cardiac output until very high end diastolic volumes are reached. At this point cardiac output will not increase with any further increase in preload, and may even decrease after a certain preload is reached. Also, any increase or decrease in the contractility of the cardiac muscle for a given end diastolic volume will act to shift the curve up or down, respectively.
Hemodynamic monitoring has evolved considerably over the last 30 years. It is now widely accepted that bedside clinical examination and routine hemodynamic observations are not sufficient to evaluate the adequacy of either resuscitation or the metabolic status of a patient. These monitoring tools allow us to understand the physiology of the circulation at the bedside and, thereby, to direct therapies appropriately. Doubts concerning the use of the pulmonary artery catheter have driven the development of newer less invasive devices. These devices can be as accurate as the pulmonary artery catheter and are often easier to use.
The interaction of mechanical ventilation and left ventricular function is complex. Both ventilator issues - tidal volume, positive end-expiratory pressure, chest and lung compliance - and cardiovascular issues - heart rate and rhythm, ventricular function, cardiac after load, arterial compliance - may affect functional preload indices.
The measurement of changes in aortic blood flow following a passive leg raising maneuver, which has been reported to have a high sensitivity and specificity in a critically ill population including patients with spontaneous respiratory efforts and arrhythmias, may be of particular interest. In addition, the recently proposed systolic variation test may in the future provide further helpful information on fluid responsiveness.
Most studies performed on sedated and mechanically ventilated patients have demonstrated the superiority of dynamic indices in predicting fluid responsiveness. However, very few published studies have been conducted in spontaneously breathing patients and most studies were performed on spontaneous breathing movements. The results of these studies showed a potential value of dynamic indices. However, validation of these indices requires further studies, comprising larger patient populations and comparative studies with static indices.
In spontaneously breathing patients (with or without mechanical ventilation), the prediction of volume responsiveness can be a difficult challenge, in particular in those who have already been resuscitated in the preceding hours or days and in whom continuation of fluid infusion carries risks of pulmonary edema. In these cases, static markers of cardiac preload are generally in the normal range and are rarely helpful for determination of volume responsiveness. Since absolute measures of preload cannot be used effectively to assess volume responsiveness, more dynamic tests need to be employed to improve the utility of these measures. Because of the presence of spontaneous breathing, the indices of volume responsiveness that use heart-lung interactions, such as respiratory variation in arterial pressure and in stroke volume are no longer reliable. Careful analysis of the hemodynamic consequences of passive leg raising using real-time aortic blood flow monitoring may be helpful for predicting the beneficial effects of volume administration. In the most difficult cases, a fluid challenge strategy can still be applied provided that clinicians carefully follow the recommended rules in terms of the type of fluid, rate of infusion, clinical end-points, and safety limits in order to minimize the risks of fluid overload.