Second, the two still most applied measures in this context, i.e. , pulmonary capillary wedge pressure and central venous pressure, do not at all predict volume responsiveness, in clear contrast to the common assumption. First, there is no proof that this circulatory surrogate, enabling the clinician to maximize stroke volume, really achieves the optimum. Measuring volume responsiveness, occasionally referred to as a “goal-directed” approach, 30,38,39seems at first to be an interesting alternative to directly measuring blood volume, but it has several limitations. Unfortunately, they remain impractical in everyday routine. Therefore, direct blood volume measurements are possible in principle and are frequently used to answer scientific questions. 35Hematocrit dilution is often based on estimated basic values and can only assess changes in the circulating part of the blood volume, 33,36,37ignoring a considerable noncirculating portion of the plasma (see section titled The Endothelial Glycocalyx: The Gateway to the Interstitial Space). 31–34Alternative methods that do not use sampling lack calibration and are, therefore, imprecise. Importantly, optimizing does not necessarily mean maximizing , despite frequently being interpreted in this way, 30and blood volume cannot be assessed routinely: Double-label blood volume measurement, the current standard to assess total body blood volume, is invasive, complex, and personnel intensive. An important determinant, total body blood volume, should be optimized to achieve this. The principal goal is to optimize cardiac preload. In addition, the exact target remains unclear, and many theoretically possible targets cannot be measured in clinical routine. Under normal circumstances, the individual patient’s hydration and volume state before surgery is unknown. The attending anesthesiologist is faced daily with several principal and practical problems when arranging perioperative fluid handling. 11,27This statement is mainly based on four generally unquestioned pathophysiologic “fundamentals”: (1) The preoperatively fasted patient is hypovolemic because of ongoing insensible perspiration and urinary output 10 (2) the insensible perspiration increases dramatically when the surgeon starts cutting the skin barrier 27 (3) an unpredictable fluid shift toward the third space requires generous substitution 28 and (4) hypervolemia is harmless because the kidneys regulate the overload. 21,22Rather, preoperative volume loading is considered indispensable by many, 15,19,23–26and fluid boluses are part of most recommendations for perioperative care. 14–20Most perioperative fluid overload is regarded as a minor problem, and studies showing increased fluid accumulation in tissue have not changed this attitude. 8–13The discussion is still dominated by the advocates of a more liberal regimen. After the ongoing controversy on colloids versus crystalloids 1–3and proposing the ideal composition of saline fluids, 4–7the main focus is now on the amount of applied fluids in general. PERIOPERATIVE fluid application has been a topic of debate in past years. Using the right kind of fluid in appropriate amounts at the right time might improve patient outcome. Therefore, undifferentiated fluid handling may increase the shift toward the interstitial space. The endothelial glycocalyx plays a key role and is destroyed not only by ischemia and surgery, but also by acute hypervolemia. Crystalloids physiologically load the interstitial space, whereas colloidal volume loading deteriorates a vital part of the vascular barrier. Blood volume after fasting is normal, and a fluid-consuming third space has never been reliably shown. This concept brings into question common liberal infusion regimens. Because the intravascular blood volume remains unchanged and insensible perspiration is negligible, the fluid must accumulate inside the body. The consequence is a positive fluid balance and weight gain of up to 10 kg, which may be related to severe complications. Replacement of assumed preoperative deficits, in addition to generous substitution of an unsubstantiated increased insensible perspiration and third space loss, plays an important role in current perioperative fluid regimens.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |