With the exception of cardiogenic shock, the primary reason for inadequate cardiac output in the shock patient is low ventricular filling pressure. As a result, volume expansion, often referred to as resuscitation, has been the most successful treatment. Volume expanders fall into three categories: whole blood; cell-free fluids that have a colloid included, such as plasma; or colloid-free fluids, such as lactated Ringer's solution. Surprisingly, administration of colloid-free fluids has yielded the best outcomes. As injury to the peripheral circulation increases, the capillaries become very permeable to the plasma proteins, the colloids that are responsible for osmoti-cally pulling water back into the capillary lumen (see Chapter 16). The failure of the capillaries to restrict the movement of plasma proteins causes massive pooling of fluids in the interstitial spaces. As a result, the colloid becomes ineffective and lactated Ringer's (basically, salt water with some lactic acid added as a nutrient) is just as effective in expanding the patient's blood volume. The body can tolerate dilution of the blood to a hematocrit as low as 20% without serious consequences. As would be expected, fluid resuscitation causes extensive peripheral edema in these patients, but the benefits of increased cardiac output far outweigh any consequence of the swelling.
Current research in shock therapy is focusing on the role that many of the circulating cytokines play in the shock state. It may be that selective blockade of some of them, such as nitric oxide or tumor necrosis factor, may further improve current therapy. Another important field of investigation involves an attempt to understand why the capillaries become so permeable to macromole-cules and how this could be reversed. Mechanical attempts at increasing venous return, such as inflatable pants or simple elevation of the legs, have had a minor clinical impact. The amount of blood that can be translocated to the thorax by these devices is small compared to what can be accomplished with simple fluid resuscitation.
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