Physiology of Treatment in Shock
Blood and Plasma Transfusion. If a person is in shockcaused by hemorrhage, the best possible therapy is usually transfusion of whole blood. If the shock is caused by plasma loss, the best therapy is administra-tion of plasma; when dehydration is the cause, admin-istration of an appropriate electrolyte solution can correct the shock.
Whole blood is not always available, such as under battlefield conditions. Plasma can usually substitute adequately for whole blood because it increases the blood volume and restores normal hemodynamics. Plasma cannot restore a normal hematocrit, but the human body can usually stand a decrease in hemat-ocrit to about half of normal before serious con-sequences result, if cardiac output is adequate. Therefore, in emergency conditions, it is reasonable to use plasma in place of whole blood for treatment of hemorrhagic or most other types of hypovolemic shock.
Sometimes plasma is unavailable. In these instances, various plasma substitutes have been developed that perform almost exactly the same hemodynamic func-tions as plasma. One of these is dextran solution.
Dextran Solution as a Plasma Substitute. The principalrequirement of a truly effective plasma substitute is that it remain in the circulatory system—that is, not filter through the capillary pores into the tissue spaces.
In addition, the solution must be nontoxic and must contain appropriate electrolytes to prevent derange-ment of the body’s extracellular fluid electrolytes on administration.
To remain in the circulation, the plasma substitute must contain some substance that has a large enough molecular size to exert colloid osmotic pressure. One of the most satisfactory substances developed for this purpose is dextran, a large polysaccharide polymer of glucose. Certain bacteria secrete dextran as a by-product of their growth, and commercial dextran can be manufactured using a bacterial culture procedure. By varying the growth conditions of the bacteria, the molecular weight of the dextran can be controlled to the desired value. Dextrans of appropriate molecular size do not pass through the capillary pores and, there-fore, can replace plasma proteins as colloid osmotic agents.
Few toxic reactions have been observed when using purified dextran to provide colloid osmotic pressure; therefore, solutions containing this substance have proved to be a satisfactory substitute for plasma in most fluid replacement therapy.
A sympathomimetic drug is a drug that mimics sym-pathetic stimulation. These drugs include norepineph-rine, epinephrine, and a large number of long-actingdrugs that have the same effect as epinephrine and norepinephrine.
In two types of shock, sympathomimetic drugs have proved to be especially beneficial. The first of these is neurogenic shock, in which the sympathetic nervoussystem is severely depressed. Administering a sympa-thomimetic drug takes the place of the diminished sympathetic actions and can often restore full circula-tory function.
The second type of shock in which sympatho-mimetic drugs are valuable is anaphylactic shock, in which excess histamine plays a prominent role. The sympathomimetic drugs have a vasoconstrictor effect that opposes the vasodilating effect of histamine. Therefore, either norepinephrine or another sympa-thomimetic drug is often lifesaving.
Sympathomimetic drugs have not proved to be very valuable in hemorrhagic shock. The reason is that in this type of shock, the sympathetic nervous system is almost always maximally activated by the circulatory reflexes already; so much norepinephrine and epi-nephrine are already circulating in the blood that sym-pathomimetic drugs have essentially no additional beneficial effect.
Treatment by the Head-Down Position. When the pressurefalls too low in most types of shock, especially in hemorrhagic and neurogenic shock, placing the patient with the head at least 12 inches lower than the feet helps tremendously in promoting venous return, thereby also increasing cardiac output. This head-down position is the first essential step in the treat-ment of many types of shock.
Oxygen Therapy. Because the major deleterious effect ofmost types of shock is too little delivery of oxygen to the tissues, giving the patient oxygen to breathe can be of benefit in many instances. However, this frequently is far less beneficial than one might expect, because the problem in most types of shock is not inadequate oxy-genation of the blood by the lungs but inadequate transport of the blood after it is oxygenated.
Treatment with Glucocorticoids (Adrenal Cortex Hormones That Control Glucose Metabolism). Glucocorticoids are fre-quently given to patients in severe shock for several reasons: (1) experiments have shown empirically that glucocorticoids frequently increase the strength of the heart in the late stages of shock; (2) glucocorticoids stabilize lysosomes in tissue cells and thereby prevent release of lysosomal enzymes into the cytoplasm of the cells, thus preventing deterioration from this source; and (3) glucocorticoids might aid in the metabolism of glucose by the severely damaged cells.