Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

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two main causes of the loss of vascular tone: a decrease in the sympathetic control of vasomotor tone, or the release of excessive vasodilator substances. It can also occur as a complication of vessel damage resulting from prolonged and severe hypotension due to hemor- rhage, known as irreversible or late-stage hemorrhagic shock. 61 Three shock states share the basic circulatory pattern of distributive shock: neurogenic shock, ana- phylactic shock, and septic shock. Neurogenic Shock. Neurogenic shock is caused by decreased sympathetic control of blood vessel tone due to a defect in the vasomotor center in the brain stem or the sympathetic outflow to the blood vessels. 6 The term spinal shock describes the neurogenic shock that occurs in persons with spinal cord injury (see Chapter 36). Output from the vasomotor center can be interrupted by brain injury, the depressant action of drugs, general or spinal anesthesia, hypoxia, or lack of glucose (e.g., insulin reaction). Fainting due to emotional causes is a transient form of impaired sympathetic outflow. Many general anesthetic agents can cause a neurogenic shock- like reaction, especially during induction, because of interference with sympathetic nervous system function. Spinal anesthesia or spinal cord injury above the mid- thoracic region can interrupt the transmission of out- flow from the vasomotor center. In contrast to other shock states, the heart rate in neurogenic shock often is slower than normal, and the skin is dry and warm. These findings are considered the cardinal signs of neurogenic shock. This type of distrib- utive shock is rare and usually transitory. 62 Anaphylactic Shock. Anaphylaxis is a clinical syn- drome that represents the most severe systemic allergic reaction. 63 Anaphylactic shock results from an immu- nologically mediated reaction in which vasodilator sub- stances such as histamine are released into the blood. These substances cause vasodilation of arterioles and venules along with a marked increase in capillary per- meability. The vascular response in anaphylaxis is often accompanied by life-threatening laryngeal edema and bronchospasm, circulatory collapse, contraction of gas- trointestinal and uterine smooth muscle, and urticaria (hives) or angioedema. 63 Among the most frequent causes of anaphylactic shock are reactions to medications, such as penicillin; foods, such as nuts and shellfish; and insect venoms. The most com- mon cause is stings from insects of the order Hymenoptera (i.e., bees, wasps, and fire ants). Latex allergy causes life- threatening anaphylaxis in a growing segment of the pop- ulation (see Chapter 16). Health care workers and others who are exposed to latex are developing latex sensitivities that range from mild urticaria, contact dermatitis, and mild respiratory distress to anaphylactic shock. 64 Children with spina bifida also are at extreme risk for this serious and increasingly common allergy. 64 The onset and severity of anaphylaxis depend on the sensitivity of the person and the rate and quantity of antigen exposure. Signs and symptoms associated with impending anaphylactic shock include abdominal

cramps; apprehension; warm or burning sensation of the skin, itching, and urticaria (i.e., hives); and respira- tory distress such as coughing, choking, wheezing, chest tightness, and difficulty in breathing. After blood begins to pool peripherally, there is a precipitous drop in blood pressure and the pulse becomes so weak that it is dif- ficult to detect. Life-threatening airway obstruction may ensue as a result of laryngeal angioedema or bronchial spasm. Early signs appear with a more severe reaction. Anaphylactic shock often develops suddenly; death can occur within minutes unless appropriate medical inter- vention is promptly instituted. Treatment includes immediate discontinuation of the inciting agent or action to decrease its absorption (e.g., application of ice to the site of an insect bite); close monitoring of cardiovascular and respiratory function; and maintenance of respiratory gas exchange, cardiac output, and tissue perfusion. Epinephrine, which stimu- lates alpha and beta receptors, is given in an anaphylac- tic reaction because it causes systemic vasoconstriction and relaxes the smooth muscle in the bronchioles, thus restoring cardiac and respiratory function. 65 Other treat- ment measures include the administration of oxygen, antihistamine drugs, corticosteroids, and bronchodi- lators. The person should be placed in a supine posi- tion because the sitting position can produce a severe decrease in venous return. 65 The prevention of anaphylactic shock is preferable to treatment. Once a person has been sensitized to an antigen, the risk of repeated anaphylactic reactions with subsequent exposure is high. All health care providers should question patients regarding previous drug reac- tions and inform patients as to the name of the medi- cation they are to receive before it is administered or prescribed. Persons with known hypersensitivities should wear MedicAlert jewelry and carry an identifi- cation card to alert medical personnel if they become unconscious or unable to relate this information. Persons who are at risk for anaphylaxis should be provided with emergency medications (e.g., epinephrine autoinjector), instructed to avoid the allergen, and given procedures to follow in case they are inadvertently exposed to the offending antigen. 65 Sepsis and Septic Shock. Septic shock, which is the most common type of distributive shock, is associated with the systemic immune response to severe infection (Fig. 20-10). 66 The nomenclature related to sepsis and septic shock has been evolving. Sepsis has been defined as a suspected or proven infection plus the clinical mani- festations of what has been termed the systemic inflam- matory response (e.g., fever, tachycardia, and elevated white blood cell count [leukocytosis]). 66 Severe sepsis is sepsis plus evidence of sepsis-induced organ dysfunc- tion or tissue hypoxia (e.g., hypotension, hypoxemia, oliguria, metabolic acidosis, thrombocytopenia). 67,68 Septic shock is defined as severe sepsis with hypoten- sion, despite fluid resuscitation. 67–69 Severe sepsis accompanied by acute organ dysfunc- tion is a frequently occurring condition in critically ill patients. The growing incidence has been attributed to