A 25-year-old woman is brought to the emergency department 40 minutes after being stung by several wasps. She reports throat tightness and dizziness. She has no chronic medical conditions and takes no medication. Blood pressure is 80/40 mm Hg, pulse is 120/min, and respirations are 32/min. Examination shows diffuse erythematous plaques over the trunk and 1+ pitting edema of the ankles. Which of the following is the most likely cause of this patient's hypotension?

Question

A 25-year-old woman is brought to the emergency department 40 minutes after being stung by several wasps.  She reports throat tightness and dizziness.  She has no chronic medical conditions and takes no medication.  Blood pressure is 80/40 mm Hg, pulse is 120/min, and respirations are 32/min.  Examination shows diffuse erythematous plaques over the trunk and 1+ pitting edema of the ankles.  Which of the following is the most likely cause of this patient's hypotension?

Quizplus · Accepted Answer

This patient with throat tightness, dizziness, hypotension, and urticaria (ie, diffuse erythematous plaques) has anaphylaxis, a type I (immediate) hypersensitivity reaction that occurs in response to an allergen (eg, wasp venom). In patients who will eventually develop anaphylaxis, the initial exposure to an allergen results in antibody class switching and the production of allergen-specific IgE by plasma cells. The allergen-specific IgE then binds to an IgE receptor on mast cells or basophils. Subsequent reexposure leads to allergen interaction with cell-bound IgE, IgE receptor aggregation, mast cell or basophil degranulation, and release of chemical mediators (histamine, prostaglandin, leukotrienes) that can cause anaphylactic shock. Anaphylactic shock is characterized by profound peripheral vasodilation. Arteriolar vasodilation causes decreased systemic vascular resistance and consequent hypotension; venular vasodilation causes decreased central venous pressure and reduces venous return to the right atrium. In addition, the inflammatory chemical mediators increase vascular permeability, leading to large fluid shifts from the intravascular to the extravascular space; this decreases intravascular volume and worsens hypotension. (Choice A) Chemical mediators (eg, histamine) released from cardiac mast cells may exert negative ionotropic effects on the heart, decreasing cardiac contractility. However, this is typically counterbalanced by the body's sympathetic response to anaphylaxis; increased sympathetic activity has a positive inotropic effect and increases the heart rate in an attempt to compensate for widespread peripheral vasodilation. (Choice C) Increased, not impaired, endogenous sympathetic nerve activity is the body's early compensatory response in anaphylaxis. In response to anaphylaxis-induced hypotension, an intact baroreceptor reflex leads to increased sympathetic activity, tachycardia, and compensatory vasoconstriction. (Choices D and E) A toxin-mediated decrease in myocardial contractility and an increase in capillary permeability occur in toxic shock syndrome (TSS). Most cases are linked to a prolonged use of tampons and wound/nasal packing (not wasp envenomation), which provide a medium for Staphylococcus aureus to replicate and release an exotoxin (eg, TSS toxin 1) that is capable of nonspecifically activating T cells (ie, a superantigen). The subsequent dramatic release of inflammatory cytokines results in increased capillary permeability and hypotension, as well as high fever and a diffuse, erythematous rash. In addition, the exotoxin can directly inhibit myocardial function. Educational objective: In anaphylaxis, allergen exposure triggers widespread IgE-mediated release of inflammatory chemical mediators (eg, histamine, prostaglandin, leukotrienes). These chemical mediators cause peripheral vasodilation and increase vascular permeability, leading to hypotension. __________ References: Regulation of vascular permeability in anaphylaxis. (http://www.ncbi.nlm.nih.gov/pubmed/29671869)

A 25-Year-Old Woman Is Brought to the Emergency Department 40