Dyspnea, hypotension, and tachycardia soon after administration of β-lactam antibiotics are suggestive of anaphylactic shock.  Hypotension occurs in anaphylactic shock secondary to collapse of peripheral vascular resistance, increases in vascular permeability, and leakage of capillary fluid.  Stimulation of the smooth muscle tone within the bronchial wall, along with an increase in bronchial secretion, accounts for the dyspnea seen in anaphylaxis.  Skin symptoms (urticaria and angioedema) may occur secondary to vasodilatation and increased vascular permeability of skin capillaries.  Increases in GI smooth muscle tone may result in vomiting, abdominal cramps, and diarrhea.

Epinephrine is the drug of choice for the treatment of anaphylactic shock due to its ability to reverse all of the pathophysiologic mechanisms of anaphylaxis.  Stimulation of α1 receptors counteracts the vasodilatation of cutaneous and viscera vasculature, thus increasing blood pressure.  Epinephrine-mediated increases in cardiac contractility (β1 effect) and cardiac output also increase blood pressure and improve peripheral perfusion.  Epinephrine-induced stimulation of β2 receptors results in bronchodilatation, making it also a popular choice for the treatment of severe asthmatic reactions.

(Choice A)  Steroids inhibit inflammation by reducing capillary permeability and suppressing neutrophil activity.  Steroids also inhibit phospholipase A2, resulting in decreased formation of prostaglandin inflammatory mediators.  Because steroids anti-inflammatory effects are not acute, they are not effective in the acute treatment of life-threatening anaphylaxis.  Epinephrine should be given prior to steroids and antihistamines in the treatment of anaphylaxis.

(Choice C)  Norepinephrine has a predominantly alpha-1 adrenergic effect; thus, it can cause intense vasoconstriction, which may limit cardiac output.  Furthermore, it has little effect on the beta-2 adrenoceptor, so it has little or no bronchodilator action.

(Choice D)  Dobutamine is a synthetic drug with primary beta-1 adrenergic action that can cause an increased cardiac output without the other effects of epinephrine.

(Choice E)  Diphenhydramine is a first generation antihistamine drug that competitively inhibits peripheral H1 receptors in the GI tract, blood vessels, and respiratory tract.  Diphenhydramine may be used for the treatment of anaphylaxis after the patient is stabilized with epinephrine.

Educational Objective:
Anaphylactic shock is characterized by vasodilatation, increased vascular permeability, and bronchoconstriction.  Epinephrine counteracts these physiological mechanisms and is the drug of choice for the treatment of anaphylaxis.