Situation: A 62-year-old man who weighs 80 kg is receiving his first dose of an intravenous (IV) antibiotic on the medical ward. Ten minutes into the infusion, he develops generalized hives, hoarseness, and wheezing; his blood pressure is 80/50 mmHg and heart rate 96/min. He takes metoprolol daily for hypertension. The infusion has been stopped and help has been called. After two doses of adrenaline (epinephrine) and IV fluid boluses, his blood pressure remains 78/48 mmHg. Which drug should the nurse anticipate next?

Beta blockers such as metoprolol can make anaphylaxis more severe and blunt the response to adrenaline. Glucagon raises heart rate and contractility through a pathway that does not use beta receptors, so it is the drug for anaphylaxis that does not respond to adrenaline in a client on a beta blocker. Corticosteroids act over hours and are adjuncts only.

Pathophysiology: Why metoprolol changes the game

Anaphylaxis is driven by massive release of mediators such as histamine, tryptase, and leukotrienes from mast cells and basophils. These mediators cause vasodilation, increased capillary permeability, bronchoconstriction, and laryngeal edema, producing the classic picture of hives, wheezing, hoarseness, and hypotension. The first-line treatment is adrenaline (epinephrine), which acts on alpha-1 receptors to constrict blood vessels and on beta-1 and beta-2 receptors to increase heart rate, contractility, and bronchodilation.

This client takes metoprolol, a cardioselective beta-1 blocker, daily. In anaphylaxis, beta blockade creates a dangerous triad: it blunts the beta-receptor-mediated response to adrenaline, reduces the cardiovascular compensatory mechanisms that normally help maintain blood pressure, and may even promote unopposed alpha-mediated vagotonic reflexes that worsen bradycardia and hypotension ^[1]. The result is anaphylaxis that is more severe and resistant to standard adrenaline dosing.

Watch out! A blood pressure of 78/48 mmHg after two doses of adrenaline and IV fluid boluses is not simply “needs more adrenaline.” It signals beta-receptor blockade is preventing adrenaline from doing its beta-1 job, and pushing more adrenaline can worsen the paradoxical vagotonic response ^[1].

Why glucagon is the next drug

Glucagon is a polypeptide hormone that increases heart rate and myocardial contractility by activating glucagon receptors on cardiac myocytes. These receptors are coupled to the same intracellular cyclic AMP pathway that beta-1 receptors use, but they are completely independent of beta-adrenergic receptors. Therefore, metoprolol cannot block glucagon’s inotropic and chronotropic effects. In a patient whose beta receptors are occupied or desensitized, glucagon provides an alternative route to stimulate the failing heart ^[2].

This is not a theoretical point. In a reported case of refractory anaphylactoid shock during coronary angiography in a patient on beta-blockers, aggressive standard therapy failed, and significant clinical improvement occurred only after glucagon was administered ^[2]. The same principle applies to anaphylaxis from any trigger, including antibiotics, contrast media, or insect stings .

Drug	Mechanism in refractory anaphylaxis	Role here
Glucagon	Activates cardiac glucagon receptors; bypasses beta receptors to raise heart rate and contractility	First choice when adrenaline fails in a patient on a beta blocker
Calcium gluconate	Stabilizes cardiac membranes; no direct reversal of beta blockade or mediator release	Not indicated for anaphylactic hypotension
Atropine	Blocks vagal muscarinic effects; may help bradycardia but does not restore beta-mediated contractility	Not the primary drug for beta-blocker–refractory anaphylaxis
Methylprednisolone	Anti-inflammatory; onset over hours; reduces late-phase response	Adjunct only; will not fix acute hypotension

Why not the other options

Calcium gluconate is used for hyperkalemia, hypocalcemia, or calcium channel blocker toxicity. It does not address the beta-receptor blockade or the vasodilation of anaphylaxis. Atropine can raise heart rate by blocking vagal tone, but in this scenario the primary problem is not excessive vagal stimulation; it is the inability of the heart to respond to catecholamines. Atropine would not restore contractility or reverse the vasodilation. Methylprednisolone is an important adjunct in anaphylaxis to prevent biphasic or protracted reactions, but its onset of action is 4 to 6 hours, far too slow to rescue a patient with ongoing hypotension .

Clinical application for the nursing licensure exam

When a client on a beta blocker presents with anaphylaxis that does not respond to adrenaline and fluids, the nurse should anticipate an order for IV glucagon. The usual adult dose is 1 to 5 mg IV bolus over 5 minutes, followed by an infusion if needed. Glucagon can cause nausea and vomiting, so the nurse should monitor for aspiration, especially in a patient with airway compromise. Blood glucose may rise transiently, but this is not a reason to withhold the drug in a life-threatening situation.

Key point! Beta blockers make anaphylaxis harder to treat because they block the very receptors adrenaline needs. Glucagon bypasses that blockade. Corticosteroids are never the answer for acute, adrenaline-resistant hypotension.