Glucagon

Glucagon is a polypeptide hormone commonly used in the treatment of severe hypoglycemia with FDA approval for the treatment of severe hypoglycemia and as a diagnostic aid in imaging of the GI tract. Glucagon has several off-label indications, including beta-blocker overdose therapy, calcium channel blocker overdose adjunctive therapy, and medical treatment of esophageal food impaction. Glucagon's indications have continued to develop since its discovery, and research is ongoing for use to treat asthmatic bronchospasm, as a constituent in bi-hormonal artificial pancreases along with more traditional insulin, and the treatment of arrhythmias secondary to cardiac anaphylaxis.[1][2][3]

Severe hypoglycemia is a life-threatening event treated with oral carbohydrate intake, IV glucose, or glucagon by various routes. Glucagon has attractive traits in the diabetic population due to its simplicity of use and safe administration, not requiring patent IV access.  Patients with decreased levels of consciousness cannot safely consume the oral carbohydrates needed to raise their blood sugar without risk of aspiration, and obtaining IV access can be problematic in the diabetic population, which can prevent prompt administration of IV glucose.  IV glucose also runs the risk of severe tissue necrosis in the setting of IV medication extravasation. Glucagon is a reliable method of raising the patient's glucose and relieving severe hypoglycemia long enough for more definitive correction of the patient's glucose levels by mouth, particularly when IV access is unavailable to the provider or has failed.[4]

Glucagon's role in gastrointestinal imaging is to induce bowel and upper gastrointestinal tract hypotonia, which permits more precise visualization for studies and procedures; this is the same mechanism by which it helps esophageal food boluses pass. The primary indicated use is to decrease peristalsis during abdominal vascular procedures such as treatment of esophageal varices and other GI bleeds. Glucagon is also utilized in biopsies, abscess drainage, GI stenting, and gastrostomy tube placement. Previously, radiologists used anticholinergic agents for the same indication but switched to glucagon in the 1970s due to a safer side effect profile.[5][6]

Beta-blocker overdose and calcium channel blocker overdose can be treated with high-dose glucagon infusion in addition to beta-agonists such as epinephrine, because of the logistical difficulties associated with obtaining large amounts of glucagon needed for infusion, the recommendation that high dose insulin euglycaemic therapy (HIET) and catecholamine therapy be optimized before adding glucagon infusion unless readily available in adequate quantities. Calcium channel blocker overdose can also receive treatment with glucagon; in this instance, glucagon is a second line therapy behind calcium, epinephrine, and high dose insulin and dextrose.[7][8]

Glucagon binds G-coupled surface receptors found throughout the body in varying concentrations; binding to the glucagon receptors in the liver, GI tract, heart, pancreas, fat, adrenal glands, and kidneys activate adenylate cyclase which in turn raises cAMP levels. cAMP stimulates glycogenolysis and glucogenesis, resulting in the release of glucose, primarily from liver glycogen stores. The extrahepatic effects of glucagon are also mediated by adenylate cyclase, including relaxation of GI smooth muscle and positive inotropic effects.[9][10]

Glucagon can be administered IV as a bolus or infusion, IM, subcutaneously, and intranasally. Glucagon is available as a dehydrated powder termed a "Glucagon Emergency Kit," which is reconstituted with supplied sterile water or as a purpose formulated intranasal spray. Similar to epinephrine auto-injectors, a pre-filled glucagon injector has received approval, which injects into the patient's thigh. Healthcare providers will most often encounter the emergency kit formulation necessitating reconstitution before injecting the medication, but the intranasal spray may be encountered with increasing frequency in hypoglycemia kits for layperson use. The intranasal powder requires no preparation, and administration is via a spray into the patient's nose while holding the other nare closed.[11][12][13][14]

Nausea is the most frequently encountered adverse effect of glucagon administration, with an incidence as high as 35% in some studies. Hypertension has been described up to 2 hours following administration for GI cases due to the inotropic effects of glucagon. Additionally, severe anaphylactic reactions, including hypotension, rash, and vomiting, have been observed due to the protein structure. Hypersensitivity reactions are rare, and most have occurred in the endoscopic setting with IV administration. Adverse reactions from glucagon administration become more likely with the IV route and when giving higher doses.[14][15]

Less common adverse reactions include rebound hypoglycemia in the setting of insulinoma and worsening hyperglycemia in the setting of pancreatic glucagon secreting tumor. Additionally, the positive inotropic effects of glucagon can precipitate severe hypertension when administered to patients with pheochromocytoma and should be avoided in favor of oral or IV glucose for emergency use and the clonidine suppression test for pheochromocytoma diagnosis.[16]

The only absolute contraindication to glucagon administration is known hypersensitivity to the medication. Most hypersensitivity reactions occur in GI imaging patients.[17]

Relative contraindications include use in neonates or children, which may not have sufficient glycogen stores, patients with known insulinoma, pheochromocytoma, or glucagon secreting tumor, and known lactose allergy (some formulations contain lactose).[18][19][20]