Beta-Blocker Toxicity Clinical Presentation

Ideally, the clinician should determine the specific beta-blocker involved, the quantity, and the time of the overdose. Unfortunately, these details are often not immediately available. When a history of intentional overdose is lacking, beta-blocker toxicity can go unrecognized as a cause of bradycardia and hypotension

Information regarding the patient's underlying medical condition may be a clinical clue to the possibility of an overdose.

Initial evaluation

The initial evaluation of a comatose patient should include consideration of an occult overdose. If a patient is bradycardic and hypotensive, the clinician should consider a beta-blocker or calcium channel blocker overdose. Other associated symptoms may include hypothermia, hypoglycemia, and seizures. Hypoglycemia is relatively uncommon, but it is described in patients with unstable diabetes and in children; beta-blocking drugs may cause hypoglycemia by inhibiting glycogenolysis.

Myocardial conduction delays with decreased contractility typify the acute beta-blocker ingestion.

Cardiac output may diminish with resulting hypotension from bradycardia and negative inotropy. Hypotension due to the beta2-receptor blockade can be profound and jeopardize myocardial perfusion, creating a downward spiral of events.

Beta-blockers that are not sustained-release formulations are all rapidly absorbed from the gastrointestinal tract. The first critical signs of overdose can appear 20 minutes postingestion but are more commonly observed within 1-2 hours. In all clinically significant beta-blocker overdoses, symptoms develop within 6 hours.

Although the half-life of these compounds is usually short (2-12 h), half-lives in the overdose patient may be prolonged because of a depressed cardiac output, reduced blood flow to the liver and kidneys, or because of the formation of active metabolites.

Saturation kinetics prolong elimination at the type of high plasma concentrations that typically occur with overdose. Delayed absorption from long-acting preparations can significantly increase the apparent elimination half-life. Thus, prolonged effects (>72 h) after massive overdoses are not uncommon.

Asymptomatic intoxication occurs mainly in healthy persons with tolerance to these drugs who ingest beta-blockers that lack membrane-stabilizing effects or have a partial agonist effect (eg, pindolol). Individual sensitivity to beta-blockade may be significantly reduced in those patients who have tolerated therapeutic doses of up to 4 g of propranolol daily and in patients who have sustained deliberate overdoses of both practolol and propranolol without serious adverse effects.

Conversely, circulatory collapse may occur in patients with preexisting cardiac failure when sympathetic drive is inhibited by even a small dose of a particular beta-blocker.

Intermediate toxicity results in a moderate drop in blood pressure (systolic BP >80 mm Hg) and/or bradycardia (heart rate < 60 bpm).

Bradycardia

Bradycardia with associated hypotension and shock (systolic BP < 80 mm Hg, heart rate < 60 bpm) defines severe beta-blocker toxicity. Patients with severe toxicity often manifest extracardiac manifestations of intoxication. Bradycardia, by itself, is not necessarily helpful as a warning sign because slowing of the heart rate and damping of tachycardia in response to stress is observed at therapeutic doses.

Although case reports have documented hypotension in the absence of bradycardia, blood pressure usually does not fall before the onset of bradycardia. Bradycardia may be isolated or accompanied by mild conduction disturbances.

CNS symptoms

A depressed level of consciousness and seizures may occur as a result of cellular hypoxia from poor cardiac output, a direct CNS effect caused by sodium channel blocking, or even as a result of hypoglycemia. The lipid-soluble agents have increased distribution into the brain, and these agents are associated with severe CNS toxicity.

Patients who have taken lipid-soluble beta-blockers, such as propranolol, frequently present with seizures after an overdose. Seizures are generalized and may be multiple but are usually brief, lasting seconds to minutes. Seizures occasionally have been reported after therapeutic use of esmolol and with overdose of alprenolol, metoprolol, and oxprenolol. Seizures are far more common after propranolol overdose.

Coma may be prolonged, depending on the half-life of the agent involved and the coexisting morbidity.

Severe memory impairment developed in an 81-year-old woman taking propranolol 20 mg 3 times per day. Effects were associated with an elevated propranolol blood level (163 mcg/L) and resolved after discontinuation of the drug.

Bronchospasm

Bronchospasm is a rare complication of beta-blocker therapy or overdose but is more likely in patients who already have bronchospastic disease. Sudden fatality following administration of therapeutic doses of beta-blocker has been reported in 4 patients with asthma. Pulmonary edema had been reported to occur as a result of cardiac failure. Respiratory arrest has also been described with beta-blocker intoxication, especially with propranolol, and is thought to be secondary to a central drug effect.