Toxicology – β-Receptor Blocking Drugs (β-Blockers)

Core concept

β-Blockers are competitive antagonists of β-adrenergic receptors used for hypertension, ischemic heart disease, dysrhythmias, heart failure, migraine, thyrotoxicosis, essential tremor, portal hypertension, and glaucoma.

The classic severe overdose produces:

Bradycardia + hypotension + decreased myocardial contractility → cardiogenic shock

Additional important features include:

• AV conduction block
• Hypoglycemia
• Bronchospasm
• CNS depression
• Seizures

Certain β-blockers have additional toxic properties that create distinctive syndromes.

Common β-Blockers

Examples include:

• Atenolol
• Bisoprolol
• Carvedilol
• Esmolol
• Labetalol
• Metoprolol
• Nadolol
• Propranolol
• Sotalol
• Timolol

Ophthalmic preparations such as timolol eye drops can undergo systemic absorption and occasionally cause clinically important bradycardia, hypotension, or bronchospasm.

Pathophysiology

β-Blockers competitively inhibit:

β₁ receptors

Blockade causes:

• Decreased heart rate
• Decreased AV-node conduction
• Decreased myocardial contractility
• Reduced cardiac output
• Decreased renin release

β₂ receptors

Blockade may cause:

• Bronchoconstriction
• Impaired glycogenolysis
• Impaired gluconeogenesis

The overall severe poisoning syndrome is therefore:

β-receptor blockade → bradycardia + negative inotropy + conduction disturbance → hypotension and cardiogenic shock

AHA guidance identifies decreased contractility, bradycardia, and AV nodal blockade as the major mechanisms underlying severe β-blocker poisoning.

Agent-Specific Toxicity

Not all β-blockers behave identically.

Propranolol

Particularly important because it is:

• Highly lipophilic
• Able to readily enter the CNS
• A cardiac sodium-channel blocker at toxic concentrations

Therefore, propranolol overdose may cause:

• Severe CNS depression
• Seizures
• QRS widening
• Ventricular dysrhythmias

Sotalol

In addition to β-blockade, sotalol blocks cardiac potassium channels.

This can cause:

QT prolongation → torsades de pointes

Labetalol and Carvedilol

Also block α₁ receptors, potentially producing greater peripheral vasodilation and hypotension.

Atenolol, Nadolol, and Sotalol

These are relatively hydrophilic and substantially renally eliminated.

They may:

• Accumulate in renal impairment
• Be more amenable to extracorporeal removal than highly lipophilic β-blockers

Toxic Dose

There is no single toxic dose applicable to all β-blockers.

Severity depends on:

• Specific drug
• Formulation
• Amount ingested
• Patient age
• Renal function
• Cardiovascular disease
• Coingestants

Even apparently modest doses may produce clinically important toxicity in:

• Small children
• Older adults
• Patients with severe cardiac disease
• Patients with renal impairment

Risk Factors

Increased risk occurs with:

• Sustained-release formulations
• Renal impairment
• Significant cardiovascular disease
• Advanced age
• Reactive airway disease
• Diabetes
• Coingestion of calcium-channel blockers
• Digoxin
• Clonidine
• Other antihypertensive or cardiodepressant medications

Clinical Features

Cardiovascular

The classic findings are:

Bradycardia + hypotension

Severe poisoning may cause:

• Sinus bradycardia
• First-, second-, or third-degree AV block
• Intraventricular conduction delay
• Reduced contractility
• Cardiogenic shock
• Ventricular dysrhythmias
• Cardiac arrest

Neurologic

Possible effects include:

• Dizziness
• Confusion
• Somnolence
• Coma

Seizures

Seizures are particularly associated with propranolol, reflecting its CNS penetration and membrane-stabilizing/sodium-channel-blocking properties.

Endocrine / Metabolic

Hypoglycemia

β-Blocker poisoning may cause:

Hypoglycemia, particularly in:

• Children
• Patients with diabetes
• Propranolol poisoning

β-blockade may also blunt warning symptoms of hypoglycemia such as:

• Tremor
• Tachycardia

Therefore, glucose should be checked repeatedly in significant poisoning.

AHA guidance recognizes hypoglycemia as a characteristic metabolic complication of β-blocker toxicity.

Pulmonary

β₂ blockade may produce:

• Bronchospasm
• Wheezing

Risk is greater in patients with:

• Asthma
• Other reactive airway disease

Severe cardiogenic shock may additionally produce:

• Pulmonary edema
• Respiratory failure

Characteristic Toxic Syndromes

General β-Blocker Overdose

Bradycardia + hypotension + cardiogenic shock

Propranolol

Bradycardia + hypotension + seizures + QRS widening

Sotalol

Bradycardia + prolonged QT + torsades de pointes

Diagnosis

Diagnosis is generally based on:

Exposure history + characteristic cardiovascular findings

β-blocker concentrations are rarely available rapidly enough to guide treatment and generally have limited clinical utility.

Essential Investigations

For significant exposure obtain:

• 12-lead ECG
• Continuous cardiac monitoring
• Blood glucose
• Serum electrolytes
• Potassium
• Magnesium
• Calcium
• BUN
• Creatinine

Additional Investigations

Depending on severity:

• Blood gas
• Lactate
• Serum CK after prolonged seizures or shock
• Chest radiograph for pulmonary edema or aspiration

In intentional overdose consider:

• Acetaminophen concentration
• Salicylate concentration
• Assessment for other coingestants

ECG Assessment

Specifically evaluate:

• Heart rate
• PR interval
• QRS duration
• QT/QTc
• AV block
• Ventricular dysrhythmias

QRS widening

Think particularly of propranolol.

QT prolongation

Think particularly of sotalol.

Differential Diagnosis

Other causes of bradycardia and hypotension include:

Toxicologic

• Calcium-channel blockers
• Digoxin
• Clonidine
• Class I antiarrhythmics
• Amiodarone
• Sedative agents

Non-toxicologic

• Acute myocardial infarction
• Hyperkalemia
• Hypothermia
• Sinus-node dysfunction
• AV conduction disease

β-Blocker vs Calcium-Channel Blocker Poisoning

The syndromes can overlap considerably.

A useful metabolic clue is:

β-blocker → hypoglycemia may occur

whereas severe calcium-channel blocker toxicity more commonly produces:

hyperglycemia

This distinction is helpful but not absolute.

Treatment

1. Initial Stabilization

Management begins with:

• Airway assessment
• Oxygenation
• Ventilatory support when required
• IV/IO access
• Continuous ECG monitoring
• Frequent blood pressure measurements
• Repeated blood glucose measurements

Patients with life-threatening toxicity benefit from early medical-toxicology or poison-center consultation. The AHA specifically emphasizes early specialist consultation in critically poisoned patients.

2. IV Fluids

Careful isotonic crystalloid administration may be appropriate for hypotension.

However, severe β-blocker poisoning is often primarily cardiogenic, so excessive fluid administration may:

• Fail to improve blood pressure
• Worsen pulmonary edema

Fluid therapy should therefore be reassessed frequently.

3. Atropine

Atropine may be attempted for symptomatic bradycardia.

However:

Severe β-blocker-induced bradycardia often responds poorly to atropine.

Failure of atropine should not delay more effective hemodynamic therapy.

Vasopressors

For life-threatening β-blocker-induced hypotension:

Vasopressors should be administered.

Possible agents include:

• Norepinephrine
• Epinephrine

Selection may be guided by the predominant physiology:

• Cardiogenic shock
• Vasodilatory shock
• Mixed shock

Current AHA guidance gives vasopressor therapy a Class 1 recommendation for life-threatening β-blocker poisoning.

High-Dose Insulin Euglycemia Therapy

Role

High-dose insulin therapy is one of the most important modern treatments for severe β-blocker poisoning.

It improves:

• Myocardial contractility
• Cardiac output
• Myocardial carbohydrate utilization

The AHA recommends high-dose insulin for hypotension refractory to vasopressors in life-threatening β-blocker poisoning.

Typical Regimen

A commonly recommended regimen is:

Regular insulin 1 unit/kg IV bolus

followed by:

1–10 units/kg/hour IV infusion

with concurrent glucose supplementation as needed to maintain euglycemia.

Treatment is titrated to:

• Blood pressure
• Perfusion
• Cardiac output
• Clinical response

Monitoring During High-Dose Insulin

Closely monitor:

• Blood glucose
• Serum potassium
• Fluid balance

Major complications include:

• Hypoglycemia
• Hypokalemia
• Volume overload

Protocolized therapy reduces the risk of serious hypoglycemia.

Importantly, the fall in serum potassium during insulin therapy usually reflects an intracellular shift rather than true total-body potassium depletion, so potassium replacement should be careful and guided by serial measurements.

Glucagon

Mechanism

Glucagon activates adenylate cyclase through a receptor independent of the β-adrenergic receptor.

This can increase:

Intracellular cAMP → heart rate + myocardial contractility

Role

Glucagon has historically been regarded as the classic β-blocker antidote.

Modern evidence is less robust than older textbooks imply, but current AHA guidance states that a glucagon bolus followed by infusion is reasonable for symptomatic bradycardia or hypotension in life-threatening β-blocker poisoning.

Typical Adult Dose

A commonly used regimen is approximately:

5–10 mg IV bolus

followed, if an initial response occurs, by a continuous infusion titrated to effect.

Older weight-based regimens such as 50–150 μg/kg are also described.

Adverse Effects

Glucagon commonly causes:

• Nausea
• Vomiting
• Hyperglycemia

Because vomiting is common and the patient may already have depressed consciousness, airway protection is important.

Tachyphylaxis may occur during prolonged infusion.

Practical Point

Glucagon should generally be considered an adjunct, not a substitute for:

• Vasopressors
• High-dose insulin
• Appropriate critical-care support

Calcium

Although calcium is much more strongly associated with treatment of calcium-channel blocker toxicity, it may provide some hemodynamic benefit in severe β-blocker poisoning.

The AHA states that calcium may be reasonable in life-threatening β-blocker toxicity.

It should be regarded as adjunctive therapy.

Propranolol – Sodium Bicarbonate

Propranolol can block fast cardiac sodium channels.

Therefore:

Propranolol overdose + wide QRS / ventricular conduction abnormality → consider IV sodium bicarbonate

Sodium bicarbonate is used similarly to other sodium-channel-blocking poisonings to:

• Increase extracellular sodium
• Alkalinize serum
• Improve cardiac conduction

Current toxicology references specifically recommend sodium bicarbonate for propranolol-associated QRS widening.

Sotalol – QT Prolongation and Torsades

Sotalol blocks potassium channels and can cause marked QT prolongation.

If torsades de pointes develops:

• Correct potassium
• Correct magnesium
• Give IV magnesium
• Manage according to standard torsades protocols

Overdrive pacing or chronotropic therapy may occasionally be required for recurrent bradycardia-dependent torsades.

Seizures

Treat seizures with:

Benzodiazepines

while simultaneously correcting:

• Hypoglycemia
• Hypotension
• Hypoxia

This is particularly relevant to propranolol toxicity.

Gastrointestinal Decontamination

Do Not Induce Vomiting

Emesis should not be induced because:

• Bradycardia
• Hypotension
• Seizures
• Altered consciousness

may develop abruptly.

Activated Charcoal

A single dose of activated charcoal may be considered after a significant recent ingestion when:

• Presentation is early
• The airway is intact or protected
• Aspiration risk is acceptable

Routine use in a severely unstable or obtunded patient with an unprotected airway is inappropriate.

Gastric Lavage

The older source recommends gastric lavage after substantial early ingestion.

Routine gastric lavage is not part of contemporary β-blocker overdose management.

It should be reserved, if ever used, for exceptional circumstances after specialist toxicology consultation and appropriate airway protection.

Sustained-Release Preparations

Sustained-release preparations may:

• Delay onset of toxicity
• Prolong toxicity

Whole-bowel irrigation with polyethylene glycol may be considered in selected substantial sustained-release ingestions, particularly before severe instability develops.

Intravenous Lipid Emulsion

Because some β-blockers, especially propranolol, are highly lipophilic, IV lipid emulsion has been used as rescue therapy.

However:

Evidence of benefit is uncertain.

The AHA states that the usefulness of IV lipid emulsion for refractory β-blocker shock remains uncertain.

It should therefore generally be considered only in severe refractory poisoning with specialist toxicology guidance.

Cardiac Pacing

Temporary pacing may be attempted for severe bradycardia or AV block.

However, electrical capture does not guarantee adequate mechanical cardiac output in profound myocardial depression.

Therefore:

Pacing should not delay high-dose insulin, vasopressors, and other hemodynamic therapies.

VA-ECMO / Extracorporeal Life Support

For refractory cardiogenic shock despite maximal pharmacologic therapy, venoarterial extracorporeal membrane oxygenation may be lifesaving.

The AHA considers ECLS/VA-ECMO reasonable for adults and children with β-blocker poisoning and cardiogenic shock refractory to pharmacologic therapy.

This is particularly relevant when:

• The poisoning is potentially reversible
• Profound myocardial depression persists
• Conventional therapies fail

Hemodialysis

Dialyzability varies substantially between β-blockers.

Potentially Dialyzable

Hemodialysis may be useful in severe poisoning with:

• Atenolol
• Nadolol
• Sotalol

The AHA states that hemodialysis may be reasonable for life-threatening poisoning from these agents.

Atenolol

EXTRIP suggests extracorporeal treatment for severe atenolol poisoning with renal impairment when refractory:

• Bradycardia
• Hypotension

are present.

Sotalol

EXTRIP suggests extracorporeal treatment in severe sotalol poisoning with renal impairment when there is:

• Refractory bradycardia/hypotension
• Recurrent torsades de pointes

Propranolol

Hemodialysis is not useful for propranolol, because it is highly protein-bound and has a large volume of distribution.

EXTRIP specifically recommends against extracorporeal treatment for severe propranolol poisoning as an addition to standard care.

Monitoring

Patients with significant toxicity require:

• Continuous ECG
• Continuous hemodynamic monitoring
• Serial glucose
• Serial potassium
• Magnesium
• Renal function
• Frequent neurologic assessment

During high-dose insulin therapy:

• Glucose should initially be monitored very frequently
• Potassium should be checked repeatedly
• Fluid administration should be closely tracked

Admission

Hospital admission is indicated for:

• Symptomatic bradycardia
• Hypotension
• AV block
• QRS widening
• QT prolongation
• Dysrhythmias
• Seizures
• Hypoglycemia
• Sustained-release overdose
• Significant intentional overdose

Patients with significant cardiovascular toxicity generally require ICU management.

Observation

Patients with immediate-release exposures who remain entirely asymptomatic with a normal ECG after an appropriate observation period may be considered for discharge.

However, observation should be individualized according to:

• Specific β-blocker
• Dose
• Immediate- vs sustained-release formulation
• Renal function
• Coingestants

Sustained-release preparations and sotalol generally require longer monitoring because delayed or prolonged toxicity is possible.

Pregnancy

The historical FDA pregnancy letter categories A, B, C, D, and X are no longer used.

Current medication labeling provides individualized information on:

• Pregnancy risk
• Clinical considerations
• Available human and animal data

Therefore, the older Category B/C classifications in the source should not be used as current pregnancy guidance.

Prognosis

Most mild exposures have a good outcome with observation and supportive care.

Poor prognostic features include:

• Profound hypotension
• Cardiogenic shock
• Severe conduction disturbances
• Ventricular dysrhythmias
• Recurrent seizures
• Sustained-release overdose
• Significant cardiodepressant coingestion
• Advanced cardiovascular disease

Even profound toxicity can be reversible with aggressive critical-care management.

Important Pitfalls

1. Treating all β-blockers as identical

Remember:

Propranolol → seizures + QRS widening

Sotalol → QT prolongation + torsades

Atenolol/nadolol/sotalol → potentially dialyzable

2. Relying only on glucagon

Glucagon may help, but severe shock often requires:

Vasopressors + high-dose insulin ± glucagon

3. Delaying high-dose insulin

High-dose insulin is a major modern therapy for life-threatening β-blocker-induced cardiogenic shock.

4. Missing hypoglycemia

Check glucose repeatedly, particularly in:

• Children
• Diabetics
• Propranolol poisoning

5. Missing sodium-channel blockade

A wide QRS after propranolol overdose should prompt consideration of sodium bicarbonate.

6. Missing QT toxicity

Sotalol overdose can produce delayed or recurrent torsades de pointes.

7. Assuming eye drops cannot cause systemic toxicity

Ophthalmic β-blockers such as timolol can be systemically absorbed and cause significant cardiovascular or respiratory effects.

8. Excessive IV fluids

Profound hypotension may be due primarily to myocardial depression rather than volume depletion.

Excessive fluids may worsen pulmonary edema.

9. Using routine gastric lavage

Aggressive GI decontamination is generally less important than rapid cardiovascular stabilization.

High-Yield Toxicology Pearls

β-Blocker overdose = bradycardia + hypotension + cardiogenic shock

Think:

Bradycardia + hypotension ± hypoglycemia

Important agent-specific clues:

Propranolol → seizures + wide QRS

Sotalol → prolonged QT + torsades

Key points:

• Mechanism: competitive β-adrenergic receptor blockade
• Main toxicity: bradycardia, negative inotropy, and hypotension
• Severe poisoning can cause AV block and cardiogenic shock
• Hypoglycemia is particularly important in children
• Bronchospasm may occur
• Continuous ECG and glucose monitoring are essential
• Vasopressors are first-line hemodynamic support
• High-dose insulin is a major therapy for severe refractory hypotension
• Typical high-dose insulin regimen: 1 U/kg IV bolus → 1–10 U/kg/h infusion
• Monitor glucose and potassium closely during insulin therapy
• Glucagon is a reasonable adjunct, but is no longer viewed as sufficient monotherapy for severe poisoning
• Sodium bicarbonate is important for propranolol-associated QRS widening
• IV magnesium is important for sotalol-associated torsades
• Calcium may provide adjunctive benefit
• IV lipid emulsion has uncertain benefit
• VA-ECMO may be lifesaving in refractory cardiogenic shock
• Hemodialysis may help severe atenolol, nadolol, or sotalol poisoning
• Hemodialysis is not useful for propranolol