There are a number of ways to classify β-blockers. A simple functional classification is described in the table below [1].
| β-1 selective | β1/β2 selective (non-selective) | |
|---|---|---|
| No vasodilatory activity | • Bisoprolol • Esmolol • Atenolol • Metoprolol | • Propranolol • Sotalol • Pindolol • Nadolol • Timolol |
| Vasodilatory activity | • Nebivolol • Celiprolol | • Labetalol • Carvedilol |
Antagonism of β-receptors can cause bradycardia and hypotension for all drugs in this class.
β1 adrenergic antagonism
β2 adrenergic antagonism
Other effects In addition to their cardiac effects, beta blocking drugs may also cause hypoglycaemia (by inhibiting glycogenolysis) and /or bronchospasm (in susceptible individuals.) Propranolol and sotalol have the highest relative toxicity due to other pharmacological properties of these drugs: sodium channel blockade (propranolol) and potassium channel blockade (sotalol).
Note that this section has bullets with exposition, and then subheadings with exposition. Please can you compare and we can decide how to move forward with similar situations. Options: - Bullet points only for headers + description below with subheadings (like the original https://www.wikitox.org/doku.php?id=wiki:3.4.3.4.2_beta_blockers#mechanism_of_toxic_effects) - Bullet points for headers + description in bullet points - Subheadings with description, no bullet points to signpost
Individual drugs in this class differ based on their unique pharmacological properties, which include:
Cardioselectivity (β1-selectivity)
While β1-selectivity can influence adverse effects in therapeutic use, it becomes less relevant in overdose situations because selectivity is lost at high drug concentrations.
Intrinsic sympathomimetic activity (ISA)
Some β-blockers have ISA due to partial β agonism and may result in tachycardia and hypertension. This partial agonist effect rarely leads to significant problems and probably protects to some extent from the more serious class I and III antiarrhythmic effects. Drugs with ISA include acebutolol, pindolol, labetalol, and celiprolol.
Membrane-stabilizing activity / class I antiarrhythmic effects
The membrane-stabilizing activity of some β-blockers is due to the inhibition of fast Na+ channels (class I anti-arrhythmic activity). These effects usually only occur at high drug concentrations. Propranolol has the most membrane-stabilizing activity of the β-blockers and can result in impaired AV conduction, widened QRS interval, ventricular tachyarrhythmias, coma, and seizures.
K+ channel blockade / class III antiarrhythmic effects
Some β-blockers block the delayed rectifier outward K+ channel which is responsible for cell repolarization. This prolongs the action potential duration and prolongs the QT interval, which can predispose to arrhythmias. Examples of these β-blockers include sotalol and acebutolol.
Vasodilatory activity
The vasodilatory activity of certain β-blockers can theoretically enhance the hypotensive effects in cases of β-blocker overdose.
Lipid solubility
Only lipid soluble drugs will lead to direct CNS effects as they are able to penetrate the blood brain barrier, though CNS symptoms may occur secondary to cardiac effects and decreased cerebral perfusion. Lipid solubility alone will not lead to CNS effects and they may relate to Na+ channel blocking effects as they are particularly common with propranolol.
Renal/hepatic clearance
This is occasionally important in therapeutics but is largely irrelevant to overdose.
The prognosis correlates best with the degree of heart block/bradycardia. Factors in the history that increase the severity of the overdose are:
Most beta-blockers are rapidly absorbed from the small intestine except esmolol (IV administration) and atenolol (~50% GI absorption). However, most have relatively low oral bioavailability due to high first-pass metabolism except sotalol and bisoprolol (both 90% bioavailable). In overdose, bioavailability increases because the enzymes responsible for first-pass metabolism become saturated. Under therapeutic conditions, peak drug concentrations are typically reached within 1-4 hours.
All β-blockers have moderate to large volumes of distribution, roughly proportional to their lipid solubility. The drug's lipid solubility also determines the degree of CNS penetration. Most are also relatively highly protein-bound.
Most β-blockers undergo extensive hepatic metabolism except atenolol, sotalol, and esmolol. The half-life of most beta-blockers at therapeutic doses is less than 12 hours. In overdose, the half life of β-blockers vary and are prolonged due to reduced cardiac output (reduced blood flow to liver and kidneys) and/or the formation of active metabolites.
The more water-soluble β-blockers (atenolol, sotalol) are primarily excreted unchanged by the kidneys. The more lipid-soluble β-blockers undergo extensive hepatic metabolism, and their metabolites are excreted via the urine or bile.
The principal clinical effects of β-blocker toxicity are hypotension and bradycardia.
The cardiovascular manifestations of β-antagonist poisoning typically are bradydysrhythmias, cardiac conduction defects, hypotension, and circulatory shock.
Bradyarrhythmias and cardiac conduction defects
Varying degrees of bradyarrhythmia may occur (sinus bradycardia, 1st to 3rd degree heart block, junctional or ventricular bradycardia, or asystole) and deterioration may occur rapidly and without warning. Vagal stimuli (gastric lavage, emesis, intubation) and seizures are precipitants for cardiac arrest. Atropine pretreatment should be used prior to any intervention that could enhance vagal tone.
Other ECG changes including QRS and QT prolongation occur and are a measure of severity.
Pump failure
Direct myocardial depression due to the negative inotropy effects of β-blockers can complicate circulatory shock.
Hypotension
Hypotension occurs due to a combination of bradycardia (with or without heart block) and direct myocardial depression. Toxicity develops over the first few hours. Intractable hypotension with extreme bradycardia and/or asystole is the usual mode of death.
The two primary neurologic manifestations of β-blocker toxicity are CNS depression and seizures.
CNS depression
Drowsiness is commonly due to cardiovascular depression and decreased cerebral perfusion, and may respond to correction of hypotension.
Seizures
Seizures are primarily linked to overdoses of the lipophilic β-blockers with propranolol being disproportionately implicated. Risk factors for seizures in propranolol overdose include ingestion of > 2 g of propranolol and QRS width >100 ms [2].
Bronchospasm
β-blocker overdose can result in bronchospasm as a result of β2 antagonism, particularly in individuals with underlying reactive airway disease. It is unlikely
Hypoglycaemia
β-blocking drugs may cause hypoglycemia by inhibiting glycogenolysis.
Hyperglycaemia, due to a combination of glucagon treatment and impaired insulin release (due to beta blockade) may also occur.
Need to check this - never seen this elsewhere: There are some reports of patients responding to glucose with “normal” blood glucose measurements. Therefore, it is worth giving a bolus of 50% glucose to any patient with CNS effects.
Blood glucose level
β-blockers can cause hypoglycemia in overdose.
Serum biochemistry and blood gas
A basic biochemistry panel is used to assess for electrolyte derangements and renal function.
Serum paracetamol and salicylate levels
This is done to rule out common co-ingestants. Serum β-blocker concentrations are generally unhelpful because they cannot be obtained in time to be clinically useful.
ECG
Serial 12-lead ECGs with continuous cardiac monitoring is used to identify signs of cardiotoxicity. These include:
There are a number of drugs that can lead to a patient presenting with profound hypotension and bradycardia. Correct diagnosis is important as these drugs have different specific treatments.
Propranolol
Propranolol is the only beta-blocker that frequently causes seizures. In one series, of those who ingested more than 2 g of propranolol, two thirds had a seizure. It also causes more severe cardiovascular effects and death more commonly than other widely used beta blocking drugs. Propranolol also appears to be over represented in beta-blocker poisoning when corrected for frequency of prescription (Reith et al 1996). This presumably relates to propranolol being taken by a younger age group for predominantly non-cardiac indications.
Seizures are a common and serious complication of poisoning with lipophilic beta-blockers such as propranolol and may precipitate cardiac complications..
Compared with the other β-adrenergic antagonists, propranolol accounts for a disproportionate number of reported cases of self-poisoning50,204 and deaths.130,156 This may be explained by the fact that propranolol is frequently prescribed to patients with diagnoses such as anxiety, stress, and migraine who may be more prone to suicide attempts. Propranolol is more lethal because of its lipophilic and membrane-stabilizing properties
Sotalol
Sotalol may frequently cause significant QT prolongation and torsade de pointes (occasionally reported with propranolol) as well as the usual manifestations of beta blockade. Other factors relate to:
Intrinsic sympathomimetic activity (partial agonist activity) Lipid solubility (CNS effects)
IV access with IV fluids (normal saline) should be secured as soon as possible. ECG monitoring in intensive care is indicated for all but the most trivial propranolol or sotalol poisonings. Glucose should be given to any patient with decreased consciousness or seizures regardless of a normal blood sugar.
Syrup of ipecac should not be used to decontaminate beta-blocker poisonings under any circumstances. Gastric lavage should be considered in large ingestions of propranolol or sotalol if patients present within one hour of ingestion. Atropine should be given prior to lavage and in any patient who is vomiting. Oral activated charcoal should be given to all patients ingesting any overdose of a beta-blocking drug who present within 2 hours.
The drugs that are water soluble are predominantly renally cleared. Among these drugs, only sotalol has significant “antiarrhythmic” effects and frequently causes life threatening poisoning. Thus, haemodialysis is unlikely to be useful except perhaps for sotalol in patients with life threatening toxicity and impaired renal function.
There are a number of drugs that will antagonize some of the cardiac effects of beta-blockers. All these treatments may be used simultaneously, if this is required
Atropine Glucagon Isoprenaline Dextrose & Insulin
Atropine
This should be tried in all patients with bradycardia. It should be given prior to intubation, lavage, or any other procedure that might increase vagal tone and in patients who are nauseated or vomiting.
Glucagon
Glucagon had been used as antidote for beta-blocker poisoning in the past but its use has been largely superseded by insulin dextrose. The rationale for its use is that it increases cyclic AMP and activates myosin kinase independent of beta-receptors. The dose is 5 - 10 mg IV as a bolus and then an infusion titrated against heart rate and blood pressure (starting at 5 - 10 mg/hour).
Isoprenaline This is a non-selective competitive beta agonist. Doses should also be titrated against cardiac parameters and the dose required may be ten or twenty fold larger than normally used. As both the agonist and antagonist are competing for the same receptors, much larger doses are needed to reach the same level of receptor occupancy. Dose requirements will fall rapidly as the beta-blocking drug is metabolised.
Patients who require inotropics support should be commenced on Dextrose & Insulin.
This should be implemented in patients not responding to isoprenaline.
Seizures Glucose should be given regardless of a normal blood sugar. Otherwise, they should be treated conventionally with benzodiazepines(eg diazepam). If seizures are refractory-use phenobarbitone.
Arrhythmias Ventricular tachycardia (torsades de pointes) may occur with sotalol or occasionally propranolol. Conventional treatment is with magnesium, isoprenaline, or cardiac pacing. Magnesium has calcium channel blocking effects and is potentially hazardous as it may further impair cardiac conduction and contractility. It should be used with great caution if at all. Isoprenaline or cardiac pacing to achieve a heart rate of 120-140 bpm is the safest option.
Occasional late complications/deterioration have been reported generally in patients who have had significant poisoning. It is likely that these relate to too rapid withdrawal of treatment. Long term sequelae have not been reported and no follow up is required after resolution of the clinical signs - ECG findings unless the patient has been profoundly hypotensive.
Useful general references: