Pathophysiology
Clinical meaning
Beta-blocker overdose produces life-threatening cardiovascular toxicity through excessive blockade of beta-1 and beta-2 adrenergic receptors. Understanding the mechanism guides the NP in selecting appropriate antidotal therapies. Beta-1 receptors in the heart are coupled to stimulatory G-proteins (Gs) that activate adenylyl cyclase, increasing intracellular cAMP. cAMP activates protein kinase A (PKA), which phosphorylates L-type calcium channels (increasing calcium influx for stronger contraction), phospholamban (enhancing sarcoplasmic reticulum calcium reuptake for faster relaxation), and troponin I (modulating calcium sensitivity). In overdose, excessive beta-1 blockade profoundly reduces cAMP, causing severe bradycardia (SA node depression), AV conduction block, and decreased myocardial contractility (cardiogenic shock). Beta-2 blockade in overdose causes bronchospasm (loss of bronchodilatory tone), hypoglycemia (inhibited hepatic glycogenolysis and gluconeogenesis โ especially dangerous in children and diabetics), and peripheral vasoconstriction. Propranolol deserves special attention: it is the most dangerous beta-blocker in overdose due to lipophilicity (crosses the blood-brain barrier causing seizures and coma), sodium channel blockade (widened QRS like tricyclic antidepressant toxicity), and membrane-stabilizing activity. Sotalol is also uniquely dangerous because it blocks potassium channels (Class III antiarrhythmic effect), causing QT prolongation and torsades de...