Clinical meaning
Opioid overdose is a life-threatening medical emergency caused by excessive activation of mu-opioid receptors in the central nervous system, resulting in profound respiratory depression, central nervous system depression, and cardiovascular collapse. Opioids include both prescription medications (morphine, hydromorphone, oxycodone, fentanyl, codeine, methadone) and illicit substances (heroin, illicitly manufactured fentanyl). Opioids exert their effects by binding to three primary receptor types: mu receptors (responsible for analgesia, euphoria, respiratory depression, and miosis), kappa receptors (analgesia, sedation, dysphoria), and delta receptors (analgesia, mood modulation). In overdose, excessive mu receptor activation in the brainstem respiratory centers (the pre-Botzinger complex and the nucleus tractus solitarius) suppresses the medullary response to rising CO2 levels, causing the respiratory drive to diminish and eventually cease. Normal breathing is driven by chemoreceptors that detect elevated carbon dioxide (PaCO2) in the blood; opioids raise the threshold at which these chemoreceptors trigger a breath, meaning higher and higher CO2 levels are required to stimulate breathing until the threshold becomes unreachable and apnea occurs. The classic opioid toxidrome consists of three clinical findings: pinpoint pupils (miosis from parasympathetic stimulation of the Edinger-Westphal...