Patient-Controlled Analgesia (PCA) delivers parenteral opioids on-demand via a programmable infusion pump, allowing patients to self-titrate to their individual minimum effective analgesic concentration (MEAC). The MEAC concept explains why PCA is superior to PRN dosing: there is a narrow serum opioid concentration range (unique to each patient) where analgesia occurs without excessive sedation. Below the MEAC, patients experience pain; above the ceiling concentration, side effects increase without additional analgesia. With PRN dosing, serum levels oscillate widely above and below the MEAC due to dosing delays (patient requests → nurse availability → preparation → administration = 30-60 minute gaps). PCA eliminates these delays, maintaining more consistent serum levels near the MEAC. PCA programming parameters: demand dose (amount delivered per button press), lockout interval (minimum time between doses, prevents stacking), continuous/basal rate (optional background infusion - increases respiratory depression risk, generally NOT recommended for opioid-naive patients), and 1-hour/4-hour maximum dose limit (safety ceiling). The inherent safety of PCA: if the patient becomes overly sedated, they stop pressing the button (self-limiting mechanism), allowing serum levels to decrease. This safety mechanism is DEFEATED by...