Population Pharmacokinetics of Remifentanil in Low-Weight Critically Ill Patients

The goal of this prospective clinical study is to learn about the population pharmacokinetics (PopPK), safety, and efficacy of remifentanil in low-body-weight patients in the Intensive Care Unit (ICU) who require mechanical ventilation. The main questions it aims to answer are: 1. What are the pharmacokinetic characteristics of remifentanil in low-weight ICU patients? 2. How does remifentanil (and concurrent propofol use) affect hemodynamic stability (such as blood pressure) and sedation efficacy in this population? Participants receiving mechanical ventilation will receive remifentanil for analgesia and sedation according to a standardized protocol. They will undergo arterial blood sampling at specific time points to measure drug concentrations. Researchers will also record hemodynamic parameters. Additionally, a subset of patients receiving propofol as a rescue sedative will have plasma propofol concentrations measured to evaluate its influence on blood pressure changes.