This study investigates how well fentanyl is absorbed through the skin when delivered via a transdermal patch in critically ill surgical ICU patients. It compares hemodynamically stable patients with unstable patients who require vasopressors to maintain adequate blood pressure. Fentanyl blood levels will be measured over time to assess whether absorption is impaired in unstable patients. The goal is to determine whether transdermal fentanyl is a viable option for pain management in resource-limited ICUs or in situations where intravenous fentanyl is unavailable.