This risk is higher in specific groups, like those with obstructive sleep apnea.Other factors that increase the risk of respiratory depression are:
High fentanyl doses
Sleep
Older age
Simultaneous use of CNS depressants like benzodiazepines, barbiturates, alcohol, and inhaled anesthetics
Hyperventilation
Decreased CO2 levels in the serum
Respiratory acidosis
Decreased fentanyl clearance from the body
Decreased blood flow to the liver
Renal insufficiencySustained release fentanyl preparations, such as patches, may also produce unexpected delayed respiratory depression. The precise reason for sudden respiratory depression is unclear, but there are several hypotheses:

Saturation of the body fat compartment in people with rapid and profound body fat loss (people with cancer, cardiac or infection-induced cachexia can lose 80% of their body fat). Early carbon dioxide retention causing cutaneous vasodilation (releasing more fentanyl), together with acidosis, which reduces protein binding of fentanyl, releasing yet more fentanyl. Reduced sedation, losing a useful early warning sign of opioid toxicity and resulting in levels closer to respiratory-depressant levels.Another related complication of fentanyl overdoses includes the so-called wooden chest syndrome, which quickly induces complete respiratory failure by paralyzing the thoracic muscles, explained in more detail in the Muscle rigidity section below. Heart and blood vessels
Bradycardia: Fentanyl decreases the heart rate by increasing vagal nerve tone in the brainstem, which increases the parasympathetic drive. Vasodilation: It also vasodilates arterial and venous blood vessels through a central mechanism, by primarily slowing down vasomotor centers in the brainstem. To a lesser extent, it does this by directly affecting blood vessels.
State use
In August 2018, Nebraska became the first American state to use fentanyl to execute a prisoner. Carey Dean Moore, at the time one of the longest-serving death row inmates in the United States, was executed at the Nebraska State Penitentiary. Moore received a lethal injection, administered as an intravenous series of four drugs that included fentanyl citrate, to inhibit breathing and render the subject unconscious. The other drugs included diazepam as a tranquilizer, cisatracurium besylate as a muscle relaxant, and potassium chloride to stop the heart. The use of fentanyl in execution caused concern among death penalty experts because it was part of a previously untested drug cocktail. The execution was also protested by anti-death penalty advocates at the prison during the execution and later at the Nebraska capitol building.Russian Spetsnaz security forces are suspected to have used a fentanyl analogue, or derivative, to incapacitate people rapidly in the Moscow theater hostage crisis in 2002. The siege was ended, but some hostages may have died from the gas after their health was severely taxed during the days long siege. The Russian Health Minister later stated that the gas was based on fentanyl, but the exact chemical agent has not been identified. Veterinary use
Fentanyl in injectable formulation is commonly used for analgesia and as a component of balanced sedation and general anesthesia in small animal patients. Its potency and short duration of action make it particularly useful in critically ill patients.