Fentanyl-induced changes in cardiorespiratory responses to PBG. A: A representative recording showing that fentanyl (FEN, 8 μg/kg, iv) converted the PBG (4.5 μg/kg)-induced RSB (left) into a long-lasting apnea (middle) and this facilitating effect disappeared 2 h later (right). The traces from the top to bottom are arterial blood pressure (BP) and tidal volume (VT). B: Group data of the effects of FEN on the cardiorespiratory responses to PBG. N = 7; mean ± SE. Note: all the cardiorespiratory responses to PBG were significant (P < 0.01). * P < 0.05 and ** P < 0.01 compared with before fentanyl. TE, expiratory duration; MBP, mean arterial blood pressure; HR, heart rate.