Fig. 3.

Example data from experimental protocol. A) Following a baseline period, 10% carbon dioxide was administered for 5 minutes, which significantly increased respiratory drive. After a recovery period, morphine was administered at a dose (4 mg/kg) that produced respiratory depression. Dashed line indicates zero-point of signal. B) At intervals, anesthetized animals exhibited sighs (arrowheads), brief increases in inspiration that open alveoli and prevent atelectasis (Orem and Trotter, 1993). Recordings in this figure were taken from the same animal, using all three methods simultaneously.