Two-minute captures of traces from a bioimpedance-based respiratory volume monitor over the course of the peri-operative stay, with average minute ventilation, tidal volume and respiratory rate. (A) Normal pre-operative breathing. (B) Pre-operative apnea. (C) Apnea prior to opioid administration in the post-anesthesia care unit (PACU). (D) Reduced ventilation with obstructed breathing after an initial opioid administration. Predicted minute ventilation (MVPRED) based on ideal body weight for the patient was 7.9L/min. (E) Time course of the patient’s minute ventilation (MV) over his entire PACU stay. Dashed horizontal lines represent (from top to bottom) 100%, 80% and 40% of MVPRED. Purple lines indicate opioid administrations (hydromorphone). Ventilation decreased following opioid administration and was persistently low until discharge. Respiratory rate (RR) and oxygen saturation levels, as documented by clinical personnel in the PACU flowchart, are shown below. (F) Mean and standard deviation for MV measurements recorded by the respiratory volume monitor (RVM) during 5 minutes of patient rest before and after an initial opioid administration (15:32) in the PACU (periods shown in gray in (E)). The RVM data depict a sudden drop in ventilation that is not reflected in either RR or oxygen saturation levels. TV, Tidal volume.