With midazolam and alfentanil iv the synergistic effects knock out the ability to sense CO levels in the blood  so all voluntary respiration stops  its used in Intensive Care for people on mechanical ventilators to stop them fighting the machine but is adminsitered in the NHS on side wards mainly  CO just builds up in the blood but the midazolam and morphine levels are so high they dont twitch and upset the bitches of nurses who know exactly what they are doing when they inject this stuff and dont really give a toss one way or the other