Atelectasis can be prevented by intermittent deep breathing. Patients with severe abdominal pain take occasional deep breaths, but these differ from those of normal volunteers. When morphine is given, deep breathing ceases. It appears that morphine, in doses which do not grossly depress ventilation and exchange of gases, may depress the reflex urge to maintain normal pulmonary mechanics. This may explain the increased incidence of postoperative atelectasis following large doses of narcotics. Since severe postoperative pain must be alleviated, measures to reduce or prevent pulmonary complications should be considered. These include vigorous encouragement to do breathing exercises, use of forced deep inspiration with bag and mask, and regional anesthesia to block pain locally.
Egbert LD, Bendixen HH. Effect of Morphine on Breathing PatternA Possible Factor in Atelectasis. JAMA. 1964;188(6):485–488. doi:10.1001/jama.1964.03060320007001