A 19-year-old G1P0 was hospitalized in her 31st week of pregnancy with a 2-day history of abdominal pain which was diagnosed as acute appendicitis. While receiving MgSO4infusion to treat preterm labor, she underwent appendectomy under uneventful spinal anesthesia. 12 hours later, she developed marked tachypnea of 40–50/min and ABGs showed significant hypoxemia (PaO2= 42 mmHg on room air). In the SICU, PaO2on 100% oxygen by mask was 83 mmHg and after CPAP failed to improve ventilation, she was intubated. Fetal monitoring showed good fetal parameters. With PEEP settings of 10 cmH2O, PaO2was 60 mmHg with an FIO2of 0.4, with chest radiogram indicating extensive bilateral pulmonary edema despite a PCWP of 15 mmHg. She was delivered vaginally with IV meperidine only for analgesia 62 hours after appendectomy. Despite a transient episode of desaturation at delivery, requiring an FIO2of 1.0 for correction, the patient's ventilatory status improved steadily, permitting extubation 18 hours postpartum. At delivery oxygen consumption and intrapulmonary shunting were maximal. After delivery Qs/Qt fell from 28% to 8–13%, SaO2rose from 86.1 to 97.4% with an FIO2of 0.4, and oxygen consumption fell from 209 ml O2/min/m2to 125 ml O2/min/m2.