A 30-year-old gravida 2 para woman 1001 at 36 weeks of gestation presents to you with shortness of breath

Intubate and start mechanical ventilation, with a goal PaCO2 of 30-32 mm Hg and a goal PaO2 of = 70 mm Hg

Educational Objective:

Summarize the basic concepts of caring for a pregnant patient with acute respiratory failure.

Key Point:

Acute respiratory failure in pregnancy has many causes; regardless of its cause, the first step in management is adequate maternal oxygenation. Adequate fetal oxygenation requires a PaO2 of at least 70 mm Hg, which corresponds to a maternal oxygen saturation of 95%. There is normal, mild, compensated respiratory alkalosis during pregnancy that facilitates fetal CO2 unloading. Maternal hypoventilation can quickly result in hypoxia, and CO2 retention can lead to maternal acidosis and, as a result, fetal acidemia.

Explanation:

This patient is in acute respiratory failure due to a severe asthma attack, as observed by the hypercapnia and hypoxemia on ABG. Her respiratory rate has decreased to normal, but it is due to respiratory fatigue and hypoventilation. Pregnant women can quickly develop hypoxia due to hypoventilation because of an increase in oxygen consumption of 20% to 33% above baseline in the third trimester. She needs mechanical ventilation. A PaCO2 between 30 and 32 mm Hg is normal for pregnancy, and a PaO2 of at least 70 mm Hg is necessary to maintain adequate fetal oxygenation.

Oxygenation with the non-rebreather mask has been insufficient for this patient and continuing it will only worsen her status as well as the oxygen status of the fetus. ABG values are within normal limits for a nonpregnant patient and continuing the mask would be appropriate if she was not becoming fatigued.

FHT is nonreassuring most likely due to maternal hypoxemia. Improving her oxygenation will likely improve the FHT, so immediate cesarean delivery instead of maternal stabilization is inappropriate nor will delivery improve the patient’s respiratory failure by itself. If FHT remains nonreassuring or if it worsens despite adequate maternal ventilation and oxygenation, then cesarean delivery would be appropriate.

These ventilation goals are appropriate for a nonpregnant patient. Although 40 mm Hg is normal PaCO2 outside of pregnancy, even a mild degree of hypercapnia is "permissible" in nonpregnant patients if it is necessary to maintain the lower tidal volumes of a "lung-protective" strategy. Hypercapnia puts the obstetric patient at risk for acidemia and, by extension, fetal acidemia.

References:

Bhatia PK, Biyani G, Mohammed S, Sethi P, Bihani P. Acute respiratory failure and mechanical ventilation in pregnant patient: a narrative review of literature. J Anaesthesiol Clin Pharmacol. 2016;32:431-439.

Clardy PF, Reardon CC. Acute respiratory failure during pregnancy and the peripartum period. Accessed October 22, 2018.

Lapinsky SE. Acute respiratory failure in pregnancy. Obstet Med. 2015;8:126-132.

Pacheco LD, et al. Critical Care Obstetrics. 6th ed., 2019.