A 34-year-old woman at 32 weeks gestation presents to the emergency department with cough, dyspnea and hypoxemia. She rapidly progresses to severe ARDS despite lung protective ventilation, paralysis, PEEP titration and inhaled epoprostenol. P/F ratio is 99 mm Hg. PCR for SARS-CoV-2 returns positive. Is prone positioning safe to perform in pregnant patients with severe ARDS? If so, are modifications necessary to offload the abdomen and monitor the fetus? A recently published review in Obstetrics and Gynecology discusses this important topic.

ARDS is an acute diffuse, inflammatory lung injury, leading to capillary leak and loss of aerated lung tissue. Alveolar opacification appears homogenous on chest x-ray, but CT scanning reveals that alveolar filling, consolidation, and atelectasis occur predominantly in dependent lung zones, sparing the ventral areas, giving rise to concept of the “baby lung” as termed by Gattinoni. Therefore, the smallest fraction of gas content is found in dorsal portions of the lungs where the majority of blood flow is directed due to increased surface area and dependent perfusion in the supine position. Thus, the force of gravity diminishes the oxygen carrying capacity of blood by increasing shunt and V/Q mismatch. Conversely, when a patient is placed in the prone position, oxygenation and ventilation are enhanced through several mechanisms: 

1. The compressive force exerted by the heart is redirected toward sternum, taking the weight of the heart off the lungs. 

2. The diaphragm moves down, improving diaphragmatic excursion and increasing functional residual capacity (FRC), or the oxygen reserve of the lungs. 

3. Suspending the dorsal lung over increased surface area increases the transpulmonary pressure of the dorsal alveoli, producing more homogeneous distribution of inspired gas during each tidal breath as the dorsal lung is recruited (i.e. reduces intrapulmonary shunt fraction).  

4. Distribution of tidal volume is more uniform, reducing lung stress and strain, protecting against ventilator-induced lung injury (VILI). 

5. Increased lung volume reduces the heterogeneity of pulmonary vascular resistance by reducing pulmonary vasoconstriction and redistributing perfusion to better ventilated regions of the lung (i.e. improved V/Q matching). 

6. Prone positioning decreases shunting across a patent foramen that occurs in up to 1 in 5 ARDS patients who develop RV dysfunction or cor pulmonale. 

7. Prone positioning facilitates drainage of secretions toward the central airways to facilitate clearance and, in some studies, has reduced the incidence of ventilator-associated pneumonia (VAP).  

Can prone positioning be performed in the pregnant patient? A review published in the August 2020 issue of Obstetrics and Gynecology describes the successful use of prone positioning in pregnant patients with ARDS due to COVID-19 through careful placement of pillows under the chest, pelvis and below the knees, keeping the abdomen free. The safety of prone positioning for mechanically ventilated pregnant patients has previously been reported in several case reports and series, where improved oxygenation has been observed. Prone positioning is particularly effective in pregnancy through relief of diaphragmatic compression caused by upward displacement of the diaphragm by the gravid uterus.

The authors present two videos demonstrating the method of assisted proning in the awake gravid patient (video 1) as well as the intubated pregnant patient (video 2, Tolcher et al.). After placement of pillows on the chest, pelvis and below the knees in the supine position, the standard “envelop maneuver” can be performed in the intubated patient. Lines are tucked on top and wrapped around the patient to facilitate a 180-degree turn toward the ventilator. Although challenging, fetal monitoring can be performed in the prone position and is advised beyond 24 weeks gestational age. The authors conclude that prone positioning can be accomplished safely and effectively by an experienced prone team for pregnant patients with ARDS. Daily rounding and management of proned pregnant patients should involve multidisciplinary collaboration between obstetrics, critical care and neonatology.

References: 

Kallet RH, et al. A comprehensive review of prone positioning in ARDS. Respiratory Care 2015;60(11):1660-1687.  

Tolcher MC, et al. Prone positioning for pregnant women with hypoxemia due to Coronavirus disease 2019 (COVID-19). Obstetrics and Gynecology 2020;136(2):259-261.