With severe lung injury and high intrapulmonary shunt fractions, it may not be possible to provide adequate oxygenation with usual ventilatory pressures, even with 100-percent oxygen. Positive end-expiratory pressure is usually successful in decreasing the shunt by recruiting collapsed alveoli. At low levels of 5 to 15 mm Hg, positive pressure can typically be used safely. At higher levels, impaired right-sided venous return can result in decreased cardiac output, lowered uteroplacental perfusion, alveolar overdistention, falling compliance, and barotrauma (Schwaiberger, 2016; Slutsky, 2013).
As discussed in Chapter 33 (p. 620), extracorporeal membrane oxygenation (ECM 0) has been successfully used for neonatal meconium aspiration syndrome. Preliminary observation suggests that it may be useful in adults with ARDS (Brodie, 2011; Levy, 2015; Peek, 2009). ECMO use has been reported in pregnant women. In one study, 12 patients with inluenza-induced lung failure were treated with ECMO, and of four maternal deaths, three were due to anticoagulation-related hemorrhage (Nair, 2011). In another study, the duration of support in four survivors was 2 to 28 days (Cunningham, 2006). In a review of 29 treated gravidas, 80 percent of cases were due to ARDS, and the maternal and perinatal mortality rate was 28 percent (Anselmi, 2015). Technical aspects of ECMO were reviewed by Brodie and Bacchetta (2011).
he propensity of the hemoglobin molecule to release oxygen is described by the oyhemoglobin dissociation curve (Fig. 47-2). For clinical purposes, the curve can be divided into an upper oxygen association curve representing the alveolar-capillary environment and a lower oxygen dissociation portion representing the tissue-capillary environment. Shifts of the curve have their greatest efect at the steep portion because they afect oxygen delivery. A rightward shit is associated with decreased hemoglobin ainity for oxygen and hence increased tissue-capillary oxygen :0 60= :l
FIGURE 47-2 Oxyhemoglobin dissociation curve. With higher oxygen tension (Pao2) in the pulmonary alveoli, adult hemoglobin is maximally saturated compared with that at the lower oxygen tension in the tissue capillaries. Note that at any given oxygen tension, fetal hemoglobin carries more oxygen than adult hemoglobin, as indicated by percent saturation.
interchange. Rightward shifts are produced by hypercapnia, metabolic acidosis, fever, and increased 2,3-diphosphoglycerate levels. During pregnancy, the erythrocyte concentration of 2,3-diphosphoglycerate is increased by approximately 30 percent. This favors oxygen delivery to both the fetus and maternal peripheral tissues.
Fetal hemoglobin has a higher oxygen ainity than adult hemoglobin. As seen in Figure 47-2, its curve is positioned to the left of the adult curve. To achieve 50-percent hemoglobin saturation, the Paoz must be 27 mm Hg in the mother compared with only 19 mm Hg in the fetus. Under normal physiological conditions, the fetus is constantly on the dissociation, or tissue, portion of the curve. Even with severe maternal lung disease and very low Paoz levels, oxygen displacement to fetal tissues is favored. Another example of this comes from pregnant women who live at high altitudes. Here, despite a maternal Paoz of only 60 mm Hg, the fetal Paoz is equivalent to that of fetuses at sea level (Subrevilla, 1971).