Hg pC02 increment will lower the pH by 0.08 units (Eisen berg, 1987). hus, in a mixed respiratory-metabolic acidemia, the benign respiratory component can be calculated. As an example, acute cord prolapse during labor prompts cesarean delivery of a neonate 20 minutes later. The umbilical artery blood gas pH was 6.95 and the pC02 was 90 mm Hg. The degree to which the cord compression and subsequent impair ment of CO2 exchange afected the pH is calculated using the relationship given earlier and shown below.
To correct pH: (40 -10) X 0.08 = 0.32; 6.95 + 0'.32 = 7.27
Therefore, the pH before cord prolapse was approximately 7.27, well within normal limits. Thus, the low pH resulted from respiratory acidosis.
The fetus begins to develop metabolic acidemia when oxygen deprivation is suiciently long and severe to require anaerobic metabolism for cellular energy needs. Low and associates (1997) defined fetal acidosis as a base deicit ::12 mmoUL, and severe fetal acidosis as a base deicit ::16 mmoUL. In the Parkland study of more than 150,000 newborns cited earlier, metabolic acidemia was defined using umbilical cord blood gas thresholds that were two standard deviations below the mean (Casey, 2001 b). Thus, metabolic acidemia was an umbilical artery blood pH <7.00 accompanied by a pC02 ;76.3 mm Hg, with higher values indicating a respiratory component; HC03-concentration ;17.7 mmoUL; and base deficit ::10.3 mEq/L. From the standpoint of possible neurological injury, the American College of Obstetricians and Gynecologists (2014) defines metabolic acidosis as umbilical arterial pH <7.0 and a base deficit ::12 mmoUL.
Metabolic acidemia is associated with a high rate of multiorgan dysfunction. In rare cases, such hypoxia-induced metabolic logical impairment-hypoxic-ischemic encephalopathy (Chap.
33, p. 621). In fact, a fetus without such acidemia cannot by definition have sufered recent hypoxic-induced injury. hat said, severe metabolic acidosis is poorly predictive of subse quent neurological impairment in the term neonate (King, 1998; Socol, 1994). In very-Iow-birthweight neonates, that is, those < 1000 g, newborn acid-base status may be more closely linked to intraventricular hemorrhage and possibly long-term neurological outcome (Lavrijsen, 2005; Salhab, 2005; Victory, 2003).
Casey and coworkers (2001 b) described the association between metabolic acidemia, low Apgar scores, and neonatal death in term and preterm newborns. Regarding term neonates, the risk of neonatal death was more than 3200-fold greater in term neonates with metabolic acidemia and 5-minute scores ;3 compared with those with a 5-minute Apgar score ::7.
In some centers, cord gas analysis is performed in all neonates at birth (Casey, 2001 b; Sabol, 2016). Cost-efectiveness analysis potential cost savings (White, 2010, 2016). It seems reasonable of cesarean delivery for fetal compromise, abnormal fetal heart rate tracing, fever, and low 5-minute Apgar score. Multifetal gestation and severely growth-restricted fetuses are others.