Assuming that maternal pH and blood gases are normal, the actual pH of fetal blood is dependent on the proportion of carbonic and organic acids and the amount of bicarbonate, which is the major bufer in blood. This can best be illustrated by the Henderson-Hasselbalch equation: [base]pH = pK + Iog -or, pH = pK + Iog
For clinical purposes, HC03-represents the metabolic component and is reported in mEq/L. The H2C03 concentration reflects the respiratory component and is reported as the pC02 in mm Hg. Thus:
The result of this equation is a pH value. Because pH is a logarithmic term, it does not give a linear measure of acid accumulation. For example, a change in hydrogen ion concentration associated with a fall in pH from 7.0 to 6.9 is almost twice that which is associated with a fall in pH from 7.3 to 7.2. For this reason, the change in base-termed delta base-ofers a more linear measure of the degree of accumulation of metabolic acid (Armstrong, 2007). The delta base is a calculated number used as a measure of the change in bufering capacity of bicarbonate (HC03 -). he formula for calculating the base excess (BE) is as follows:
BE = 0.02786 X pC02 X lO(pH -6.,1) X 13.77 X pH -124.58 32-4 is a nomogram developed from which these can be calculated if only two parameters are known. For example, the HC03 -concentration declines with a metabolic acidemia as it is consumed to maintain a normal pH. A base deicit develops when the HC03-concentration drops below 6.8 reported for preterm neonates (Dick8 pH 015 ? ,:00 ,0ED 70 inson, 1992; Ramin, 1989; Riley, 7.2 7.1 Fetal oxygenation and pH generally decline during the course of normal 7.0 labor. Normal umbilical cord blood pH and blood gas values at delivery 6.9 60 in term newborns are summarized in
Table 32-3. Similar values have been 6.7 1993). he lower limits of normal pH in the newborn have been found to range from 7.04 to 7.10 (horp, 6.6 1996). Thus, these values should be 6 considered to deine neonatal acidemia. Even so, most fetuses will tol erate intrapartum acidemia with a 110 pH as low as 7.00 without incurring neurological impairment (Freeman, 1988; Gilstrap, 1989). That said, in FIGURE 32-4 Nomogram for determining the delta base. (Adapted with permission from Siggaarda study of newborns with a pH <7.0 Anderson 0: Blood acid-base alignment nomogram, Scand J (lin Lab Invest. 1963;15:21o1-7.) from Parkland Hospital, there were
TABLE 32-3. Umbilical Cord Blood pH and Blood Gas Values in Normal Term Newborns