Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 40Acidification of the Urine & Bicarbonate Excretion 685

in clinical situations. This nomogram has PCO 2 plotted on a
log scale on the vertical axis and pH on the horizontal axis.
Thus, any point to the left of a vertical line through pH 7.40 in-
dicates acidosis, and any point to the right indicates alkalosis.
The position of the point above or below the horizontal line
through a PCO 2 of 40 mm Hg defines the effective degree of
hypoventilation or hyperventilation.
If a solution containing NaHCO 3 and no buffers were
equilibrated with gas mixtures containing various amounts of
CO 2 , the pH and PCO 2 values at equilibrium would fall along
the dashed line on the left in Figure 40–7 or a line parallel to
it. If buffers were present, the slope of the line would be
greater; and the greater the buffering capacity of the solution,
the steeper the line. For normal blood containing 15 g of
hemoglobin/dL, the CO 2 titration line passes through the 15-
g/dL mark on the hemoglobin scale (on the underside of the
upper curved scale) and the point where the PCO 2 = 40 mm
Hg and pH = 7.40 lines intersect, as shown in Figure 40–7.
When the hemoglobin content of the blood is low, there is sig-
nificant loss of buffering capacity, and the slope of the CO 2
titration line diminishes. However, blood of course contains
buffers in addition to hemoglobin, so that even the line drawn

from the zero point on the hemoglobin scale through the nor-

mal PCO 2 –pH intercept is steeper than the curve for a solu-

tion containing no buffers.

For clinical use, arterial blood or arterialized capillary

blood is drawn anaerobically and its pH measured. The pHs

of the same blood after equilibration with each of two gas

mixtures containing different known amounts of CO 2 are also

determined. The pH values at the known PCO 2 levels are plot-

ted and connected to provide the CO 2 titration line for the

blood sample. The pH of the blood sample before equilibra-

tion is plotted on this line, and the PCO 2 of the sample is read

off the vertical scale. The standard bicarbonate content of the

sample is indicated by the point at which the CO 2 titration

line intersects the bicarbonate scale on the PCO 2 = 40 mm Hg

line. The standard bicarbonate is not the actual bicarbonate

concentration of the sample but, rather, what the bicarbonate

concentration would be after elimination of any respiratory

component. It is a measure of the alkali reserve of the blood,

except that it is measured by determining the pH rather than

the total CO 2 content of the sample after equilibration. Like

the alkali reserve, it is an index of the degree of metabolic aci-

dosis or alkalosis present.

FIGURE 40–7 Siggaard–Andersen curve nomogram. (Courtesy of O Siggaard–Andersen and Radiometer, Copenhagen, Denmark.)

30

25

20

19

18

17

16

15

35

40

(^45505560)
65
70
75
80
0 10
(^10152025)
6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8
30 40 50
25
CO 2 titration line
of normal blood
CO 2 titration line
solution containing
NaHCO 3 , 15 meq/L,
and no buffers
Hemoglobin
(g/dL)
Buffer base
(meq/L)
0 +^5
− 5 +^10
− 10
− 15
− 20
− 22

• 15


• 20
  Standard bicarbonate
  (meq/L)
  Base excess
  (meq/L)
  pH
  110
  100
  90
  80
  70
  60
  50
  40
  35
  30
  25
  20
  15
  10
  PCO
  (mm Hg) 2