Page 159 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Assessment of Acid–Base Status lines) at normal P CO 2 = 5.33 (! B and C, points D
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and d). [HCO 3 ] Act is read from the [HCO 3 ] line
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The Henderson–Hasselbalch equation for the intersected by the P CO 2 /pH line at the level of
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HCO 3 /CO 2 buffer system states: the actually measured P CO 2 . Since the normal
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pH ! pK a + log ([HCO 3 ]/[CO 2]). [6.5] and measured P CO 2 values agree in normals,
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Since [CO 2] = α · P CO 2 (! p. 126), Equation 6.5 their [HCO 3 ] Act is usually equal to [HCO 3 ] St. If
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contains two constants (pK a and α) and three P CO 2 deviates from normal (! B, C, point c),
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variables (pH, [HCO 3 ], and P CO 2 ). At 37 "C in [HCO 3 ] Act is read at point e on the HCO 3 line
plasma, pK a = 6.1 and α = 0.225 mmol · L –1 · (! B, C, interrupted 45" line) on which the ac-
–1
kPa (cf. p. 126). When one of the variables re- tually measured P CO 2 lies (! B, C, point c).
mains constant (e.g., [HCO 3 ]), the other two Blood P CO 2 and pH measurement. When
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(e.g., P CO 2 and pH) are interdependent. In a using the equilibration method (Astrup
graphic representation, this dependency is re-
method), three pH measurements are taken:
Acid–Base Homeostasis P CO 2 is plotted against the pH (! A–C and equilibration with a high P CO 2 (e.g., 10 kPa
flected as a straight line when the logarithm of
(1) in the unchanged blood sample; (2) after
[75 mmHg]; ! C, points A and a), and (3) after
p. 382).
equilibration with a low P CO 2 (e.g., 2.7 kPa
When the P CO 2 varies in a bicarbonate solution
[20 mmHg]; ! C, points B and b). The P CO 2 of
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(without other buffers), the pH changes but [HCO 3 ]
the original blood sample can then be read
remains constant ( ! A, solid line). One can also plot
from lines A–B and a–b using the pH value ob-
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the lines for different HCO 3 concentrations, all of
which are parallel (! A, B, dotted orange lines).
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upper case letters, green), [HCO 3 ] Act
6 Figures A through C use a scale that ensures that the tained in measurement 1. In normals (! C, =
[HCO 3 ] St = 24 mmol/L (! C, points E and D).
bicarbonate lines intersect the coordinates at 45"
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angles. The Siggaard–Andersen nomogram (! C) Example 2 (! C, lower case letters, red) shows
does not use the lines, but only the points of intersec- an acid–base disturbance: The pH is too low
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tion of the [HCO 3 ] lines with the normal P CO 2 of
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5.33 kPa (40 mmHg). (7.2) and [HCO 3 ] St (! C, point d) has dropped
to 13 mmol/L (metabolic acidosis). This has
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The blood contains not only the HCO 3 /CO 2 been partially compensated (! p. 142) by a re-
buffer but also non-bicarbonate buffers, NBB duction in P CO 2 to 4 kPa, which led to a con-
(! p. 138). Thus, a change in the P CO 2 does not sequent reduction in [HCO 3 ] Act to 11 mmol/L
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alter the pH as much as in a solution contain- (! C, point e).
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ing the HCO 3 /CO 2 buffer alone (! p. 144). In Total buffer bases (BB) and base excess (BE)
the P CO 2 /pH nomogram, the slope is therefore (! p. 142) can also be read from the Siggaard–
steeper than 45" (! B, green and red lines). Andersen nomogram (! C). The base excess
Hence, the actual bicarbonate concentration, (points F and f on the curve) is the difference
[HCO 3 ] Act, in blood changes and shifts in the between the measured buffer base value
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same direction as the P CO 2 if the pH varies (points G or g) and the normal buffer base
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(! p. 144). Therefore, both the [HCO 3 ] Act and value (point G). Point G is dependent on the
the standard bicarbonate concentration, hemoglobin concentration of the blood (! C;
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[HCO 3 ] St, can be determined in clinical blood [Hb]/BB comparison). Like [HCO 3 ] St, deviation
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tests. By definition, [HCO 3 ] St represents the of BB from the norm (0 # 2.5 mEq/L) is diag-
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[HCO 3 ] at a normal P CO 2 of 5.33 kPa (40 mmHg). nostic of primary non-respiratory acid–base
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[HCO 3 ] St therefore permits an assessment of disturbances.
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[HCO 3 ] independent of P CO 2 changes.
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[HCO 3 ] St and [HCO 3 ] Act are determined The P CO 2 /pH line of the blood sample in plate C can
using measured P CO 2 and pH values obtained also be determined if (1) the P CO 2 (without equilibra-
tion), (2) the pH, and (3) the hemoglobin concentra-
with a blood gas analyzer. When plotted on the tion are known. One point (! C, point c) on the un-
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Siggaard–Andersen nomogram, [HCO 3 ] St is known line can be drawn using (1) and (2). The line
read from the line as indicated by the points of must be drawn through the point in such a way that
intersect of the [HCO 3 ] line (! B, orange lines) BB (point g) – BB normal (dependent on Hb value) = BE
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146 and the P CO 2 /pH line (B and C, green and red (point f).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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