L

Renal compensation of acidosis is achieved: pH

NBB is regenerated

But: [HC03 ]act and PC02 are increased

Assessment of Acid-Base Status

The Henderson-Hasselbalch equation for the HCO3 /CO2 buffer system states:

Since [CO2] = a •PCO2 (^ p. 126), Equation 6.5 contains two constants (pKa and a) and three variables (pH, [HCO3], and Pco2). At 37 °C in plasma, pKa = 6.1 and a = 0.225 mmol • L1 • kPa-1 (cf. p. 126). When one of the variables remains constant (e.g., [HCO3]), the other two (e.g., Pco2 and pH) are interdependent. In a graphic representation, this dependency is reflected as a straight line when the logarithm of PCO2 is plotted against the pH (^ A-C and p. 382).

When the Pco2 varies in a bicarbonate solution (without other buffers), the pH changes but [HCO3-] remains constant (^ A, solid line). One can also plot the lines for different HCO3- concentrations, all of which are parallel (^A, B, dotted orange lines). Figures A through C use a scale that ensures that the bicarbonate lines intersect the coordinates at 45° angles. The Siggaard-Andersen nomogram (^ C) does not use the lines, but only the points of intersection of the [HCO3-] lines with the normal PCO2 of 5.33 kPa (40 mmHg).

The blood contains not only the HCO3 /CO2 buffer but also non-bicarbonate buffers, NBB (^ p. 138). Thus, a change in the PCO2 does not alter the pH as much as in a solution containing the HCO3 /CO2 buffer alone (^ p. 144). In the PCO2/pH nomogram, the slope is therefore steeper than 45° (^ B, green and red lines). Hence, the actual bicarbonate concentration, [HCO3-]Act, in blood changes and shifts in the same direction as the Pco2 if the pH varies (^ p. 144). Therefore, both the [HCO3W and the standard bicarbonate concentration, [HCO3-]st, can be determined in clinical blood tests. By definition, [HCO3]St represents the [HCO3-! at a normal Pco2 of5.33 kPa(40 mmHg). [HCO3-]st therefore permits an assessment of [HCO3 ] independent of PCO2 changes.

[HCO3-]st and [HCO3-]Act are determined using measured PCO2 and pH values obtained with a blood gas analyzer. When plotted on the Siggaard-Andersen nomogram, [HCO3 ]St is read from the line as indicated by the points of intersect of the [HCO3] line (^ B, orange lines) and the PCO2/pH line (B and C, green and red lines) at normal PCO2 = 5.33 (^ B and C, points D and d). [HCC>3-]Act is read from the [HCO3 ] line intersected by the PCO2/pH line at the level of the actually measured Pco2. Since the normal and measured PCO2 values agree in normals, their [HCO3-]Act is usually equal to [HCO3-]st. If PCO2 deviates from normal (^ B, C, point c), [HCO3]Act is read at point e on the HCO3- line (^ B, C, interrupted 45° line) on which the actually measured PCO2 lies (^ B, C, point c).

Blood Pco2 and pH measurement. When using the equilibration method (Astrup method), three pH measurements are taken: (1) in the unchanged blood sample; (2) after equilibration with a high Pco2 (e.g., 10 kPa [75 mmHg]; ^ C, points A and a), and (3) after equilibration with a low Pco2 (e.g., 2.7 kPa [20 mmHg]; ^C, points B and b). The Pco2 of the original blood sample can then be read from lines A-B and a-b using the pH value obtained in measurement 1. In normals (^ C, upper case letters, green), [HCO3-]Act = [HCO3]St = 24 mmol/L (^ C, points E and D). Example 2 (^ C, lower case letters, red) shows an acid-base disturbance: The pH is too low (7.2) and [HCO3-]st (^C, point d) has dropped to 13 mmol/L (metabolic acidosis). This has been partially compensated (^ p. 142) by a reduction in Pco2 to 4 kPa, which led to a consequent reduction in [HCO3 ]Act to 11 mmol/L (^ C, point e).

Total buffer bases (BB) and base excess (BE) (^ p. 142) can also be read from the Siggaard-Andersen nomogram (^ C). The base excess (points F and f on the curve) is the difference between the measured buffer base value (points G or g) and the normal buffer base value (point G). Point G is dependent on the hemoglobin concentration of the blood (^ C; [Hb]/BB comparison). Like [HCO3-]st, deviation of BB from the norm (0 ± 2.5 mEq/L) is diagnostic of primary non-respiratory acid-base disturbances.

The Pco2/pH line of the blood sample in plate C can also be determined if (1 ) the PCO2 (without equilibration), (2) the pH, and (3) the hemoglobin concentration are known. One point (^ C, point c) on the unknown line can be drawn using (1) and (2). The line must be drawn through the point in such a way that BB (point g) - BBnormai (dependent on Hb value) = BE (point f).

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