Plasma base excess

Plasma Protein Buffer System and Plasma Base Excess The buffer value (p) of the nonbicarbonate buffers of plasma is about 7.7 mmol/L at pH 7.40 and a normal plasma protein concentration of 72 g/L. Proteins, especially albumin, account for the greatest portion (95%) of the nonbicarbonate buffer value of the plasma. The most important buffer groups of proteins in the physiological pH range are the imidazole groups of histidines (pimolecule contains 16 histidines. 

Plasma base excess is defined as the initial concentration of titratable base when titrating the plasma with strong acid or base to pH (Std) - 7.4 at PCO2 = 40 mm Hg and 37 °C. The equation for the CO2 equilibration curve of plasma and calculation of plasma base excess can be written ... 

The treatment of acid-base disturbances should be directed at the imderlying cause and the specific plasma constituent imbalance. It is possible to determine the relative contributions of sodium, chloride, unidentified anions (principally lactate in horses) and protein to the metabolic component of acid-base disturbances by the use of equations based on the calculated base excess (Corley Marr 1998, Whitehair et al 1995). However, decisions on treatment can often be based on the absolute values of these blood constituents and it is only in complex disturbances, with changes in multiple plasma constituents, that the equations are usually necessary. 

The pH of the plasma may be considered to be a function of two independent variables (1) the PCO2, which is regulated by the lungs and represents the acid component of the carbonic acid/bicarbonate buffer system, and (2) the concentration of titratable base (base excess or deficit, which is defined later), which is regulated by the kidneys. The plasma bicarbonate concentration is generally taken as a measure of the base excess or deficit in plasma and ECF, although it is recognized tliat conditions exist in which bicarbonate concentration may not accurately reflect the true base excess or deficit. 

As in plasma, CO2 equilibration of whole blood depends on the buffer value of nonbicarbonate buffers. Thus CO2 equilibration in whole blood is dependent on hemoglobin concentration and also on pH and oxygenation status. It is possible to derive an approximate equation for whole blood CO2 equilibration and calculation of whole blood base excess as follows ... 

Metabolic acidosis is readily detected by decreased plasma bicarbonate (or a negative extracellular base excess), the primary perturbation in this acid-base disorder. Bicarbonate is lost in the buffering of excess acid. Causes include the foUowing ... 

In 90 women who underwent cesarean deliveries under spinal anesthesia, phenylephrine, phenylephrine + ephedrine, and ephedrine were used to maintain the blood pressure near baseline by adjusting the infusion rates [40 f. Fetal heart rates increased significantly after infusion of phenylephrine - -ephedrine and ephedrine alone but did not change after phenylephrine alone. After ephedrine, umbilical arterial and venous pH and base excess were lower than after phenylephrine alone and phenylephrine + ephedrine. Umbilical arterial PCO2 and plasma concentrations of lactate and glucose after ephedrine were greater than after phenylephrine. The authors concluded that phenylephrine may be better than ephedrine for treating the hypotension of spinal anesthesia for cesarean delivery. 

A strong acid, such as hydrochloric acid, is now added to normal blood and to normal separated plasma. In each case, 20 mM acid is added to one litre of blood or plasma. Each fluid now has a base excess of — 20mM. Next both fluids are equilibrated with a gas mixture containing carbon dioxide at a partial pressure of 40 mmHg and the pH values are measured. Although the blood is a good buffer, its pH falls its status is shown by point A in Figure 4.7. 

The separated plasma is an inferior buffer so that the pH falls further than for blood the status of the acidotic plasma is shown by point B in Figure 4.7. If the PCO2 is varied and the resultant pH is measured, each fluid gives a relationship similar to that of its partner obtained before adding the strong acid but now the two lines cross at point C, with a value of Pcoj below 40 mmHg. This point is labelled — 20, to indicate the base excess of this blood and plasma. 

In Figure 4.8, the data obtained so far has been re-plotted, together with measurements made when a similar procedure is performed as in the last paragraph, except that base has been added instead of acid. This gives the three pairs of intersecting straight lines. The points of intersection are labelled with the base excess values. When the procedure is repeated for intermediate values of added acid or base, the intercepts of the lines for blood and separated plasma give the curved dashed scale labelled in units of base excess. 

Figure 4.8. PCO2 (logarithmic scale) as a function of pH. Three pairs of lines for blood and plasma, each pair corresponding to a particular value of base exeess. The points of intersection of the pairs of lines are shown as the dashed curve, calibrated in units of base excess.

D. Base excess is near zero in plasma from an anaemic patient. 

D. Yes. An anaemic person operates with arterial plasma at a normal Pco, [HCO3 ] and pH the base excess is therefore zero. 

Ag, Cl, and N to six-figure accuracy.1 This Nobel Prize-winning research allowed the accurate determination of atomic masses of many elements. In combustion analysis, a sample is burned in excess oxygen and products are measured. Combustion is typically used to measure C, H, N, S, and halogens in organic compounds. To measure other elements in food, organic matter is burned in a closed system, the products and ash (unburned material) are dissolved in acid or base, and measured by inductively coupled plasma with atomic emission or mass spectrometry. 

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