Chapter 50. Acid–Base Management

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  The strong ion difference, Pco2, and total weak acid concentration best explain acid–base balance in physiological systems.
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  The bicarbonate buffer is effective against metabolic but not respiratory acid–base disturbances.
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  In contrast to the bicarbonate buffer, hemoglobin is capable of buffering both carbonic (CO2) and noncarbonic (nonvolatile) acids.
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  As a general rule, Paco2 can be expected to increase 0.25–1 mm Hg for each 1 mEq/L increase in [HCO3−].
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  The renal response to acidemia is 3-fold: (1) increased reabsorption of the filtered [HCO3−], (2) increased excretion of titratable acids, and (3) increased production of ammonia.
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  During chronic respiratory acidosis, plasma [HCO3−] increases approximately 4 mEq/L for each 10 mm Hg increase in Paco2 above 40 mm Hg.
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  Diarrhea is a common cause of hyperchloremic metabolic acidosis.
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  The distinction between acute and chronic respiratory alkalosis is not always made, because the compensatory response to chronic respiratory alkalosis is quite variable: plasma [HCO3−] decreases 2–5 mEq/L for each 10 mm Hg decrease in Paco2 below 40 mm Hg.
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  Vomiting or continuous loss of gastric fluid by gastric drainage (nasogastric suctioning) can result in marked metabolic alkalosis, extracellular volume depletion, and hypokalemia.
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  The combination of alkalemia and hypokalemia can precipitate severe atrial and ventricular arrhythmias.
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  Changes in temperature affect Pco2, Po2, and pH. Both Pco2 and Po2 decrease during hypothermia, but pH increases because temperature does not appreciably alter [HCO3−]: Paco2 decreases, but [HCO3−] is unchanged.

Nearly all biochemical reactions in the body are dependent on maintenance of a physiological hydrogen ion concentration. The latter is tightly regulated because alterations in hydrogen ion concentration are associated with widespread organ dysfunction. This regulation—often referred to as acid–base balance—is of prime importance to anesthesiologists. Changes in ventilation and perfusion and the infusion of electrolyte-containing solutions are common during anesthesia and can rapidly alter acid–base balance.

Our understanding of acid–base balance is evolving. In the past, we focused on the concentration of hydrogen ions [H+], CO2 balance, and the base excess/deficit.

This chapter examines acid–base physiology, common disturbances, and their anesthetic implications. Clinical measurements of blood gases and their interpretation are also reviewed.

Acid–Base Chemistry

Hydrogen Ion Concentration & pH

In any aqueous solution, water molecules reversibly dissociate into hydrogen and hydroxide ions:

This process is described by the dissociation constant, KW:

The concentration of water is omitted from the denominator of this expression because it does not vary appreciably and ...