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KEY CONCEPTS
The strong ion difference, PCO2, and total weak acid concentration best explain acid–base balance in physiological systems.
The bicarbonate buffer is effective against metabolic but not respiratory acid–base disturbances.
In contrast to the bicarbonate buffer, hemoglobin is capable of buffering both carbonic (CO2) and noncarbonic (nonvolatile) acids.
As a general rule, PaCO2 can be expected to increase 0.25 to 1 mm Hg for each 1 mEq/L increase in [HCO3−].
The renal response to acidemia is three-fold: (1) increased reabsorption of the filtered [HCO3−], (2) increased excretion of titratable acids, and (3) increased production of ammonia.
During chronic respiratory acidosis, plasma [HCO3−] increases approximately 4 mEq/L for each 10 mm Hg increase in PaCO2 above 40 mm Hg.
Diarrhea is a common cause of hyperchloremic metabolic acidosis.
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 to 5 mEq/L for each 10 mm Hg decrease in PaCO2 below 40 mm Hg.
Vomiting or continuous loss of gastric fluid by gastric drainage (nasogastric suctioning) can result in marked metabolic alkalosis, extracellular volume depletion, and hypokalemia.
The combination of alkalemia and hypokalemia can precipitate severe atrial and ventricular arrhythmias.
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−] and the dissociation of water decreases (decreasing H+ and increasing pH).
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Nearly all biochemical reactions in the body are dependent on maintenance of a physiological hydrogen ion concentration, and alterations beyond normal hydrogen ion concentration are associated with widespread organ dysfunction. This regulation—usually referred to as acid–base balance—is of prime importance in critical illness. Changes in ventilation and perfusion, as well as infusion of electrolyte-containing solutions, are common during anesthesia and can rapidly alter acid–base balance.
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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. We now understand that the strong ion difference (SID), PCO2, and total weak acid concentration (ATOT) best explain acid–base balance in physiological systems.
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This chapter examines acid–base physiology and the perioperative care implications of common disturbances. Clinical measurements of blood gases and their interpretation are reviewed.
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Hydrogen Ion Concentration & pH
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In any aqueous solution, water molecules reversibly dissociate into hydrogen and hydroxide ions:
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This process is described ...