- The strong ion difference, Pco2, and total weak acid concentration
(ATOT) best explain acid–base balance in physiological
- 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
- As a general rule, Paco2 can
be expected to increase 0.25–1 mm Hg for each 1 mEq/L
increase in [HCO3–].
- 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
- 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 the most common cause of hyperchloremic
- 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.
- 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 measurements of
Pco2 and Po2 directly
and measurements of pH indirectly. Both Pco2 and
Po2 therefore decrease during
hypothermia, but pH increases because temperature does not appreciably
alter [HCO3–]: Paco2 decreases, but [HCO3–] is
Nearly all biochemical reactions in the body are dependent on
maintenance of a physiological hydrogen ion concentration. The latter
is closely regulated because changes in hydrogen ion concentration
produce 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. A thorough understanding of acid–base disturbances,
their physiological effects, and treatment is thus essential for
proper anesthetic management.
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.
This chapter examines acid–base physiology, common disturbances,
and their anesthetic implications. Clinical measurements of blood
gases and their interpretation are also discussed.
Concentration & pH
In any aqueous solution, water molecules reversibly dissociate
into hydrogen and hydroxide ions:
This process is described by ...