or actual HCO3− concentration). The anion gap is calculated to detect unidentified
anions in plasma. This approach works well clinically and is recommended for use
whenever serum total protein, albumin, and phosphate concentrations are
approximately normal. However, because the Henderson-Hasselbalch approach is
more descriptive than mechanistic, the Henderson-Hasselbalch equation frequently
provides erroneous information as to the cause of an acid-base disturbance.
The quantitive physicochemical approach to evaluating acid-base balance uses the
simplified strong ion model to categorize 6 primary acid-base disturbances:
respiratory acidosis (increased PCO2), respiratory alkalosis (decreased PCO2), strong
ion acidosis (decreased strong ion difference), strong ion alkalosis (increased strong
ion difference), nonvolatile buffer ion acidosis (increased plasma concentrations of
albumin, globulins, or phosphate), and nonvolatile buffer ion alkalosis (decreased
plasma concentrations of albumin, globulins, or phosphate). The strong ion gap is
calculated to detect unidentified anions in plasma. The simplified strong ion approach
works well clinically and is recommended for use whenever serum total protein,
albumin, or phosphate concentrations are markedly abnormal. The simplified strong
ion approach is mechanistic and is therefore well suited for describing the cause of
any acid-base disturbance.
One of Stewart’s major contributions to clinical acid-base physiology is his proposal
that plasma pH is determined by three independent variables: PCO2, SID and [ATOT].
Virtually all solutions in human biology contain water and aqueous solutions provide
a virtually inexhaustible source of [H+]. In these solutions, [H+] concentration is
determined by the dissociation of water into H+ and OH- ions. Changes in [H+]
concentration or pH occur NOT as a result of how much [H+] is added or removed,
but as a consequence of water dissociation in response to change in strong ion
difference [SID], PCO2 and weak acid.
PCO2 is changed by alveolar ventilation. Increased PCO2 results from
hypoventilation, leading to a respiratory acidosis, while decreased PCO2 is due to
hyperventilation (respiratory alkalosis).
Strong ions are fully dissociated at physiologic pH and therefore do not participate in
any chemical reactions (no buffering effect). Strong ions do however exert an
electrical effect because the sum of completely dissociated cations does not equal
the sum of the completely dissociated anions. This collective theoretical positive unit
of charge is the strong ion difference (SID).
SID = (Na+ + K+ + Mg2+ + Ca2+) – (Cl- + lactate + β-hydroxybutyrate + acetoacetate +
SO42-).
Increased SID = Strong Ion (metabolic) alkalosis
Decreased SID = Strong Ion (metabolic) acidosis
[A-tot] is the total concentration on non-volatile weak buffers. In contrast to strong
ions, buffer ions are derived from plasma weak acids and bases that are not fully
dissociated at physiologic pH. The main nonvolatile plasma buffers act as weak
acids at physiologic pH. Plasma albumin makes up the majority of [Atot]. Decreased
[Atot] (hypoalbuminaemia) therefore leads to an alkalosis (HCO3- increases to
maintain electroneutrality), while increased [Atot] (hyperalbuminaemia) leads to
acidosis (decreased HCO3-).
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
213