Acid–Base

1. Water dissociation equilibrium: $\left[{\mathrm{H}}^{\ast}\right]\times \left[{\mathrm{OH}}^{-}\right]={\mathrm{K}}_{\mathrm{w}}^{\prime }$

2. Electrical neutrality equation: $\left[\mathrm{S}\mathrm{I}\mathrm{D}\right]+\left[{\mathrm{H}}^{\ast}\right]=\left[{\mathrm{H}\mathrm{CO}}_3^{-}\right]+\left[{\mathrm{A}}^{-}\right]+\left[{\mathrm{CO}}_3^{2-}\right]+\left[{\mathrm{OH}}^{-}\right]$

3. Weak acid dissociation equilibrium: $\left[{\mathrm{K}}_{\mathrm{A}}\right]\times \left[\mathrm{H}\mathrm{A}\right]\leftrightarrow \left[{\mathrm{H}}^{+}\right]\times \left[{\mathrm{A}}^{-}\right]$

4. Conservation of mass for “A”: $\left[{\mathrm{A}}_{\mathrm{TOT}}\right]\leftrightarrow \left[{\mathrm{A}}^{-}\right]+\left[\mathrm{H}\mathrm{A}\right]$

5. Bicarbonate ion formation equilibrium: $\left[{\mathrm{PCO}}_2\right]\times \left[{\mathrm{K}}_{\mathrm{C}}\right]=\left[{\mathrm{H}}^{+}\right]\times \left[{\mathrm{H}\mathrm{CO}}_3^{-}\right]$

6. Carbonate ion formation equilibrium: $\left[{\mathrm{K}}_3\right]\times \left[{\mathrm{H}\mathrm{CO}}_3^{-}\right]=\left[{\mathrm{H}}^{+}\right]\times \left[{\mathrm{CO}}_3^{2-}\right]$

$\left[{\mathrm{H}}^{+}\right]=\mathrm{f}\left(\mathrm{S}\mathrm{I}\mathrm{D},\;{\mathrm{A}}_{\mathrm{TOT}},\;{\mathrm{PCO}}_2\right)$

These three independent parameters can all be normal, decreased or increased and thus six major acid base disturbances can be distinguished. This also implies that bicarbonate does not play any role in determining [H⁺]. Instead, [HCO₃ ⁻] is also determined by the three independent parameters.

5.2.1 Strong Ion Difference (SID)

Strong ions are always fully dissociated. Na⁺, K⁺ and Cl⁻ are the most important examples, but others include Mg⁺, Ca²⁺, sulfate and lactate. SID is the sum of strong cations minus the sum of strong anions. Its normal plasma value is about 40 mEq/L.

When SID increases, physicochemistry dictates that [H⁺] must decrease. An example is prolonged vomiting causing plasma [Cl⁻] to decrease and therefore [SID] to increase. When SID decreases, physicochemistry dictates that [H⁺] must increase. Examples include lactic acidosis and ketoacidosis. Ketones and lactate are also strong negative ions that decrease [SID] directly and therefore cause [H⁺] to rise.

5.2.2 Total Amount of Weak Acids (A_TOT)

The total amount of weak acids is expressed as A_TOT. It mainly consists of albumin and to a lesser extent of phosphate. By definition, weak acids are only partially dissociated. When A_TOT increases, physicochemistry dictates that [H⁺] must increase, for example in the case of hyperphosphatemia in renal failure. When A_TOT decreases, [H⁺] must also decrease, as is the case in hypoalbuminemia, a common problem in critically ill patients.

5.2.3 The Partial Pressure of Carbon Dioxide (PCO₂)

Tissues produce CO₂. All approaches to acid–base treat PCO₂ similarly. Physicochemistry dictates that if PCO₂ rises, for example due to increased dead space ventilation, [H⁺] must rise too. If PCO₂ decreases, [H⁺] must also decrease. An example would be psychogenic hyperventilation.

5.2.4 Strong Ion Gap (SIG) and Urine SID

Both SIG and Urine SID may be used to further differentiate between causes of acid–base abnormalities (Tables 5.2 and 5.3). Urine SID is defined as urine [Na⁺] + [K⁺] − [Cl⁻]. In metabolic acidosis, a positive urine SID may suggest renal tubular acidosis.

Table 5.2

Differential diagnosis of metabolic acidosis

Adjusted AG >8–12 mEq/L	Normal adjusted AG;
Low SID with SIG >0–2 mEq/L	Low SID with normal SIG
Lactate	NaCl 0.9 % infusion
Ketones	Diarrhea
End-stage renal failure	Early renal insufficiency
Salicylate intoxication	Acetazolamide
Methanol intoxication	Ureteroenterostomy
Ethylene glycol intoxication	Parenteral nutrition
	Anion exchange resins
	Small bowel/pancreatic drainage
	Renal tubular acidosis (Urine SID > 0)
	Type I: Urine pH >5.5
	Type II: Urine pH <5.5/low serum K⁺
	Type IV: Urine pH <5.5/high serum K⁺

Table 5.3

Differential diagnosis for increased SID metabolic alkalosis

Chloride loss <sodium loss (urine [Cl⁻] <10 mmol/L)	Vomiting, gastric drainage, chloride wasting diarrhea, postdiuretic use, posthypercapnea
Chloride loss <sodium loss (urine [Cl⁻] >10 mmol/L)	Mineralocorticoid excess (Conn’s syndrome, Cushing syndrome, Liddle syndrome, Bartter syndrome, exogenous corticoids, excessive licorice intake), ongoing diuretic use
Exogenous sodium load	Massive blood transfusions, parenteral nutrition, plasma volume expanders, sodium lactate, sodium citrate
Other	Severe deficiency of Mg²⁺ or K⁺

SIG is the difference between the apparent SID (SID_a) and the effective SID (SID_e). SID_a consists of the sum of the measured strong ions:

${\mathrm{SID}}_{\mathrm{a}}=\left[{\mathrm{Na}}^{+}\right]+\left[{\mathrm{K}}^{+}\right]+\left[{\mathrm{Ca}}^{2+}\right]+\left[{\mathrm{Mg}}^{2+}\right]\hbox{--} \left[{\mathrm{Cl}}^{-}\right]$

SID_e is an approximation of the true SID, calculating the remaining ion space after accounting for the negative charge on albumin and bicarbonate, and can be calculated from the other independent variables using the following formula, where albumin (Alb) is expressed in g/L and phosphate (P_i) in mM:

${\mathrm{SID}}_{\mathrm{e}}=\left[{\mathrm{HCO}}_3^{-}\right]+\left(0.123\ast \mathrm{p}\mathrm{H}-0.631\right)\ast \left[\mathrm{Alb}\right]+\left(\mathrm{p}\mathrm{H}-0.469\right)\ast \left[{\mathrm{P}}_{\mathrm{i}}\right]$

Thus SIG represents the sum of any unmeasured strong positive and negative ions. Its normal value is 0 ± 2 mEq/L. When positive, unmeasured anions exceed unmeasured cations.

5.2.5 Osmol Gap

This is the gap between the measured osmolality and the calculated osmolality.

$\mathrm{Calculated}\;\mathrm{osmolaity}=2\times \left[{\mathrm{Na}}^{+}\right]+\left[\mathrm{glucose}\right]+\left[\mathrm{urea}\right]$

where glucose and urea are expressed in mM. The normal value of the osmol gap is 10–15 mM. A high osmol gap suggests the presence of ethanol, ethylene glycol or methanol.

5.2.6 Effect of Resuscitation Fluids

Normal plasma has a SID of approximately 40 mEq/L. Thus, if fluids with a different SID are given, SID will change. Examples include 0.9 or 0.45 % saline with equal amounts of sodium and chloride thus a SID of 0 mM. The same is true for glucose 5 %, which does not contain any strong ions and hence also has a SID of 0 mM.

Balanced salt solutions contain variable amounts of negative ions that are metabolized, such as lactate or acetate. This explains their increased SID, which is about 28 mM for lactated Ringer’s for example. Similarly, 8.4 % NaHCO₃ is essentially sodium without strong anions, resulting in a high SID of 1,000 mM. In clinical medicine, the effect of resuscitation fluids on SID is counterbalanced by simultaneous dilution of A_TOT and renal SID handling.

5.3 The Henderson-Hasselbalch Approach

As can be seen from Table 5.1, the Henderson-Hasselbalch equation is actually one of the Stewart equations:

$\left[{\mathrm{H}}^{+}\right]\sim {\mathrm{PCO}}_2/\left[{\mathrm{H}\mathrm{CO}}_3^{-}\right]$

The ratio of [HCO₃ ⁻] and PCO₂ has a fixed relationship to [H⁺]. All acid–base disturbances are thus explained by changes in either [HCO₃ ⁻] or PCO₂. Singling out this equation is attractive as it facilitates dividing acid–base abnormalities in respiratory and non-respiratory or metabolic disorders. However, it should be noted that because PCO₂ and HCO₃ ⁻ are interdependent, relying solely on this equation might lead to circular reasoning.

Compensatory changes to acid–base disturbances may also be respiratory or metabolic and reflected in same direction changes in PCO₂ of [HCO₃ ⁻]. Respiratory compensation, either spontaneously or by altering the settings of mechanical ventilation may occur within minutes. Metabolic compensation is slower and may take hours to days, and is mainly regulated by the kidney. To determine whether the compensation is sufficient and to identify mixed acid–base disorders, rules of thumb have been proposed (Table 5.4).

Table 5.4

Rules of thumb for compensation

Primary acid–base disturbance

Compensation rule

Metabolic acidosis

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