Page 1396 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 100: Acid-Base Balance 969

3. Bicarbonate ion (HCO ) As SID becomes less positive, more [H ] is released into the solution,
−
+
3
+
4. Weak acids and their conjugate bases (HA + A = A ) (A is the and acidemia develops. As SID becomes more positive, more [H ] asso-
−
tot
tot
−
total independent variable, and HA + A are dependent variables.) ciates with [OH ], forming water, and alkalemia develops. By contrast,
−
) A , composed of weak acids, is acidifying. As A increases, the pH
tot
tot
5. Partial pressure of carbon dioxide (P CO 2 falls, and as A decreases, such as with hemodilution, the pH increases.
6. Carbonate ion (CO ) tot
2−
3
7. Hydroxyl ion (OH ) RESPIRATORY DISORDERS
−
8. Hydrogen ion (H )
+
By definition, abnormalities in P CO 2 are classified as respiratory dis-
The difference between the strong cations and strong anions (the strong orders. SID (and possibly A ) is manipulated by the human body to
tot
, and the total amount of weak acids and their compensate for chronic respiratory disorders, thus maintaining pH
ion difference [SID]), P CO 2
conjugate bases ([A ]) are the only independent variables. All the other within the normal range (7.35-7.45). SID decreases to compensate for
1
tot
components are, by definition, dependent, including [HCO ], [HA], a chronic respiratory alkalosis, and SID increases to compensate for a
−
3
[A ], [CO ], [OH ], and [H ]. Because the concentrations of each of chronic respiratory acidosis. The physiologic determinants of P CO 2 are
−
2−
−
+
3
these six variables are dependent on one or more of the independent straightforward:
variables, we must solve separate equilibrium equations for each. Water /V
itself is minimally dissociated despite the importance of [H ] and can be P CO 2 ∝ V CO 2 A
+
considered a constant. The six equations are as follows: where V CO 2 is CO production and Va is alveolar ventilation. A change in
2
Water dissociation: P CO 2 must be explained by one of these factors. Hypercarbia and hypo-
carbia usually can be explained easily at the bedside.
[H ] × [OH ] = K × [H O] (100-1)
+
–
1 2
Weak acid dissociation: ACIDEMIA AND ALKALEMIA
[H ] × [A ] = K [HA] (100-2) Even relative extremes of [H ] are remarkably well tolerated (eg, pH
+
+
–
2 7.1-7.7), at least for the short term, in otherwise healthy individuals.
Weak acid conservation: However, some authors have even suggested that acidemia itself may be
2
[HA] + [A ] = [A ] (100-3) beneficial to critically ill patients. For example, since acidemia shifts the
–
tot oxyhemoglobin curve to the right, there is better oxygen delivery under
HCO formation: acidemic conditions. Unfortunately, this “benefit” is dubious because
−
3
acidosis also reduces synthesis of 2,3-diphosphoglycerate (2,3-DPG),
+ − (100-4)
3 3 and thus chronically, acidosis does not appear to improve oxygen
[H ] × [HCO ] = K × P CO 2
CO formation: delivery. Acidosis may produce other salutary effects on the circulation
2−
3
2
[H ] × [CO ] = K × [HCO ] (100-5) that could result in benefit in certain clinical scenarios, but acidosis also
2−
−
+
3 4 3 produces numerous undesirable effects on various systems (Table 100-1),
Electrical neutrality: and we caution against “permissive acidosis.”
This is not to say that we believe that correcting an acid-base disorder
−
SID + [H ] − [HCO ] − is always appropriate. Indeed, the existing evidence does not support the
+
3
[A ] − [CO ] − [OH ] = 0 (100-6) use of sodium bicarbonate for the purpose of correcting the pH in most
2−
−
−
3
3
K through K represent constants for the individual reactions. Now that conditions of acute acidosis, and some animal experiments even suggest
4
1
4
we have six equations and six unknowns, we can arrange any unknown as harm. However, supporting respiratory compensation when feasible
a fourth-order polynomial and solve the equation. In acid-base balance, and avoiding acidosis when possible seems a prudent course of action in
the dependent variable in question is [H ]. Stated less elegantly, there is a most clinical scenarios. Frequently, when treating critically ill patients,
+
, and it is easy to blur the lines between supportive measures and therapeu-
unique value for each of the six dependent variables once SID, P CO 2 tic interventions. Common forms of acidemia may be associated with
A are known so that all the equations can be solved simultaneously.
tot significant mortality because of the disease processes that underlie them,
By taking logarithms to base 10 of Eq. (100-4) and rearranging, we get
+
the familiar Henderson-Hasselbalch equation: not necessarily because of the actual [H ]. For instance, bowel infarc-
tion is lethal, and only surgical resection is curative. Treating the lactic
− × 0.03)} acidosis without addressing the underlying cause of the lactic acidosis in
pH = pK + log{[HCO ]/(P CO 2 (100-7)
a 3
Since this equation is no more or less correct than any of the other five this scenario is certain to fail. Dissecting out the effects of acidosis itself
equations that must be solved simultaneously, there is nothing wrong from the causes that underlie it is difficult in patients. However, acidosis
itself has been shown to produce harm in animal models —especially
5-8
with using it to determine an acid-base disorder. Indeed, the fact that in models of sepsis, where decreased survival time and hypotension
all three values are readily available from a standard arterial blood gas appear to be attributable to exogenous acid loading. Nonetheless, it has
7,8
determination explains the popularity of this equation.
yet to be demonstrated that treating acidosis per se improves outcome.
METABOLIC DISTURBANCES SPECIFIC METABOLIC DISORDERS
By examining Eq. (100-7) it is possible to determine whether an acid- To diagnose a disorder leading to a change in SID, an actual account-
) or ing of strong ions occurs. A decrease in SID may be brought about by
base disturbance is present and whether it is due to respiratory (P CO 2
metabolic [HCO ] derangements. One might assume, therefore, that pH the generation of organic strong anions (eg, lactate and ketones) or the
−
3
−
is determined by the relationship between P CO 2 and [HCO ]. This pre- loss of strong cations paired with weak anions to balance charge. If
3
sumption is false. Likewise, solving Eq. (100-2) for [H ] does not mean the patient has diarrhea, he or she is losing [Na ] and [HCO ]; there-
+
−
+
3
that [H ] is determined by the HA and A . In truth, [H ] and thus pH fore, the [Cl ] will increase relative to the [Na ], leading to a decrease in
−
+
+
−
+
, SID, and A . Since we define respiratory disor- SID and, ultimately, acidosis.
are determined by P CO 2 tot
, metabolic disturbances are brought about by The organs of the gastrointestinal tract are underappreciated regula-
ders by alterations in P CO 2
changes in SID and A . They are not caused by changes in [HCO ], but tors of acid-base balance. Their ability to manipulate SID complexly
−
3
tot
rather, changes in [HCO ] occur as a result of the disturbance. is a direct result of the fact that strong ions are handled differently in
−
3
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