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Acid-Base Disorders

By

James L. Lewis III

, MD, Brookwood Baptist Health and Saint Vincent’s Ascension Health, Birmingham

Last full review/revision Jul 2021| Content last modified Jul 2021
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Acid-base disorders are pathologic changes in carbon dioxide partial pressure (Pco2) or serum bicarbonate (HCO3) that typically produce abnormal arterial pH values.

  • Acidemia is serum pH < 7.35.

  • Alkalemia is serum pH > 7.45.

  • Acidosis refers to physiologic processes that cause acid accumulation or alkali loss.

  • Alkalosis refers to physiologic processes that cause alkali accumulation or acid loss.

Actual changes in pH depend on the degree of physiologic compensation and whether multiple processes are present.

Classification

Primary acid-base disturbances are defined as metabolic or respiratory based on clinical context and whether the primary change in pH is due to an alteration in serum HCO3 or in Pco2.

  • Increased acid production

  • Acid ingestion

  • Decreased renal acid excretion

  • Gastrointestinal or renal HCO3 loss

  • Acid loss

  • HCO3 retention

  • Decrease in minute ventilation (hypoventilation)

  • Increase in minute ventilation (hyperventilation)

A simple acid-base disorder is a single acid-base disturbance with its accompanying compensatory response.

Mixed (sometimes called complex) acid-base disorders comprise ≥ 2 primary disturbances.

Pearls & Pitfalls

  • Compensatory mechanisms for acid-base disturbances cannot return pH completely to normal and never overshoot.

Table
icon

Symptoms and Signs

Diagnosis

  • Arterial blood gases (ABG)

  • Serum electrolytes

  • Anion gap calculated

  • If metabolic acidosis is present, delta gap calculated and Winters formula applied

  • Search for compensatory changes

Evaluation is with ABG and serum electrolytes. The ABG directly measures arterial pH and Pco2. HCO3 level reported on the arterial blood gas panel is calculated using the Henderson-Hasselbalch equation. The HCO3 level on serum chemistry panel is directly measured. Directly measured HCO3 levels are considered more accurate in cases of discrepancy.

Acid-base balance is most accurately assessed with measurement of pH and Pco2 in an arterial blood sample. In cases of circulatory failure or during cardiopulmonary resuscitation, measurements from a sample of venous blood may more accurately reflect conditions at the tissue level and may be a more useful guide to bicarbonate administration and adequacy of ventilation.

The pH establishes the primary process (acidosis or alkalosis), although pH moves toward the normal range with compensation. Changes in Pco2 reflect the respiratory component, and changes in HCO3 reflect the metabolic component.

Complex or mixed acid-base disturbances involve more than one primary process. In these mixed disorders, values may be deceptively normal. Thus, when evaluating acid-base disorders, it is important to determine whether changes in Pco2 and HCO3 show the expected compensation (see table Primary Changes and Compensation in Simple Acid-Base Disorders Primary Changes and Compensations in Simple Acid-Base Disorders Acid-base disorders are pathologic changes in carbon dioxide partial pressure (Pco2) or serum bicarbonate (HCO3−) that typically produce abnormal arterial pH values. Acidemia is serum pH 7... read more ). If not, then a second primary process should be suspected of causing the abnormal compensation. Interpretation must also consider clinical conditions (eg, chronic lung disease, renal failure, drug overdose).

The anion gap Calculation of the anion gap Acid-base disorders are pathologic changes in carbon dioxide partial pressure (Pco2) or serum bicarbonate (HCO3−) that typically produce abnormal arterial pH values. Acidemia is serum pH 7... read more should always be calculated; elevation almost always indicates a metabolic acidosis Metabolic Acidosis Metabolic acidosis is primary reduction in bicarbonate (HCO3−), typically with compensatory reduction in carbon dioxide partial pressure (Pco2); pH may be markedly low or slightly subnormal... read more . A normal anion gap with a low HCO3 (eg, < 24 mEq/L [< 24 mmol/L]) and high serum chloride (Cl) indicates a non-anion gap (hyperchloremic) metabolic acidosis. If metabolic acidosis is present, a delta gap is calculated to identify concomitant metabolic alkalosis, and Winters formula is applied to determine whether respiratory compensation is appropriate or reflects a second acid-base disorder (predicted Pco2 = 1.5 [HCO3] + 8 ± 2; if Pco2 is higher, there is also a primary respiratory acidosis—if lower, respiratory alkalosis).

Respiratory acidosis Respiratory Acidosis Respiratory acidosis is primary increase in carbon dioxide partial pressure (Pco2) with or without compensatory increase in bicarbonate (HCO3−); pH is usually low but may be near normal. Cause... read more is suggested by Pco2> 40 mm Hg; HCO3 should compensate by increasing 3 to 4 mEq/L ( 3 to 4 mmol/L) for each 10-mm Hg rise in Pco2 sustained for 4 to 12 hours (there may be no increase or only an increase of 1 to 2 mEq/L [1 to 2 mmol/L], which slowly increases to 3 to 4 mEq/L [3 to 4 mmol/L] over days). Greater increase in HCO3 implies a primary metabolic alkalosis; lesser increase suggests no time for compensation or coexisting primary metabolic acidosis.

Metabolic alkalosis Metabolic Alkalosis Metabolic alkalosis is primary increase in bicarbonate (HCO3−) with or without compensatory increase in carbon dioxide partial pressure (Pco2); pH may be high or nearly normal. Common causes... read more is suggested by HCO3> 28 mEq/L (> 28 mmol/L). The Pco2 should compensate by increasing about 0.6 to 0.75 mm Hg for each 1 mEq/L (1 mmol/L) increase in HCO3 (up to about 55 mm Hg). Greater increase implies concomitant respiratory acidosis; lesser increase, respiratory alkalosis.

Respiratory alkalosis Respiratory Alkalosis Respiratory alkalosis is a primary decrease in carbon dioxide partial pressure (Pco2) with or without compensatory decrease in bicarbonate (HCO3−); pH may be high or near normal. Cause is an... read more is suggested by Pco2< 38 mm Hg. The HCO3 should compensate over 4 to 12 hours by decreasing 4 to 5 mEq/L (4 to 5 mmol/L) for every 10 mm Hg decrease in Pco2. Lesser decrease means there has been no time for compensation or a primary metabolic alkalosis coexists. Greater decrease implies a primary metabolic acidosis.

Nomograms (acid-base maps) are an alternative way to diagnose mixed disorders, allowing for simultaneous plotting of pH, HCO3, and Pco2.

Equations for Calculating Acid-Base Balance

Calculation of the anion gap

The anion gap is defined as serum sodium (Na) concentration minus the sum of serum chloride (Cl) and serum bicarbonate (HCO3) concentrations.

Na+ (Cl+ HCO3)

The term “gap” is misleading, because the law of electroneutrality requires the same number of positive and negative charges in an open system; the gap appears on laboratory testing because certain cations (+) and anions (−) are not measured on routine laboratory chemistry panels. Thus,

Na++ unmeasured cations (UC) = Cl + HCO3+ unmeasured anions (UA)

and

the anion gap,

Na+ (Cl+ HCO3) = UA UC

The predominant "unmeasured" anions are phosphate (PO43), sulfate (SO4), various negatively charged proteins, and some organic acids, accounting for 20 to 24 mEq/L (20 to 24 mmol/L).

The predominant "unmeasured" extracellular cations are potassium (K+), calcium (Ca++), and magnesium (Mg++) and account for about 11 mEq/L (5.5 mmol/L).

Thus, the typical anion gap is 23 11 = 12 mEq/L (12 mmol/L). The anion gap can be affected by increases or decreases in the UC or UA.

The delta gap: The difference between the patient’s anion gap and the normal anion gap is termed the delta gap. This amount is considered an HCO3 equivalent, because for every unit rise in the anion gap, the HCO3 should lower by 1 (by buffering). Thus, if the delta gap is added to the measured HCO3, the result should be in the normal range for HCO3; elevation indicates the additional presence of a metabolic alkalosis Metabolic Alkalosis Metabolic alkalosis is primary increase in bicarbonate (HCO3−) with or without compensatory increase in carbon dioxide partial pressure (Pco2); pH may be high or nearly normal. Common causes... read more .

Example: A vomiting, ill-appearing patient with alcohol use disorder has laboratory results showing

  • Na: 137

  • K: 3.8

  • Cl: 90

  • HCO3: 22

  • pH: 7.40

  • Pco2: 41

  • Po2: 85

At first glance, results appear unremarkable. However, calculations show elevation of the anion gap:

137 (90 + 22) = 25 (normal, 10 to 12)

indicating a metabolic acidosis. Respiratory compensation is evaluated by Winters formula:

Predicted Pco2= 1.5 (22) + 8 ± 2 = 41 ± 2

Predicted = measured, so respiratory compensation is appropriate.

Because there is metabolic acidosis, the delta gap is calculated, and the result is added to measured HCO3:

25 10 = 15

15 + 22 = 37

The resulting corrected HCO3 is above the normal range for HCO3, indicating a primary metabolic alkalosis is also present. Thus, the patient has a mixed acid-base disorder.

Using clinical information, one could theorize a metabolic acidosis arising from alcoholic ketoacidosis combined with a metabolic alkalosis from recurrent vomiting with loss of acid (HCl) and volume.

Key Points

  • Acidosis and alkalosis refer to physiologic processes that cause accumulation or loss of acid and/or alkali; blood pH may or may not be abnormal.

  • Acidemia and alkalemia refer to an abnormally acidic (pH < 7.35) or alkalotic (pH > 7.45) serum pH.

  • Acid-base disorders are classified as metabolic if the change in pH is primarily due to an alteration in serum bicarbonate (HCO3) and respiratory if the change is primarily due to a change in Pco2 (increase or decrease in ventilation).

  • The pH establishes the primary process (acidosis or alkalosis), changes in Pco2 reflect the respiratory component, and changes in HCO3 reflect the metabolic component.

  • All acid-base disturbances result in compensation that tends to normalize the pH. Metabolic acid-base disorders result in respiratory compensation (change in Pco2); respiratory acid-base disorders result in metabolic compensation (change in HCO3 ).

  • More than one primary acid-base disorder may be present simultaneously. It is important to identify and address each primary acid-base disorder.

  • Initial laboratory evaluation of acid-base disorders includes measurement of arterial blood gases and serum electrolytes and calculation of the anion gap.

  • Use one of several formulas, rules-of-thumb, or an acid-base nomogram to determine if laboratory values are consistent with a single acid-base disorder (and compensation) or if a second primary acid-base disorder is also present.

  • Treat each primary acid-base disorder.

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