Acid-base disorders describe problems with blood pH caused by changes in bicarbonate, carbon dioxide, or both. This cheat sheet helps medical science students classify acidosis and alkalosis, identify the primary disturbance, and recognize expected compensation. It is useful for understanding blood gas results, electrolyte patterns, and the basic logic used in clinical interpretation.
The main values are pH, HCO3-, and pCO2. Low pH means acidosis, high pH means alkalosis, HCO3- reflects the metabolic component, and pCO2 reflects the respiratory component. The anion gap helps separate causes of metabolic acidosis, while compensation formulas estimate whether the body response is appropriate.
Key Facts
- Normal arterial pH is about 7.35 to 7.45, with pH less than 7.35 indicating acidemia and pH greater than 7.45 indicating alkalemia.
- Normal HCO3- is about 22 to 26 mEq/L, and a low HCO3- points toward metabolic acidosis while a high HCO3- points toward metabolic alkalosis.
- Normal arterial pCO2 is about 35 to 45 mmHg, and a high pCO2 points toward respiratory acidosis while a low pCO2 points toward respiratory alkalosis.
- Anion gap is calculated as Na+ - (Cl- + HCO3-), and a typical normal value is about 8 to 12 mEq/L without potassium.
- In metabolic acidosis, Winter's formula estimates expected pCO2 as expected pCO2 = 1.5 x HCO3- + 8, plus or minus 2.
- In acute respiratory acidosis, HCO3- rises about 1 mEq/L for every 10 mmHg increase in pCO2 above 40 mmHg.
- In acute respiratory alkalosis, HCO3- falls about 2 mEq/L for every 10 mmHg decrease in pCO2 below 40 mmHg.
- A mixed acid-base disorder is suspected when the measured compensation is outside the expected range for the primary disorder.
Vocabulary
- Acidemia
- Acidemia is a blood pH below 7.35, meaning the blood is more acidic than normal.
- Alkalemia
- Alkalemia is a blood pH above 7.45, meaning the blood is more basic than normal.
- Bicarbonate
- Bicarbonate, written HCO3-, is the main blood buffer measured to assess the metabolic part of acid-base balance.
- pCO2
- pCO2 is the partial pressure of carbon dioxide in blood and reflects the respiratory part of acid-base balance.
- Anion Gap
- The anion gap is an estimate of unmeasured ions in blood, calculated as Na+ - (Cl- + HCO3-).
- Compensation
- Compensation is the body's attempt to correct pH by adjusting breathing or kidney handling of acid and bicarbonate.
Common Mistakes to Avoid
- Calling a disorder acidosis just because pH is low is incomplete, because acidosis is the process and acidemia is the measured low blood pH.
- Mixing up HCO3- and pCO2 is wrong because HCO3- represents the metabolic component while pCO2 represents the respiratory component.
- Forgetting to calculate the anion gap in metabolic acidosis is a problem because high-gap and normal-gap metabolic acidosis have different likely causes.
- Assuming compensation fully normalizes pH is wrong because compensation usually moves pH toward normal but does not completely fix the primary disorder.
- Ignoring whether compensation is expected can miss a mixed disorder, because an abnormal compensation value suggests a second acid-base problem.
Practice Questions
- 1 A patient has pH 7.25, HCO3- 14 mEq/L, and pCO2 30 mmHg. Identify the primary acid-base disorder.
- 2 Calculate the anion gap for Na+ 140 mEq/L, Cl- 104 mEq/L, and HCO3- 16 mEq/L.
- 3 Use Winter's formula to estimate the expected pCO2 for a patient with metabolic acidosis and HCO3- 12 mEq/L.
- 4 Explain why a patient with low pH, high pCO2, and elevated HCO3- may have respiratory acidosis with compensation rather than a simple metabolic disorder.