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Acid-Base Disorders
pH, Compensation, and Clinical Patterns
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Acid-base disorders describe problems that change blood pH, usually through abnormal carbon dioxide handling by the lungs or bicarbonate regulation by the kidneys. These disorders matter because even small pH changes can disrupt enzyme activity, oxygen delivery, heart rhythm, and brain function. In clinical medicine, recognizing the pattern quickly helps identify emergencies such as diabetic ketoacidosis, opioid overdose, sepsis, or prolonged vomiting. A clear framework based on pH, PaCO2, and HCO3- makes these disorders much easier to classify.
The body stabilizes pH through buffers, respiratory compensation, and renal compensation. Respiratory disorders begin with a primary change in PaCO2, while metabolic disorders begin with a primary change in HCO3-. Compensation moves the other variable in the same direction as the primary disturbance, but it does not fully normalize pH. Clinical interpretation often combines blood gas values, expected compensation formulas, and the anion gap to detect mixed disorders and identify the underlying cause.
Key Facts
- Normal arterial pH = 7.35 to 7.45, normal PaCO2 = 35 to 45 mmHg, normal HCO3- = 22 to 26 mEq/L.
- Henderson-Hasselbalch relationship: pH is proportional to HCO3- / PaCO2.
- Acidemia means pH < 7.35, and alkalemia means pH > 7.45.
- Winter's formula for metabolic acidosis: expected PaCO2 = 1.5 x HCO3- + 8 ± 2.
- Anion gap = Na+ - (Cl- + HCO3-), with a typical normal value about 8 to 12 mEq/L.
- Compensation rule: metabolic disorders are compensated by lungs changing PaCO2, and respiratory disorders are compensated by kidneys changing HCO3-.
Vocabulary
- Acidemia
- Acidemia is a blood pH below 7.35, showing that the blood is too acidic.
- Alkalemia
- Alkalemia is a blood pH above 7.45, showing that the blood is too basic.
- Compensation
- Compensation is the body's physiologic response that changes PaCO2 or HCO3- to reduce the pH abnormality.
- Anion gap
- The anion gap is a calculated value used to help identify causes of metabolic acidosis.
- PaCO2
- PaCO2 is the partial pressure of carbon dioxide in arterial blood and reflects respiratory acid control.
Common Mistakes to Avoid
- Calling compensation a mixed disorder, which is wrong because appropriate compensation is an expected physiologic response rather than a second primary problem. Compare the measured value with the expected compensation formula before deciding it is mixed.
- Looking only at pH, which is wrong because near-normal pH can hide a serious mixed acid-base disorder. Always interpret pH together with PaCO2, HCO3-, and the clinical setting.
- Assuming compensation fully corrects the pH, which is wrong because compensation usually moves pH toward normal but does not overshoot into the opposite disorder. If values suggest overcorrection, suspect a second primary process.
- Ignoring the anion gap in metabolic acidosis, which is wrong because it can separate toxin or lactate related acidosis from bicarbonate loss states. Calculate the anion gap whenever HCO3- is low.
Practice Questions
- 1 An arterial blood gas shows pH 7.28, PaCO2 30 mmHg, and HCO3- 14 mEq/L. Identify the primary disorder and use Winter's formula to decide whether respiratory compensation is appropriate.
- 2 A patient has Na+ 140 mEq/L, Cl- 100 mEq/L, and HCO3- 12 mEq/L. Calculate the anion gap and state whether it is normal or elevated.
- 3 A patient with persistent vomiting has alkalemia and elevated HCO3-. Explain why the lungs and kidneys respond the way they do, and state which organ produced the primary disorder.