Epistemis

Respiratory Disorders and Mixed Acid-Base Balance Disorders

  • Analysis of Arterial Gases

Respiratory disorders of acid-base balance are characterized by the primary change induced in pulmonary ventilation, directly modifying the partial pressure of carbon dioxide (pCO2).

Mechanism

Physiological Compensation Mechanisms

Faced with a disorder of the acid-base balance, the body reflexively activates the opposing physiological system in order to return the pH to physiological values. The mathematical limits of the expected normal compensation are described below:

Primary Disorder Expected Compensatory Response Mathematical Formula for Estimating Compensation
Metabolic Acidosis Compensatory hyperventilation (decrease in pCO2) Winter's formula: expected pCO2 = (1.5 · [HCO3-]) + 8 ± 2
Metabolic alkalosis Compensatory hypoventilation (elevated pCO2) expected pCO2 = (0.7 · [HCO3-]) + 21 ± 2
Acute Respiratory Acidosis Renal (acute) bicarbonate retention For every 10 mmHg increase in pCO2 [HCO3-] there is a rise of 1 mEq/L
Chronic Respiratory Acidosis Mitochondrial adaptation of renal transport (chronic) For every 10 mmHg increase in pCO2 [HCO3-] it rises 3.5 to 4.0 mEq/L
Acute Respiratory Alkalosis Reflex renal excretion of bicarbonate (acute) For every 10 mmHg decrease in pCO2 [HCO3-] decreases 2 mEq/L
Chronic Respiratory Alkalosis Suppression of tubular bicarbonate reabsorption (chronic) For every 10 mmHg decrease in pCO2 [HCO3-] decreases 4 to 5.0 mEq/L

Clinical

Clinical Pearl: Systematic 5-Step Interpretation Algorithm of Arterial Gases

To decipher with diagnostic precision the acid-base metabolic balance in critically ill patients, the sequential application of the following analytical protocol is suggested:

  1. Step 1: Evaluate Acidemia or Alkalemia: Analyze the serum pH of the sample. If the pH is < 7.35, there is primary acidemia. If the pH is > 7.45, there is primary alkalemia.
  2. Step 2: Identify the Primary Driving Process: Analyze the behavior of carbon dioxide (pCO2) and bicarbonate concentration (HCO3-) to verify which variable causes the pH measured in the first step.
  3. Step 3: Determine the Appropriate Level of Compensation: Apply the compensation formulas described in the table above. If the value measured by the laboratory differs significantly from the calculated value expected by the formula, the concomitant presence of a mixed acid-base disorder is documented (for example, if the actual pCO2 is higher than that calculated by Winter's formula, there is a respiratory acidosis added to the base metabolic acidosis).
  4. Step 4: Mandatory Calculation of the Anion Gap: Evaluate whether there is an increased anion gap, strictly correcting for the plasma albumin concentration measured by the laboratory.
  5. Step 5: Apply the Delta/Delta Ratio in the event of a High Anion Gap: Analyze whether there is a metabolic alkalosis or hyperchloremic acidosis masked behind the acidosis with a primary high Anion Gap.

Epistemis is educational review material. It is not a medical device, does not diagnose or prescribe treatment, and does not replace formal medical training, current clinical guidelines, or professional clinical judgment.

PreviousMetabolic Alkalosis
System
Fluid therapy, Electrolytes and Acid-Base
Cluster
Analysis of Arterial Gases
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