Medicine – Acid–Base Disorders: pH, PaCO₂ and Bicarbonate Changes

The image summarises the characteristic changes in pH, PaCO₂ and bicarbonate (HCO₃⁻) in the four major acid–base disorders. The key to understanding the pattern is to identify which variable changes primarily and which variable changes as compensation.

1. Basic Principle

There are two major components controlling blood pH:

Respiratory component → PaCO₂

and

Metabolic component → HCO₃⁻.

A useful relationship is:

pH ∝ HCO₃⁻ / PaCO₂.

Therefore:

↑ HCO₃⁻ → pushes pH upward.

↓ HCO₃⁻ → pushes pH downward.

↑ PaCO₂ → pushes pH downward.

↓ PaCO₂ → pushes pH upward.

2. Metabolic Acidosis

The primary abnormality is:

↓↓ HCO₃⁻.

Loss of bicarbonate or accumulation of acid lowers the:

pH.

Therefore initially:

↓ HCO₃⁻ → ↓ pH.

3. Compensation in Metabolic Acidosis

The respiratory system compensates by:

Hyperventilation.

More CO₂ is exhaled, causing:

↓ PaCO₂.

Therefore the complete pattern is:

pH → ↓ or near normal if compensated.

PaCO₂ → ↓.

HCO₃⁻ → ↓↓.

The double arrow on bicarbonate indicates that this is the:

Primary disturbance.

4. Kussmaul Respiration

Severe metabolic acidosis may produce deep, rapid breathing called:

Kussmaul respiration.

This is particularly characteristic of:

Diabetic ketoacidosis.

The sequence is:

↓ HCO₃⁻ → acidosis → respiratory stimulation → hyperventilation → ↓ PaCO₂.

5. Metabolic Alkalosis

The primary abnormality is:

↑↑ HCO₃⁻.

This raises:

Blood pH.

Therefore:

↑ HCO₃⁻ → ↑ pH.

6. Compensation in Metabolic Alkalosis

The respiratory system attempts to compensate through:

Hypoventilation.

This retains CO₂.

Therefore:

PaCO₂ rises slightly.

The complete pattern is:

pH → ↑ or near normal if compensated.

PaCO₂ → slight ↑.

HCO₃⁻ → ↑↑.

Again, the larger bicarbonate change represents the:

Primary metabolic abnormality.

7. Why Respiratory Compensation Is Limited

The lungs cannot compensate indefinitely by hypoventilating because excessive hypoventilation would cause:

Hypoxaemia.

Therefore respiratory compensation for metabolic alkalosis is generally limited.

8. Respiratory Acidosis

The primary abnormality is:

↑↑ PaCO₂.

This occurs because of:

Hypoventilation.

CO₂ combines with water and ultimately increases hydrogen ion concentration.

Therefore:

↑ PaCO₂ → ↑ H⁺ → ↓ pH.

9. Compensation in Respiratory Acidosis

The kidneys compensate by:

Increasing H⁺ excretion

and

Retaining/generating HCO₃⁻.

Therefore bicarbonate rises.

The pattern shown in the image is:

pH → ↓ or near normal if compensated.

PaCO₂ → ↑↑.

HCO₃⁻ → ↑.

10. Acute versus Chronic Respiratory Acidosis

This distinction is important.

In acute respiratory acidosis, renal compensation has had little time to occur.

Therefore bicarbonate rises only slightly.

A useful rule is:

Every 10 mmHg ↑ PaCO₂ → HCO₃⁻ ↑ by approximately 1 mmol/L.

In chronic respiratory acidosis, the kidneys have had several days to compensate.

Therefore bicarbonate rises more substantially:

Every 10 mmHg ↑ PaCO₂ → HCO₃⁻ ↑ by approximately 3–4 mmol/L.

This is why a patient with chronic hypercapnic COPD may have a markedly elevated PaCO₂ while the pH is relatively close to normal.

11. Respiratory Alkalosis

The primary abnormality is:

↓↓ PaCO₂.

This occurs because of:

Hyperventilation.

Excess CO₂ is eliminated from the lungs.

Therefore:

↓ PaCO₂ → ↓ H⁺ → ↑ pH.

12. Compensation in Respiratory Alkalosis

The kidneys compensate by:

Reducing bicarbonate reabsorption

and increasing:

Bicarbonate excretion.

Therefore:

HCO₃⁻ falls.

The complete pattern is:

pH → ↑ or near normal if compensated.

PaCO₂ → ↓↓.

HCO₃⁻ → slight ↓.

13. Acute versus Chronic Respiratory Alkalosis

In acute respiratory alkalosis, renal compensation is limited.

For every:

10 mmHg ↓ PaCO₂

bicarbonate falls by approximately:

2 mmol/L.

In chronic respiratory alkalosis, renal compensation becomes stronger.

For every:

10 mmHg ↓ PaCO₂

bicarbonate falls by approximately:

4–5 mmol/L.

14. Why the Image Says “N or ↓” and “N or ↑”

The image shows:

Metabolic acidosis → pH N or ↓.

Metabolic alkalosis → pH N or ↑.

Respiratory acidosis → pH N or ↓.

Respiratory alkalosis → pH N or ↑.

The “N” refers to a disorder that has undergone sufficient physiological compensation for the pH to move:

Toward the normal range.

However, an important principle is:

COMPENSATION DOES NOT OVERCORRECT THE pH.

If the pH moves beyond normal in the opposite direction, consider:

A mixed acid–base disorder.

15. How to Identify the Primary Disorder

Start with the:

pH.

If:

pH < 7.35 → acidaemia.

pH > 7.45 → alkalaemia.

Then determine whether PaCO₂ or HCO₃⁻ explains the direction of the pH.

16. Acidaemia

If the patient has:

↓ pH + ↓ HCO₃⁻

the primary disorder is:

Metabolic acidosis.

If the patient has:

↓ pH + ↑ PaCO₂

the primary disorder is:

Respiratory acidosis.

17. Alkalaemia

If the patient has:

↑ pH + ↑ HCO₃⁻

the primary disorder is:

Metabolic alkalosis.

If the patient has:

↑ pH + ↓ PaCO₂

the primary disorder is:

Respiratory alkalosis.

18. The ROME Method

A useful memory aid is:

ROME

which stands for:

Respiratory Opposite, Metabolic Equal.

19. Respiratory = Opposite

In primary respiratory disorders, pH and PaCO₂ move in:

Opposite directions.

Therefore:

↓ pH + ↑ CO₂ → Respiratory acidosis.

↑ pH + ↓ CO₂ → Respiratory alkalosis.

20. Metabolic = Equal

In primary metabolic disorders, pH and HCO₃⁻ move in the:

Same direction.

Therefore:

↓ pH + ↓ HCO₃⁻ → Metabolic acidosis.

↑ pH + ↑ HCO₃⁻ → Metabolic alkalosis.

21. Metabolic Acidosis – Note Form

Primary change:

↓↓ HCO₃⁻.

↓

↓ pH.

↓

Lungs compensate by hyperventilation.

↓

↓ PaCO₂.

Therefore:

pH ↓ | PaCO₂ ↓ | HCO₃⁻ ↓↓

22. Metabolic Alkalosis – Note Form

Primary change:

↑↑ HCO₃⁻.

↓

↑ pH.

↓

Lungs compensate by hypoventilation.

↓

Slight ↑ PaCO₂.

Therefore:

pH ↑ | PaCO₂ ↑ | HCO₃⁻ ↑↑

23. Respiratory Acidosis – Note Form

Primary change:

↓↓ Ventilation.

↓

↑↑ PaCO₂.

↓

↓ pH.

↓

Kidneys retain/generate HCO₃⁻.

Therefore:

pH ↓ | PaCO₂ ↑↑ | HCO₃⁻ ↑

24. Respiratory Alkalosis – Note Form

Primary change:

↑↑ Ventilation.

↓

↓↓ PaCO₂.

↓

↑ pH.

↓

Kidneys excrete HCO₃⁻.

Therefore:

pH ↑ | PaCO₂ ↓↓ | HCO₃⁻ ↓

25. Four Disorders – Copyable Comparison

METABOLIC ACIDOSIS

pH = ↓

PaCO₂ = ↓ due respiratory compensation

HCO₃⁻ = ↓↓ primary abnormality

METABOLIC ALKALOSIS

pH = ↑

PaCO₂ = ↑ due respiratory compensation

HCO₃⁻ = ↑↑ primary abnormality

RESPIRATORY ACIDOSIS

pH = ↓

PaCO₂ = ↑↑ primary abnormality

HCO₃⁻ = ↑ due renal compensation

RESPIRATORY ALKALOSIS

pH = ↑

PaCO₂ = ↓↓ primary abnormality

HCO₃⁻ = ↓ due renal compensation

26. Compensation Rules – Copyable Note Form

Metabolic acidosis:

Primary ↓ HCO₃⁻.

Compensation → ↓ PaCO₂.

Metabolic alkalosis:

Primary ↑ HCO₃⁻.

Compensation → ↑ PaCO₂.

Respiratory acidosis:

Primary ↑ PaCO₂.

Compensation → ↑ HCO₃⁻.

Respiratory alkalosis:

Primary ↓ PaCO₂.

Compensation → ↓ HCO₃⁻.

Key Clinical Pattern

The easiest way to remember the entire image is:

METABOLIC = HCO₃⁻ IS THE PRIMARY CHANGE.

RESPIRATORY = PaCO₂ IS THE PRIMARY CHANGE.

Then:

ACIDOSIS → pH tends ↓.

ALKALOSIS → pH tends ↑.

And remember ROME:

Respiratory Opposite

Metabolic Equal.

So:

↓ pH + ↓ HCO₃⁻ → METABOLIC ACIDOSIS.

↑ pH + ↑ HCO₃⁻ → METABOLIC ALKALOSIS.

↓ pH + ↑ PaCO₂ → RESPIRATORY ACIDOSIS.

↑ pH + ↓ PaCO₂ → RESPIRATORY ALKALOSIS.