L10 (P2) - Acid-Base Disorders

What is respiratory acidosis?

A respiratory acid-base disorder caused by ↑ PaCO₂ (hypercapnia), usually from hypoventilation; ↑ CO₂ → ↑ H₂CO₃ → ↑ H⁺ → ↓ pH.

What is the ABG pattern for respiratory acidosis?

↓ pH, ↑ PaCO₂; HCO₃⁻ increases if the kidneys are compensating.

What is hypercapnia and what usually causes it?

Elevated arterial CO₂ (PaCO₂), usually caused by hypoventilation or impaired CO₂ elimination.

How is PaCO₂ related to CO₂ production and elimination?

PaCO₂ is directly proportional to CO₂ production and indirectly proportional to CO₂ elimination.

What are common causes of respiratory acidosis?

COPD, sedative use, sleep apnea, opioids, and other conditions causing hypoventilation or impaired CO₂ elimination.

Why does COPD cause respiratory acidosis?

COPD impairs expiration → ↓ CO₂ elimination → CO₂ retention → hypercapnia → respiratory acidosis.

What symptoms may occur with respiratory acidosis?

Shortness of breath, altered mental status, hypersomnolence, and respiratory decline.

How do the kidneys compensate for respiratory acidosis?

They increase HCO₃⁻ reabsorption/retention → ↑ blood HCO₃⁻ → helps raise pH toward normal.

Knowledge Check: How do the kidneys compensate for respiratory acidosis?

Reabsorption of bicarbonate (HCO₃⁻).

How is respiratory acidosis treated?

Identify and treat the underlying cause; bronchodilators may improve airway obstruction, and oxygen therapy may be used when appropriate.

What is respiratory alkalosis?

A respiratory acid-base disorder caused by ↓ PaCO₂ (hypocapnia), usually from hyperventilation; ↓ CO₂ → ↓ H₂CO₃ → ↓ H⁺ → ↑ pH.

What is the ABG pattern for respiratory alkalosis?

↑ pH, ↓ PaCO₂; HCO₃⁻ decreases if the kidneys are compensating.

What is hypocapnia and what usually causes it?

Decreased arterial CO₂ (PaCO₂), usually caused by hyperventilation/excessive CO₂ elimination.

What are common causes of respiratory alkalosis?

Anxiety, psychosis, pain, and hyperthyroidism; these can cause hyperventilation.

How can respiratory alkalosis cause hypokalemia?

↓ CO₂ → ↓ H⁺ → H⁺ shifts out of cells while K⁺ shifts into cells → ↓ serum K⁺ (hypokalemia).

How do the kidneys compensate for respiratory alkalosis?

They increase HCO₃⁻ excretion → ↓ blood HCO₃⁻ → lowers pH toward normal.

According to the lecture, how does potassium excretion assist compensation for respiratory alkalosis?

↑ K⁺ excretion is associated with ↑ H⁺ reabsorption → more H⁺ in blood → ↓ pH.

Knowledge Check: Which drug does NOT cause hypokalemia and is less likely to stimulate H⁺ reabsorption: amiloride, furosemide, HCTZ, or indapamide?

Amiloride; it is potassium-sparing and can cause hyperkalemia.

How is respiratory alkalosis treated?

Identify and address the underlying cause; breathing/reassurance techniques and psychological support may help when hyperventilation is anxiety-related.

What is the CO₂-HCO₃⁻ buffer equation?

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺.

Which organs control CO₂ and HCO₃⁻?

Lungs primarily control CO₂; kidneys primarily control HCO₃⁻.

What is normal physiologic blood pH?

7.35–7.45.

What are the three main values used to interpret an ABG?

pH, PaCO₂, and HCO₃⁻; PaCO₂ represents the respiratory component and HCO₃⁻ represents the metabolic/renal component.

What is the relationship between PaCO₂ and pH?

Inverse: ↑ PaCO₂ → ↓ pH; ↓ PaCO₂ → ↑ pH.

What is the complete pattern for metabolic acidosis?

↓ pH, ↓ HCO₃⁻ (initial effect), ↓ PaCO₂ (respiratory compensation).

What is the complete pattern for metabolic alkalosis?

↑ pH, ↑ HCO₃⁻ (initial effect), ↑ PaCO₂ (respiratory compensation).

What is the complete pattern for respiratory acidosis?

↓ pH, ↑ PaCO₂ (initial effect), ↑ HCO₃⁻ (renal compensation).

What is the complete pattern for respiratory alkalosis?

↑ pH, ↓ PaCO₂ (initial effect), ↓ HCO₃⁻ (renal compensation).

Which organ compensates for a primary respiratory disorder?

Kidneys; they alter HCO₃⁻ and H⁺ handling.

Which organ compensates for a primary metabolic disorder?

Lungs; they alter ventilation and PaCO₂.

How fast does respiratory vs renal/metabolic compensation occur?

Respiratory compensation is RAPID, typically within 24 hours; renal/metabolic compensation is SLOWER, typically within 3–5 days.

How do you recognize compensation in respiratory acidosis?

↑ PaCO₂ is accompanied by ↑ HCO₃⁻ because the kidneys retain bicarbonate.

How do you recognize compensation in respiratory alkalosis?

↓ PaCO₂ is accompanied by ↓ HCO₃⁻ because the kidneys excrete bicarbonate.

Does a normal pH rule out an acid-base disorder?

No. Compensation or certain mixed disorders can bring pH into the normal range even though PaCO₂ and HCO₃⁻ remain abnormal.

What is the pattern for metabolic acidosis + respiratory acidosis?

↓ HCO₃⁻ + ↑ PaCO₂ → both cause acidosis → pH typically VERY LOW.

What is the pattern for metabolic alkalosis + respiratory alkalosis?

↑ HCO₃⁻ + ↓ PaCO₂ → both cause alkalosis → pH typically VERY HIGH.

What is the pattern for metabolic acidosis + respiratory alkalosis?

↓ HCO₃⁻ + ↓ PaCO₂; one causes acidosis and the other alkalosis, so pH may be normal.

What is the pattern for metabolic alkalosis + respiratory acidosis?

↑ HCO₃⁻ + ↑ PaCO₂; one causes alkalosis and the other acidosis, so pH may be normal.

What is the easiest ventilation shortcut for respiratory disorders?

Hypoventilation → retain CO₂ → ↑ CO₂ → ↓ pH = respiratory acidosis; Hyperventilation → blow off CO₂ → ↓ CO₂ → ↑ pH = respiratory alkalosis.

Knowledge Check: Which condition is likely to be worsened by overuse of a β₂ receptor agonist?

Respiratory alkalosis; according to the lecture, overuse may increase ventilation → excessive CO₂ elimination → hypocapnia.