🫁 Pulmonary Aspects of Acid‑Base Balance

What are normal ABG values?

• PaO₂: ~100 mmHg (N: 75–100)

• PaCO₂: ~40 mmHg (N: 35–45)

• [HCO₃⁻]: ~24 mEq/L

• pH: ~7.4 (normal [H⁺] ~40 nEq/L)

• ↑ pH = alkalosis, ↓ pH = acidosis

Define acidosis vs alkalosis.

• Acidosis: pH < 7.35, excess H⁺ → arrhythmias, impaired cardiac function

• Alkalosis: pH > 7.45, reduced H⁺ → seizures, neuromuscular irritability, vascular collapse

What are the principal buffers in body fluids?

• Blood: Bicarbonate (pK 6.1), Proteins (pK ~7.4), Hemoglobin (pK ~7.4)

• Interstitial fluid: Bicarbonate (pK 6.1)

• Intracellular fluid: Proteins (~7.0), Phosphate (pK 6.8)

• Kidney: Phosphate, Ammonia (NH₃/NH₄⁺)

How do organs buffer H⁺?

• Blood: fastest (bicarbonate/protein) → instantaneous

• Lungs: regulate CO₂ elimination → minutes–hours

• Kidneys: excrete H⁺, reabsorb HCO₃⁻ → hours–days

• Bone: exchanges Ca²⁺/phosphate, releases carbonate → hours–days

• Ionic shift: H⁺/K⁺ exchange → stabilizes pH in 2–4 hours

What is the Henderson–Hasselbalch equation?

• Reaction: H₂O + CO₂ ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (carbonic anhydrase)

• Equation: pH = pK + log([HCO₃⁻] / (0.03 × PaCO₂))

• Key: Bicarbonate = main extracellular buffer; CO₂ controlled by lungs; HCO₃⁻ by kidneys

---

Interpretation:
• Arterial Blood Gas (ABG) is the primary method for diagnosing acid-base disorders.
• Interpretation involves pH, PaCO₂, and HCO₃⁻ to determine the type of disorder and
compensation status.

Define respiratory acidosis and alkalosis.

• Respiratory Acidosis: Hypoventilation → ↑ PaCO₂, ↓ pH; causes: airway obstruction, COPD, CNS depression; compensation: kidneys retain HCO₃⁻

• Respiratory Alkalosis: Hyperventilation → ↓ PaCO₂, ↑ pH; causes: anxiety, altitude, fever; compensation: kidneys excrete HCO₃⁻

What are causes of respiratory alkalosis?

• Central: head injury, stroke, anxiety, pain, drugs (salicylates, analeptics), progesterone, cytokines

• Peripheral chemoreceptor stimulation: hypoxemia

• Pulmonary: embolism, pneumonia, asthma, pulmonary edema

• Iatrogenic: controlled ventilation

---

MI HY: In other words, to look at that, what may cause to increase ventilation. Abnormally high ventilation that will reduce partial pressure CO2 and will result in alkalosis

What are causes of respiratory acidosis?

• Inadequate ventilation: CNS depression (opiates, trauma), neuromuscular disorders (GBS, MG), airway obstruction, COPD, chest trauma, ARDS

• Overproduction of CO₂: hypercatabolic states (malignant hyperthermia)

• Increased intake of CO₂: rebreathing, added CO₂ in inspired gas

Differentiate acute, chronic, compensated respiratory ABD.

• Acute: no compensation, pH deviates (e.g., narcotic overdose)

• Chronic: partial renal compensation, pH moves toward normal but not fully corrected

• Compensated: pH ≈ 7.4, but PaCO₂ and HCO₃⁻ abnormal

What is the stepwise approach to ABD diagnosis?

• Check pH: <7.40 = acidosis; >7.40 = alkalosis

• Check PaCO₂:

  • ↑ PaCO₂ + ↓ pH = respiratory acidosis

  • ↓ PaCO₂ + ↑ pH = respiratory alkalosis

• If PaCO₂ doesn’t match pH → metabolic disorder (Compensation evaluation covered in renal physiology)

How is a nomogram used in ABD diagnosis?

• Plot pH (X‑axis) vs [HCO₃⁻] (Y‑axis) with PaCO₂ lines

• Identify zone: respiratory acidosis/alkalosis, metabolic acidosis/alkalosis

• Example: pH 7.25, PaCO₂ 55, HCO₃⁻ 24 → acute respiratory acidosis

What are the key points of pulmonary aspects of ABD?

• Normal ABG: pH 7.4, PaCO₂ 40, HCO₃⁻ 24

• Respiratory acidosis: ↓ pH, ↑ PaCO₂ (hypoventilation)

• Respiratory alkalosis: ↑ pH, ↓ PaCO₂ (hyperventilation)

• Diagnosis: pH first, then PaCO₂ → respiratory vs metabolic

• Compensation: respiratory (fast), renal (slow)

• Tools: Henderson–Hasselbalch, flowchart, nomogram

PaCO₂ 52 mmHg, pH 7.3 → ?

Respiratory acidosis

PaCO₂ 52 mmHg, pH 7.5 → ?

Respiratory alkalosis

[HCO₃⁻] 35 mEq/L → ?

Renal compensation for respiratory acidosis

ABG pH 7.25, PaCO₂ 55, HCO₃⁻ 24 → ?

Acute respiratory acidosis