| Parameter | Reference Interval | Clinical Notes |
|---|---|---|
| pH | 7.35-7.45 | Measure of blood acidity/alkalinity |
| PaCO₂ | 35-45 mm Hg | Partial pressure of CO₂ in arterial blood |
| Bicarbonate (HCO₃⁻) | 22-26 mEq/L (mmol/L) | Primary buffer system |
| PaO₂ | 80-100 mm Hg | Decreases above sea level and with increasing age |
| SaO₂ | >95% | Arterial O₂ saturation |
| Base Excess | ±2.0 mEq/L | Metabolic component assessment |
Normal Arterial Blood Gas Values
Clinical Manifestations of Alkalosis
Respiratory Alkalosis (↓ PaCO₂)
Neurologic
- Dizziness
- Light-headedness
- Confusion
- Headache
Cardiovascular
- Tachycardia
- Dysrhythmias (related to hypokalemia from compensation)
Gastrointestinal
- Nausea, vomiting, diarrhea
- Epigastric pain
Neuromuscular
- Tetany
- Numbness
- Tingling of extremities
- Hyperreflexia
- Seizures
Respiratory
- Hyperventilation (compensatory if metabolic)
Metabolic Alkalosis (↑ HCO₃⁻)
Neurologic
- Irritability
- Lethargy
- Confusion
- Headache
Cardiovascular
- Tachycardia
- Dysrhythmias (related to hypokalemia from compensation)
Gastrointestinal
- Nausea, vomiting
- Anorexia
Neuromuscular
- Tetany
- Tremors
- Tingling of fingers and toes
- Muscle cramps, hypertonic muscles
- Seizures
Respiratory
- Hypoventilation (compensatory action by lungs)
Clinical Manifestations of Acidosis
Respiratory Acidosis (↑ PaCO₂)
Neurologic
- Lethargy
- Confusion
- Dizziness
- Headache
- Coma
Cardiovascular
- Decreased BP
- Ventricular fibrillation (related to hyperkalemia from compensation)
- Warm, flushed skin
Gastrointestinal
- No significant findings
Neuromuscular
- Seizures
Respiratory
- Hypoventilation with hypoxia
Metabolic Acidosis (↓ HCO₃⁻)
Neurologic
- Lethargy
- Confusion
- Dizziness
- Headache
- Coma
Cardiovascular
- Decreased BP
- Dysrhythmias (related to hyperkalemia from compensation)
- Cold, clammy skin
Gastrointestinal
- Nausea, vomiting, diarrhea, abdominal pain
Neuromuscular
- Muscle weakness
Respiratory
- Deep, rapid respirations
Causes, Pathophysiology, and Laboratory Findings
| Condition & Causes | Pathophysiology | Laboratory Findings |
|---|---|---|
|
Respiratory Acidosis • Chronic respiratory disease (COPD) • Barbiturate or sedative overdose • Chest wall abnormality • Severe pneumonia • Atelectasis • Respiratory muscle weakness • Mechanical hypoventilation • Pulmonary edema |
• ↑ CO₂ retention from hypoventilation • Compensatory response is ↑ HCO₃⁻ retention by kidney |
↓ Plasma pH ↑ PaCO₂ HCO₃⁻ normal (uncompensated) ↑ HCO₃⁻ (compensated) Sample ABG Uncompensated: pH 7.31 PaCO₂ 54 mm Hg HCO₃⁻ 25 mEq/L |
|
Respiratory Alkalosis • Hyperventilation (hypoxia, anxiety, fear, pain, exercise, fever) • Stimulated respiratory center (septicemia, stroke, meningitis, encephalitis, brain injury, salicylate poisoning) • Liver failure • Mechanical hyperventilation |
• ↑ CO₂ excretion from hyperventilation • Compensatory response is ↑ HCO₃⁻ excretion by kidney |
↑ Plasma pH ↓ PaCO₂ HCO₃⁻ normal (uncompensated) ↓ HCO₃⁻ (compensated) Sample ABG Uncompensated: pH 7.52 PaCO₂ 27 mm Hg HCO₃⁻ 24 mEq/L |
|
Metabolic Acidosis • Diabetic ketoacidosis • Lactic acidosis • Starvation • Diarrhea • Renal tubular acidosis • Renal failure • Gastrointestinal fistulas • Shock |
• Gain of fixed acid, inability to excrete acid or loss of base • Compensatory response is ↑ CO₂ excretion by lungs |
↓ Plasma pH PaCO₂ normal (uncompensated) ↓ PaCO₂ (compensated) ↓ HCO₃⁻ Sample ABG Uncompensated: pH 7.29 PaCO₂ 38 mm Hg HCO₃⁻ 18 mEq/L |
|
Metabolic Alkalosis • Vomiting • Nasogastric suctioning • Diuretic therapy • Hypokalemia • Excess NaHCO₃ intake • Mineralocorticoid use |
• Loss of strong acid or gain of base • Compensatory response is ↑ CO₂ retention by lungs |
↑ Plasma pH PaCO₂ normal (uncompensated) ↑ PaCO₂ (compensated) ↑ HCO₃⁻ Sample ABG Uncompensated: pH 7.50 PaCO₂ 40 mm Hg HCO₃⁻ 34 mEq/L |
Expected Compensation Formulas
| Primary Disorder | Formula | Tolerance |
|---|---|---|
| Metabolic Acidosis | Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 Simplified: HCO₃⁻ + 15 |
± 2 mm Hg |
| Metabolic Alkalosis | Expected PaCO₂ = 0.7 × (HCO₃⁻ − 24) + 40 | ± 2 mm Hg |
| Respiratory Acidosis (Acute) | Expected HCO₃⁻ = 24 + [(PaCO₂ − 40) / 10] × 1 | ± 2 mEq/L |
| Respiratory Acidosis (Chronic) | Expected HCO₃⁻ = 24 + [(PaCO₂ − 40) / 10] × 3.5 Range: 3–4 per 10 mm Hg |
± 2 mEq/L |
| Respiratory Alkalosis (Acute) | Expected HCO₃⁻ = 24 − [(40 − PaCO₂) / 10] × 2 | ± 2 mEq/L |
| Respiratory Alkalosis (Chronic) | Expected HCO₃⁻ = 24 − [(40 − PaCO₂) / 10] × 5 | ± 2 mEq/L |
Clinical Note: PaO₂ decreases above sea level and with increasing age. Always consider patient-specific factors when interpreting arterial blood gas results. Compensation mechanisms help maintain physiological pH but rarely completely normalize values, with the exception of chronic respiratory alkalosis, in which renal compensation may fully normalize pH. If measured compensation differs significantly from expected values, a mixed acid-base disorder should be suspected.