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Arterial Blood Gas (ABG) Analysis

Arterial Blood Gas (ABG) analysis is a diagnostic test performed on arterial blood to assess a patient’s acid–base balance and oxygenation status.

Normal ABG Parameters
  • pH (Power of hydrogen): 7.35–7.45
  • PCO₂ (Partial pressure of carbon dioxide): 35–45 mmHg
  • HCO₃⁻ (Bicarbonate): 22–26 mEq/L
  • PO₂ (Partial pressure of oxygen): 80–100 mmHg

Acidosis = pH < 7.35 | Normal = 7.40 | Alkalosis = pH > 7.45

PCO₂ → Respiratory component (Acid)

HCO₃⁻ → Metabolic component (Base)

Carbonic Acid–Bicarbonate Buffer System

The carbonic acid–bicarbonate reaction is the foundation of acid–base balance in the body

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

    • Carbon dioxide (CO₂) combines with water to form carbonic acid (H₂CO₃).
    • Carbonic acid then dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻).
    • Acids →  Hydrogen ions (H⁺), carbonic acid (H₂CO₃) and Carbon dioxide (CO₂)
    • Base → Bicarbonate ions 

Role of lungs and kidney:

  • Lungs →  Control carbonic acid (via CO₂ excretion or retention).
  • Kidneys → Control bicarbonate (via excretion or reabsorption).
  • This buffer system maintains pH ≈ 7.4 with a ratio of  20 parts bicarbonate : 1 part carbonic acid (20:1)
Blood Sampling for ABG

Sites for Sampling

  • Radial artery (preferred), Brachial artery, Femoral artery

Modified Allen’s Test

  • It is a simple, bedside procedure performed before radial artery puncture (for ABG sampling or arterial line insertion).
  • Its purpose is to check the adequacy of collateral circulation through the ulnar artery to ensure safe blood flow to the hand if the radial artery is damaged.

Procedure Steps

    1. Ask patient to clench fist (or open/close hand several times).
    2. Occlude both radial and ulnar arteries using firm pressure.
    3. Have patient open hand → palm will appear pale/blanched.
    4. Release ulnar artery only while keeping radial artery compressed.
    5. Check for color return:
      • Positive – Color returns within 6–15 seconds → Normal, Good collateral circulation, safe to use radial artery.
      • Negative – No color return or takes >15 sec → Abnormal, Inadequate collateral circulation,  do not use radial artery.
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Types Of Acid Base Imbalances

Any deviation in blood pH indicates acidosis (pH < 7.35) or alkalosis (pH > 7.45). Acid–base imbalances are classified as metabolic (HCO₃⁻) or respiratory (CO₂). Compensation occurs through the lungs and kidneys to restore pH balance.

1. Metabolic Acidosis
ABG Findings:
    • Low pH, Low HCO₃⁻ , 
    • PCO₂ normal (before compensation)
Common Causes:
    • Diabetic ketoacidosis (DKA)
    • Lactic acidosis (shock, sepsis)
    • Diarrhea (loss of bicarbonate)
    • Renal failure (impaired acid excretion)
Compensation:
    • Lungs increase pH by excreting more CO₂ through hyperventilation.
    • Characteristic pattern: Kussmaul’s respiration (rapid, deep breathing).

Clinical Pearl: Patients may present with fruity breath odor in DKA and signs of dehydration.

2. Metabolic Alkalosis
ABG Findings:
    • High pH , High HCO₃⁻, 
    • PCO₂ normal (before compensation)
Common Causes:
    • Severe vomiting (loss of gastric acid)
    • Excess gastric suctioning (nasogastric tube drainage)
    • Excess alkali intake (antacids, bicarbonate therapy)
Compensation:
    • Lungs retain CO₂ by hypoventilation (slowed breathing). 

Clinical Pearl:

 Metabolic alkalosis is most commonly caused by vomiting or gastric suction, leading to loss of stomach acid (H⁺).

3. Respiratory Acidosis
ABG Findings:
  • Low pH , High PCO₂ ,
  • HCO₃⁻ normal (before renal compensation)
Common Causes:
    • Chronic obstructive pulmonary disease (COPD)
    • Pneumonia (impaired gas exchange)
    • Atelectasis
    • Respiratory muscle weakness (e.g., GBS, myasthenia gravis)
    • Sedative overdose (respiratory depression)
Compensation:
    • Kidneys retain HCO₃⁻ to buffer excess acid. 
Clinical pearls:

Respiratory acidosis is seen in lung conditions causing hypoventilation (CO₂ retention)

4. Respiratory Alkalosis
ABG Findings:
    • High pH , Low PCO₂ , HCO₃⁻ normal (before renal compensation)
Common Causes:
    • Hyperventilation due to anxiety, fear, or pain
    • Exercise
    • Pulmonary embolism (hypoxia-induced hyperventilation)
    • High altitude
Compensation:
    • Kidneys excrete HCO₃⁻ to reduce alkalinity.
Clinical pearls:

Respiratory alkalosis is seen in conditions causing hyperventilation (CO₂ loss).

 

ABG Interpretation

ROME Method

The ROME Method

One of the simplest and reliable method for ABG interpretation is the ROME method:

Respiratory → Opposite (pH and PaCO₂ move in opposite directions)

Metabolic → Equal (pH and HCO₃⁻ move in the same direction)

Step 1: Assess the pH
 
  • Normal range: 7.35 – 7.45
  • pH < 7.35: Acidosis
  • pH > 7.45: Alkalosis
Step 2: Assess PaCO₂ (Respiratory Component)
 
  • Normal range: 35 – 45 mmHg
  • PaCO₂ > 45 mmHg: Respiratory Acidosis 
  • PaCO₂ < 35 mmHg: Respiratory Alkalosis 

Remember: In respiratory disorders, pH and PaCO₂ move in opposite directions.

Step 3: Assess HCO₃⁻ (Metabolic Component)
 
  • Normal range: 22 – 26 mEq/L
  • HCO₃⁻ < 22 mEq/L: Metabolic Acidosis 
  • HCO₃⁻ > 26 mEq/L: Metabolic Alkalosis

Remember: In metabolic disorders, pH and HCO₃⁻ move in the same direction

Step 4: Check for Compensation
 

1. Uncompensated – pH and either one value abnormal (PaCO₂ or HCO₃⁻).

2. Partially Compensated – All three values are abnormal

3. Fully Compensated – pH normal. Both PaCO₂ and HCO₃⁻ abnormal.

      • pH < 7.40 (but normal) → compensated acidosis
      • pH > 7.40 (but normal) → compensated alkalosis
Compensation in Acid–Base Disorders

When an acid–base imbalance occurs, the body tries to restore normal pH through compensatory mechanisms:

  • Lungs compensate for metabolic disorders by adjusting CO₂ (respiratory rate/depth).
  • Kidneys compensate for respiratory disorders by adjusting HCO₃⁻ (bicarbonate reabsorption or excretion).
    However, compensation is never perfect. The pH may approach normal but usually remains slightly abnormal, except in cases of full compensation.

Types of Compensation

1. Uncompensated

  • pH is abnormal.
  • Only one value (PaCO₂ or HCO₃⁻) is abnormal.
  • No evidence of compensation.

2. Partially Compensated

  • All three values (pH, PaCO₂, HCO₃⁻) are abnormal.
  • pH is still abnormal.
  • Indicates that the body has started to compensate but not fully.

3. Fully Compensated

  • Both PaCO₂ and HCO₃⁻ are abnormal.
  • pH is within the normal range (7.35–7.45).
  • The side of pH (closer to 7.35 or 7.45) reveals the underlying disorder:
    • < 7.40 → underlying acidosis
    • > 7.40 → underlying alkalosis
Examples for interpretation

Examples for interpretation

Case 1:     pH = 7.23, PaCO₂ = 37 mmHg, HCO₃⁻ = 18 mEq/L
  • pH ↓ → Acidosis
  • PaCO₂ → Normal
  • HCO₃⁻ ↓ (moves with pH) → Metabolic component
     Diagnosis: Uncompensated Metabolic Acidosis
Case 2:    pH = 7.20, PaCO₂ = 49 mmHg, HCO₃⁻ = 25 mEq/L
  • pH ↓ → Acidosis
  • PaCO₂ ↑ (opposite to pH) → Respiratory component
  • HCO₃⁻ → Normal

 Diagnosis: Uncompensated  Respiratory Acidosis

Case 3:   pH = 7.46, PaCO₂ = 36 mmHg, HCO₃⁻ = 32 mEq/L
  • pH ↑ → Alkalosis
  • PaCO₂ → Normal
  • HCO₃⁻ ↑ (moves with pH) → Metabolic component

 Diagnosis: Uncompensated Metabolic Alkalosis

Case 4: pH = 7.37, PaCO₂ = 33 mmHg, HCO₃⁻ = 17 mEq/L
  • pH = Normal but <7.40 ( low, leaning toward acidosis) → Acidosis
  • HCO₃⁻ low → Metabolic component (moves with pH)
  • PaCO₂ low → Respiratory system is compensating
     Diagnosis: Fully Compensated Metabolic Acidosis
Case 5: pH = 7.42, PaCO₂ = 32 mmHg, HCO₃⁻ = 18 mEq/L
  • pH = Normal but >7.40 (high, leaning toward alkalosis) → Alkalosis
  • PaCO₂ low → Respiratory component (opposite to pH)
  • HCO₃⁻ low → Kidneys compensating
     Diagnosis: Fully Compensated Respiratory Alkalosis.
Case 6:  pH = 7.56, PaCO₂ = 20 mmHg, HCO₃⁻ = 20 mEq/L
  • All 3 values abnormal ; pH ↑ = Alkalosis
  • PaCO₂ ↓ → Respiratory component (opposite to pH)
  • HCO₃⁻ ↓ → Kidneys attempting to compensate
  • But since pH is still abnormal, compensation is partial.
     Diagnosis: Partially Compensated Respiratory Alkalosis

Role of Potassium in Acid–Base Balance

 

Normal Serum Potassium Range: 3.5–5.0 mmol/L

Potassium (K⁺) shifts between intracellular and extracellular compartments during acid–base disturbances:

  1. In Acidosis,
      • K⁺ shifts out letting  H⁺ ions move into cells causing Hyperkalemia. 
      • Monitor for hypokalemia once acidosis is corrected (K⁺ shifts back into cells).
  2. In Alkalosis,
      • K⁺ moves in letting H⁺ moves out of cells causing Hypokalemia. 
      • Monitor for hyperkalemia if alkalosis is corrected rapidly.

Acidosis – Hyperkalemia; Alkalosis – Hypokalemia

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