Blood Gas Analysis

Dr. Arvind Kumar

Introduction

Blood gas analysis is a vital diagnostic tool used to evaluate:

  • Oxygenation status

  • Ventilation status

  • Acid–base balance

It helps clinicians interpret respiratory, circulatory, and metabolic disorders accurately.

A blood gas sample may be taken from:

  • Artery (Arterial Blood Gas – ABG)

  • Vein (Venous Blood Gas – VBG)

  • Capillary blood

However, arterial blood gas (ABG) remains the gold standard for assessing oxygenation and ventilation.

Components of ABG

ABG analysis measures or calculates the following parameters:

Parameter Significance
pH Acid-base balance
PaO₂ Oxygenation status
PaCO₂ Ventilation status
HCO₃⁻ Metabolic component
Base Excess/Deficit Metabolic imbalance severity
SaO₂ Oxygen saturation

Normal Values:

  • pH: 7.35–7.45

  • PaO₂: 75–100 mmHg

  • PaCO₂: 35–45 mmHg

  • HCO₃⁻: 22–26 mEq/L

  • SaO₂: 95–100%

 

Physiological Basis

Oxygenation (PaO₂)

Reflects the amount of oxygen dissolved in arterial blood.

Low PaO₂ indicates:

  • Hypoxemia

  • Ventilation-perfusion mismatch

  • Diffusion defects

  • Shunting

Ventilation (PaCO₂)

PaCO₂ reflects alveolar ventilation.

  • ↑ PaCO₂ → Hypoventilation → Respiratory acidosis

  • ↓ PaCO₂ → Hyperventilation → Respiratory alkalosis

PaCO₂ is a sensitive marker of ventilatory failure.

Acid–Base Balance

Acid–base status is interpreted using:

  • pH

  • PaCO₂ (respiratory component)

  • HCO₃⁻ (metabolic component)

Bicarbonate is calculated using the Henderson-Hasselbalch equation.

Indications of ABG Analysis

ABG is commonly used in:

  • Acute respiratory failure

  • ARDS

  • Severe sepsis and septic shock

  • Diabetic ketoacidosis

  • Renal failure

  • Cardiac arrest

  • Asthma

  • Heart failure

  • Inborn errors of metabolism

 

Specimen

Type of Specimen

  • Whole arterial blood is required.

  • Arterial blood is preferred because it accurately reflects oxygenation status.

  • Venous blood is not reliable for assessing PaO₂.

Common Sites of Arterial Puncture

  1. Radial artery – Most commonly used (superficial and easily palpable).

  2. Femoral artery – Used in emergency situations.

  3. Brachial artery – Less commonly used.

Modified Allen Test (Before Radial Puncture)

Purpose: To assess adequate collateral circulation through the ulnar artery.

Procedure:

  1. Patient clenches fist.

  2. Compress both radial and ulnar arteries.

  3. Patient opens hand (appears pale).

  4. Release pressure on ulnar artery.

  5. Normal result: Color returns within 10–15 seconds.

If color does not return → Avoid radial artery puncture.

Equipment Required

  • Sterile 1–3 mL syringe

  • Lyophilized (dry) heparin-coated syringe

  • Needle (22–25 gauge)

  • Alcohol swab

  • Ice (if delay in transport expected)

  • Gloves and sterile gauze

Collection Technique

  1. Clean the puncture site aseptically.

  2. Insert needle at 30–45° angle into artery.

  3. Allow syringe to fill passively (arterial pressure fills it).

  4. Remove needle and apply firm pressure for 5–10 minutes.

  5. Expel any air bubbles immediately.

  6. Cap the syringe securely.

  7. Mix sample gently by rolling between palms.

  8. Send for analysis immediately.

Important Precautions

  • Collect sample anaerobically (no exposure to air).

  • Avoid air bubbles — they:

    • Increase PaO₂

    • Decrease PaCO₂

    • Increase pH

  • Avoid liquid heparin dilution.

  • Analyze within 10–15 minutes.

  • Store on ice if delay is unavoidable.

Common Pre-Analytical Errors

  • Air contamination

  • Inadequate mixing

  • Clot formation

  • Delay in analysis

  • Excess liquid heparin

  • Incorrect FiO₂ documentation

 Post-Procedure Care

  • Apply pressure to puncture site.

  • Monitor for bleeding or hematoma.

  • Document oxygen therapy status and FiO₂.

 

Arterial vs Venous Blood Gas

 

Parameter Arterial Venous
PO₂ High Much lower
PCO₂ Slightly lower Slightly higher
pH Slightly higher Slightly lower

PO₂ difference is clinically significant — hence arterial sampling is required for oxygenation assessment.

Systematic Interpretation of ABG

Step 1: Check pH

  • < 7.35 → Acidemia

  • 7.45 → Alkalemia

Step 2: Check PaCO₂

  • High → Respiratory acidosis

  • Low → Respiratory alkalosis

Step 3: Check HCO₃⁻

  • Low → Metabolic acidosis

  • High → Metabolic alkalosis

Step 4: Assess Compensation

Determine if the opposite system is compensating.

Step 5: Assess Oxygenation

Evaluate PaO₂ and SaO₂.

Oxygenation Assessment

A–a Gradient

Evaluates alveolar gas exchange efficiency.

Used to differentiate:

  • Hypoventilation

  • V/Q mismatch

  • Shunt

  • Diffusion impairment

P/F Ratio (PaO₂ / FiO₂)

Used in ARDS severity classification.

Lower ratio = Worse oxygenation.

Oxygenation Index (OI)

Used especially in neonatal and pediatric ICU settings.

Incorporates:

  • Mean airway pressure

  • FiO₂

  • PaO₂

 

Acid–Base Disorders

Respiratory Disorders:

  • Respiratory acidosis

  • Respiratory alkalosis

Metabolic Disorders:

  • Metabolic acidosis

  • Metabolic alkalosis

Common causes:

Metabolic Acidosis

  • Diabetic ketoacidosis

  • Septic shock

  • Renal failure

  • Toxin ingestion

Metabolic Alkalosis

  • Prolonged vomiting

  • Diuretic use

  • Hypokalemia

 

Pulmonary Dead Space

Dead space increases when:

  • Ventilation exceeds perfusion

  • Pulmonary embolism

  • ARDS

It is a strong prognostic marker in lung injury.

Quality Control in ABG Analysis

Accurate results depend on:

  • Proper instrument maintenance

  • Frequent calibration

  • Temperature correction

  • Control material verification

  • Barometric pressure monitoring

Common Errors:

  • Air bubbles

  • Improper heparin use

  • Delayed analysis

  • Incorrect FiO₂ documentation

  • Sample dilution

 

Clinical Significance

Assessment of Oxygenation

  • Determines adequacy of oxygen delivery to tissues.

  • Identifies hypoxemia.

  • Guides oxygen therapy and ventilator settings.

  • Helps evaluate conditions like:

    • Acute respiratory distress syndrome (ARDS)

    • Pneumonia

    • Pulmonary embolism

    • Severe asthma

Evaluation of Ventilation

  • PaCO₂ reflects effectiveness of ventilation.

  • Detects:

    • Hypoventilation (respiratory acidosis)

    • Hyperventilation (respiratory alkalosis)

  • Essential in monitoring mechanically ventilated patients.

Diagnosis of Acid–Base Disorders

ABG helps identify:

  • Respiratory acidosis

  • Respiratory alkalosis

  • Metabolic acidosis

  • Metabolic alkalosis

It is essential in conditions such as:

  • Diabetic ketoacidosis

  • Septic shock

  • Renal failure

  • Drug overdose

  • Severe dehydration

Monitoring Critically Ill Patients

  • Guides ICU management

  • Evaluates response to treatment

  • Assesses progression of cardiopulmonary disease

  • Assists in resuscitation decisions

Ventilator Management

  • Adjusts tidal volume and respiratory rate

  • Evaluates adequacy of mechanical ventilation

  • Prevents respiratory failure complications

Prognostic Indicator

  • Helps assess severity of ARDS

  • High dead space fraction indicates poor prognosis

  • Assists in determining need for advanced therapies (e.g., ECMO)

Detection of Mixed Disorders

ABG can reveal complex acid–base disturbances where both respiratory and metabolic components are involved.