Arterial Blood Gas (ABG) Interpretation

Introduction

Arterial blood gas (ABG) analysis is essential in assessing and managing patients with respiratory and metabolic disorders. It provides information on the acid-base balance, oxygenation status, and ventilation. Key components of an ABG report include:

• pH: Indicates the acidity or alkalinity of the blood (normal range: 7.35-7.45).
• PaCO₂: Partial pressure of carbon dioxide (normal range: 4.7-6.0 kPa or 35-45 mmHg).
• HCO₃: Bicarbonate concentration (normal range: 22-26 mmol/L).
• PaO₂: Partial pressure of oxygen (normal range: 10-13 kPa or 75-100 mmHg).
• SaO₂: Oxygen saturation (normal range: 94-100%).
• Base Excess (BE): Indicates the amount of excess or insufficient bicarbonate in the system (normal range: -2 to +2 mmol/L).

Steps to Interpret ABGs

• Assess the pH to determine if the patient is acidotic (<7.35) or alkalotic (>7.45).
• Evaluate the PaCO₂ to determine if the respiratory system is contributing to the acid-base imbalance.
• Examine the HCO₃ to see if the metabolic (renal) system is contributing to the acid-base imbalance.
• Look for compensation:
  • If pH is normal but PaCO₂ and HCO₃ are abnormal, this suggests complete compensation.
  • If pH is abnormal, look for partial compensation.

Hypoxaemia

Definition

Hypoxaemia refers to low levels of oxygen in the blood, typically defined as PaO₂ < 10 kPa (75 mmHg).

Causes

• V/Q mismatch (e.g., pulmonary embolism)
• Shunt (e.g., congenital heart disease)
• Hypoventilation (e.g., CNS depression)
• Diffusion impairment (e.g., pulmonary fibrosis)
• Reduced FiO₂ (e.g., high altitude)

ABG Findings

• PaO₂: < 10 kPa (75 mmHg)
• Other parameters depend on the underlying cause

Type 1 vs Type 2 Respiratory Failure

Type 1 Respiratory Failure

Characterised by hypoxaemia with normal or low PaCO₂.

Causes

• Pneumonia
• Pulmonary embolism
• Acute respiratory distress syndrome (ARDS)
• Asthma

ABG Findings

• PaO₂: < 10 kPa (75 mmHg)
• PaCO₂: Normal or low

Type 2 Respiratory Failure

Characterised by hypoxaemia and hypercapnia.

Causes

• COPD
• Severe asthma
• Drug overdose
• Neuromuscular disorders

ABG Findings

• PaO₂: < 10 kPa (75 mmHg)
• PaCO₂: > 6.0 kPa (45 mmHg)

Mixed Acidosis and Alkalosis

Definition

Occurs when there are simultaneous disorders causing both acidosis and alkalosis.

Causes

• Chronic renal failure with vomiting
• Cardiac arrest with vomiting
• Severe sepsis with vomiting

ABG Findings

• Variable pH
• PaCO₂ and HCO₃ may both be abnormal in different directions

Respiratory Acidosis

Causes

• COPD
• Respiratory muscle weakness (e.g., myasthenia gravis, Guillain-Barré syndrome)
• CNS depression (e.g., sedatives, opiates)
• Severe pneumonia or asthma

ABG Findings

• pH: < 7.35
• PaCO₂: > 6.0 kPa (45 mmHg)
• HCO₃: may be increased if compensation is occurring
• PaO₂: variable

Respiratory Alkalosis

Causes

• Hyperventilation (e.g., anxiety, pain, hypoxia)
• Central nervous system disorders (e.g., stroke, infection)
• Severe anaemia
• Salicylate overdose

ABG Findings

pH: > 7.45

PaCO₂: < 4.7 kPa (35 mmHg)

HCO₃: may be decreased if compensation is occurring

PaO₂: variable

Metabolic Acidosis

Causes

• Lactic acidosis (e.g., sepsis, shock)
• Ketoacidosis (e.g., diabetic ketoacidosis)
• Renal failure
• Diarrhoea (loss of bicarbonate)

ABG Findings

• pH: < 7.35
• HCO₃: < 22 mmol/L
• PaCO₂: may be decreased if compensation is occurring
• PaO₂: variable

Metabolic Alkalosis

Causes

• Vomiting or nasogastric suction
• Diuretic use
• Excessive bicarbonate intake
• Hypokalaemia

ABG Findings

• pH: > 7.45
• HCO₃: > 26 mmol/L
• PaCO₂: may be increased if compensation is occurring
• PaO₂: variable

Mixed Respiratory and Metabolic Acidosis

Causes

• Cardiac arrest
• Severe sepsis
• Multi-organ failure

ABG Findings

• pH: < 7.35
• PaCO₂: > 6.0 kPa (45 mmHg)
• HCO₃: < 22 mmol/L
• PaO₂: variable