Arterial Blood Gas (ABG) Analysis

15-Mark Answer - MPT Level

Definition / Introduction

• Partial pressure of oxygen (PaO₂) and carbon dioxide (PaCO₂) in arterial blood
• Blood pH (acid-base status)
• Bicarbonate (HCO₃⁻), Base Excess (BE), and electrolytes (Na⁺, K⁺, Ca²⁺, Lactate)

Normal ABG Parameters

Key Parameters Explained

1. pH

• Measures acidity or alkalinity of blood
• Inversely proportional to H⁺ ion concentration
• pH < 7.35 = Acidosis; pH > 7.45 = Alkalosis
• Extremes (< 6.8 or > 7.8) cause protein denaturation, enzymatic shutdown, and death

2. PaO₂ (Partial Pressure of Oxygen)

• Reflects efficiency of O₂ transfer from lungs to blood
• Normal: 80-100 mmHg
• Age-adjusted formula: PaO₂ = 104 - (age × 0.27) mmHg
• Decreased PaO₂ (hypoxemia) causes: tissue hypoxia, tachycardia, tachypnoea, confusion, cyanosis
• Causes of low PaO₂: V/Q mismatch, diffusion defects, hypoventilation, shunting

3. PaCO₂ (Partial Pressure of CO₂)

• Reflects effectiveness of alveolar ventilation
• Normal: 35-45 mmHg
• Increased PaCO₂ (hypercapnia) → Respiratory acidosis → headache, confusion, drowsiness
• Decreased PaCO₂ (hypocapnia) → Respiratory alkalosis → dizziness, tingling, muscle spasms

4. HCO₃⁻ (Bicarbonate)

• Main blood buffer; represents the metabolic component of acid-base balance
• Regulated by the kidneys (slow control)
• Normal: 22-26 mEq/L
• Low HCO₃⁻ → Metabolic acidosis; High HCO₃⁻ → Metabolic alkalosis
• Buffer equation: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

Indications for ABG

• Respiratory failure (Type I and Type II)
• Ventilated / mechanically ventilated patients
• Cardiac failure, Renal failure, Sepsis, Burns
• Poisoning, Altered consciousness
• Pre/post-intubation and pre/post-extubation
• Metabolic disorders

Purpose

1. To determine presence and type of acid-base imbalance
2. To check for severe breathing problems and lung diseases
3. To assess response to therapeutic interventions (e.g., mechanical ventilation)

Acid-Base Disorders

A. Respiratory Acidosis

• Definition: pH < 7.35 with PaCO₂ > 45 mmHg
• Cause: Any condition causing hypoventilation
• CO₂ accumulates → combines with H₂O → forms carbonic acid → lowers pH
• Manifestations (Hypercapnic Encephalopathy):
  • Headache, cloudiness, hallucinations, CO₂ narcosis
  • Papilloedema, myoclonic jerks, tremor
  • Systemic vasodilation, vasoconstriction, cardiac arrhythmia
• Management:
  • Dialysis (to clear toxins)
  • O₂ therapy (to prevent hypoxia)
  • Ventilatory support
  • Pharmacological management of underlying cause

B. Respiratory Alkalosis

• Definition: pH > 7.45 with PaCO₂ < 35 mmHg
• Cause: Hyperventilation causing excessive CO₂ washout → alkalinizes body fluids → adaptive decrease in HCO₃⁻
• Manifestations:
  • Paraesthesia of extremities, circumoral numbness
  • Chest discomfort, light-headedness, tetany
  • Cerebral hypoxia, reflex tachycardia, decreased venous return, coronary vasospasm (Prinzmetal's angina)
• Management:
  • Breathe into a paper bag (re-breathe CO₂)
  • Restrict O₂ intake
  • If intubated: reduce minute ventilation by adjusting rate and tidal volume

C. Metabolic Acidosis

• Definition: pH < 7.35 with HCO₃⁻ < 22 mEq/L
• Cause: Accumulation of non-volatile acids or loss of base; tissue hypoperfusion → increased lactic acid
• Compensatory increase in minute ventilation (Kussmaul breathing)
• Manifestations:
  • Myocardial depression, decreased cardiac output
  • Decreased blood pressure, decreased hepatic and renal blood flow
  • Rapid breathing, joint pain, diarrhoea
• Management:
  • Administer bicarbonates
  • Ringer's lactate
  • Diuretics

D. Metabolic Alkalosis

• Definition: pH > 7.45 with HCO₃⁻ > 26 mEq/L
• Causes: Excess base / loss of acid; excess bicarbonate use; dialysis with lactate; excess diuretics; high aldosterone
• Manifestations:
  • Hypokalaemia, hypophosphataemia, hypotension
  • Cyanosis, rapid HR, irritability
• Management:
  • Dialysis, antacid laxative
  • Potassium-sparing diuretics
  • Infuse HCl (in severe cases)

Stepwise Interpretation of ABG (Clinical Approach)

Step 1: Check Source of Oxygen

• Is the patient on room air or O₂ support?
• Note FiO₂ concentration
• P/F Ratio = PaO₂ / FiO₂ - assesses severity of hypoxemia (ARDS diagnosis)
• PEEP maintains alveolar patency during expiration, improving gas exchange

Step 2: See PaO₂ and SpO₂

• Determine if respiratory failure is present
• PaO₂ falls with age: PaO₂ = 104 - (age × 0.27) mmHg

Step 3: If Failure Present, Determine Type

• PCO₂ normal or reduced → Type 1 Respiratory Failure (hypoxaemia only)
• PCO₂ increased → Type 2 Respiratory Failure (hypoxaemia + hypercapnia)

Step 4: See pH

• Acidosis < 7.35
• Normal: 7.35-7.45
• Alkalosis > 7.45

Step 5: Determine Respiratory vs. Metabolic Origin

Step 6: If Respiratory - Was it Acute, Chronic, or Acute-on-Chronic?

Step 7: Evaluate Compensation (Rules)

• If observed bicarbonate matches expected → simple disorder
• If observed does NOT match → mixed disorder

Step 8: Calculate Anion Gap (for Metabolic Acidosis)

• AG = [Na⁺] - [HCO₃⁻] - [Cl⁻]
• Normal AG = 8 ± 3 mEq/L
• Normal AG → NAGMA (Normal Anion Gap Metabolic Acidosis)
• Elevated AG → HAGMA (High Anion Gap Metabolic Acidosis)

Step 9: Delta-Delta Ratio (Dalal's Formula)

• Delta-Delta = Na - Cl - 36
• If > +6: Metabolic alkalosis with HAGMA
• If < -6: Non-anion gap metabolic acidosis with HAGMA
• If between -6 and +6: Pure HAGMA

Compensatory Mechanisms

Key principle: Compensation restores pH but does NOT treat the root cause of the imbalance.

Degrees of Compensation

1. Uncompensated - pH abnormal, compensatory parameter still normal
2. Partially Compensated - pH abnormal, compensatory parameter has begun to change
3. Fully Compensated - pH returned to normal range, both parameters remain abnormal

Complications of ABG Sampling

• Bleeding / haematoma at puncture site
• Infection at puncture site
• Thrombus in artery
• Local pain
• Numbness of hand
• Vasovagal syncope (feeling faint)

Contraindications

• Coagulopathy
• Atherosclerosis at the site
• Infection at insertion site
• Use of thrombolytic agents

Summary Table: Acid-Base Disorders at a Glance

ABG Analysis - Simple Explanation 🩸

What is ABG?

• How much oxygen is in the blood
• How much CO₂ is in the blood
• Whether the blood is too acidic or too alkaline

Normal Values (Just Remember These 4)

The Big Picture - Two Systems, Two Problems

ACID side ←————— pH 7.4 —————→ BASE (Alkaline) side
  (< 7.35)                              (> 7.45)

• 🫁 Lungs → control CO₂ (fast, within minutes)
• 🫘 Kidneys → control HCO₃⁻ (slow, takes days)

4 Acid-Base Disorders (The Heart of ABG)

Easy trick to remember:

"If CO₂ goes up → acid goes up (pH down)" "If HCO₃⁻ goes up → base goes up (pH up)"

1. 🔴 Respiratory Acidosis

• pH ↓, CO₂ ↑
• Cause: COPD, asthma, drug overdose (slow breathing)
• Symptoms: Headache, confusion, drowsy, flapping hand tremors
• Treatment: Help them breathe (ventilator, O₂)

2. 🟡 Respiratory Alkalosis

• pH ↑, CO₂ ↓
• Cause: Anxiety, panic attack, hyperventilation
• Symptoms: Tingling fingers, dizziness, muscle cramps, tetany
• Treatment: Breathe into a paper bag (re-inhale CO₂)

3. 🔵 Metabolic Acidosis

• pH ↓, HCO₃⁻ ↓
• Cause: Kidney failure, diabetes (DKA), diarrhea, lactic acidosis
• Symptoms: Fast deep breathing (body tries to blow out CO₂), low BP
• Treatment: Treat the cause, give sodium bicarbonate

4. 🟢 Metabolic Alkalosis

• pH ↑, HCO₃⁻ ↑
• Cause: Excess vomiting (lose HCl), too many antacids, diuretics
• Symptoms: Muscle weakness, low potassium, irritability
• Treatment: Replace potassium, stop the cause

Quick Memory Table

Compensation - Body's Self-Repair

• Respiratory problem? → Kidneys fix it (slowly)
• Metabolic problem? → Lungs fix it (quickly, by changing breathing rate)

1. Uncompensated - problem just started, body hasn't reacted yet
2. Partially compensated - body is trying to fix it, pH still abnormal
3. Fully compensated - pH back to normal, but both values still abnormal

How to Read an ABG - 5 Simple Steps

Step 1 → Is patient on oxygen? (Check FiO₂)
Step 2 → Check PaO₂ → Is there respiratory failure?
Step 3 → Check pH → Acidosis or Alkalosis?
Step 4 → Check CO₂ and HCO₃⁻ → Respiratory or Metabolic?
Step 5 → Is the body compensating?

Types of Respiratory Failure (Bonus)

Bottom Line

ABG = Oxygen + CO₂ + pH + HCO₃⁻ Lungs control CO₂ (respiratory) Kidneys control HCO₃⁻ (metabolic) When pH is off → find who's the culprit → treat them