Respiratory Failure -

Respiratory Failure

Respiratory failure is one of the most common reasons for ICU admission and a major cause of mortality in critically ill patients. Understanding the pathophysiology, classification, evaluation, and management is fundamental for NEET-SS / INI-SS Critical Care preparation and real ICU practice.

Content below is synthesized from Harrison’s Principles of Internal Medicine, Oh’s Intensive Care Manual, ATS/ERS guidelines, and major critical care literature.

1. Definition

Respiratory failure is the inability of the respiratory system to maintain adequate gas exchange, resulting in:

Parameter	Value
PaO₂	< 60 mmHg
PaCO₂	> 45–50 mmHg
pH	Often < 7.35 in hypercapnic failure

This occurs despite adequate oxygen supply and normal atmospheric conditions.

Two key functions fail:

Oxygenation
Carbon dioxide elimination

2. Classification of Respiratory Failure

Type	Name	Main Problem	ABG
Type I	Hypoxemic respiratory failure	Oxygenation failure	PaO₂ <60, normal/low PaCO₂
Type II	Hypercapnic respiratory failure	Ventilatory failure	PaCO₂ >45
Type III	Perioperative respiratory failure	Atelectasis	Hypoxemia
Type IV	Shock-related respiratory failure	Respiratory muscle hypoperfusion	Hypercapnia ± hypoxemia

3. Mechanisms of Respiratory Failure

Mechanism	Causes / Conditions	Important Notes
Hypoventilation	CNS depression, neuromuscular disease, chest wall disorders, severe airway obstruction	A–a gradient usually normal because lungs are structurally normal, Improves with oxygen therapy
Ventilation–Perfusion (V/Q) Mismatch	COPD, asthma, pneumonia, pulmonary embolism, ARDS (early)	Most common cause of hypoxemia in ICU,Corrects with oxygen therapy
Shunt(Perfusion without ventilation)	ARDS, pneumonia, pulmonary edema, atelectasis, intracardiac shunts	ARDS — alveoli filled with fluid preventing gas exchange,Does NOT improve with oxygen therapy
Diffusion Limitation	Interstitial lung disease, pulmonary fibrosis, emphysema	Worse during exercise and high altitude because diffusion time decreases,Partial response to oxygen
Low Inspired Oxygen (Low FiO₂)	High altitude, closed environments, equipment malfunction	A–a gradient normal,Improves with oxygen

4. Etiology Types of Respiratory Failure

Type	Causes / Conditions	ABG Pattern
Type I Respiratory Failure (Hypoxemic)	ARDS, pneumonia, pulmonary edema, pulmonary embolism, interstitial lung disease	PaO₂ ↓, PaCO₂ normal or ↓, pH normal or alkalosis
Type II Respiratory Failure (Hypercapnic)	Airway disease:COPD, asthma Neuromuscular:myasthenia gravis, Guillain-Barré syndrome, ALS CNS depression:opioids, sedatives, stroke Chest wall disorders:kyphoscoliosis, flail chest, obesity hypoventilation	PaCO₂ ↑, PaO₂ ↓, pH ↓ in acute states
Type III	Mechanisms include: • anesthesia • pain • atelectasis • diaphragmatic dysfunction	Hypoxemia may develop due to atelectasis
Type IV	Shock states (septic shock, cardiogenic shock)	Hypercapnia ± hypoxemia may develop

ABG Pattern of Type 2 Respiratory failure

Parameter	Acute	Chronic
PaCO₂	↑	↑
pH	↓	near normal
HCO₃⁻	Normal	↑

Alveolar–Arterial Oxygen Gradient (A–a Gradient)

Age	A–a gradient
Young adults	5–10 mmHg
Elderly	Age/4 + 4

Interpretation

A–a Gradient	Cause
Normal	Hypoventilation
Increased	V/Q mismatch, shunt, diffusion defect