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Respiratory failure

Respiratory Failure is defined as a PaO2 <60 mmHg

Types of respiratory failure

  1. Type I respiratory failure
    1. It is defined as hypoxia (PaO2 <60 mmHg) with a normal or low PaCO2.
    2. It is caused primarily by ventilation/perfusion (V/Q) mismatch. PAo2 – Pao2 is increased
    3. It is a diffusion or perfusion defect due to
      1. Parenchymal disease (V/Q mismatch)          
      2. Disease of blood vessel = Pulmonary embolism
      3. Interstitial lung Diseases (ARDS) 


  1. Pneumonia              
  2. Pulmonary oedema              
  3. Pulmonary Embolism            
  4. Asthma
  5. ARDS.
  1. Type II respiratory failure
    1. It is defined as hypoxia (PaO2 <60 mmHg) with hypercapnia (PaCO2 is >45 mmHg).
    2. This is caused by alveolar hypoventilation, with or without V/Q mismatch. PAo2 – Pao2 is Normal 

Causes include:

  1. COPD (Chronic bronchitis is the commonest cause) Q, obstructive sleep apnoea (OSA).
  2. Reduced respiratory drive: sedative drugs, CNS tumour, or trauma.
  3. Neuromuscular disease: cervical cord lesion, diaphragmatic paralysis, poliomyelitis, myasthenia gravis, Guillain-Barre syndrome.
  4. Thoracic wall disease: flail chest, kyphoscoliosis.

Sign / Symptom of Hypoxia

  1. Dyspnea         
  2. Agitation             
  3. Confusion                    
  4. Central cyanosis.

If longstanding hypoxia'    

  1. Polycythemia              
  2. Pulmonary hypertension       
  3. Cor pulmonale.  


  1. Headache;   
  2. peripheral vasodilatation;    
  3. tachycardia;
  4. bounding pulse,
  5. tremor/flap;   
  6. papilledema;                        
  7. Drowsiness;
  8. Coma. 


Important Points:

Management depends on the cause:

  1. Type I respiratory failure:
    1. Treat underlying cause.
    2. Give high flow oxygen Q
  2. Type II respiratory failure the respiratory centre may be relatively insensitive to CO2 and respiration could be driven by hypoxia.
    1. Oxygen therapy should be given with care. Give low flow Q oxygen
    2. Treat underlying cause.      
    3. Assisted ventilation

  1. Type III Respiratory Failure: (Ref. Hari. 18th ed., Pg - 2200)
    1. This form of respiratory failure occurs as a result of lung atelectasis.
    2. Because atelectasis occurs so commonly in the perioperative period, this is also called perioperative respiratory failure.
    3. After general anesthesia, decreases in functional residual capacity lead to collapse of dependent lung units.
    4. Such atelectasis can be treated by frequent changes in position, chest physiotherapy, upright positioning, and aggressive control of incisional and/or abdominal pain. (Conservative management). 
    5. Noninvasive positive-pressure ventilation may also be used to reverse regional atelectasis.
  2. Type IV Respiratory Failure:
    1. This form results from hypoperfusion of respiratory muscles in patients in shock. Normally, respiratory muscles consume <5% of the total cardiac output and O2 delivery.
    2. Patients in shock often experience respiratory distress due to pulmonary edema (e.g., patients in cardiogenic shock), lactic acidosis, and anemia.
    3. In this setting, up to 40% of the cardiac output may be distributed to the respiratory muscles. Intubation and mechanical ventilation can allow redistribution of the cardiac output away from the respiratory muscles and back to vital organs while the shock is treated. 
Summary of respiratory failure based on pathophysiologic derangement in respiratory function
  1. Type I or acute hypoxemic respiratory failure
    Often secondary to pulmonary edema and subsequent intrapulmonary shunting
  2. Type II respiratory failure
    Secondary to alveolar hypoventilation, resulting in the inability to effectively eliminate carbon dioxide
  3. Type III perioperative respiratory failure
    Secondary to lung atelectasis
  4. Type IV respiratory failure
    Secondary to hypoperfusion of respiratory muscles in patients in shock


  1. Hypoxia is defined as a paO2 <60mm Hg.
  2. Ventilatory efficiency:
    1. Normal paCO2 is 40 to 45 mmHg Q
    2. PaCO2 is directly related to alveolar ventilation.
    3. A paCO2 <40 mmHg indicates hyperventilation
    4. PaCO2 > 45 mmHg indicates hypoventilation.

Causes of hypoxemia -


The four basic, mechanisms of hypoxemia are -           

  1. Decrease in an inspired PO2 (FiO2)
  2. Hypoventilation
    1. Decreased respiratory drive
    2. Neuromuscular disease (Myasthenia Gravis)
    3. Drug overdose causes respiratory depression resulting in hypoxemia.
  3. Shunt
    1. Alveolar collapse (atelectasis)  
    2. Intra alveolar filling (pneumonia)
    3. Intra cardiac shunt                    
    4. Vascular shunt within lungs
  4. V/Q Mismatch
    1. Airway disease (asthma, COPD)        
    2. Interstitial lung disease
    3. Alveolar disease                        
    4. Pulmonary vascular disease (pulmonary embolism) 

Important Points: Alveolar Gas Equation

  1. Alveolar-arterial O2 concentration gradient : This is the difference in th O2 partial pressure between the alveolar and arterial sides.
  2. This gradient is normal Q in type II respiratory failure.
  3. The gradient is increased in type I respiratory failure.
  4. If the gas exchange at alveolar capillary levels occurs normally, the alveolar arterial difference in PaO2 is not elevated.
  5. In infiltrative lung disease the alveolar capillary bed is damaged and there is a decrease in diffusion capacity.
  6. This results in a increase in alveolar arterial differences in PaO2.

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