Oxygen transport, respiratory failure, oxygen therapy

• Many biochemical reactions in the body depend on oxygen utilisation
• Factors affecting oxygen supply to tissues

  • Ventilation
  • Diffusion across alveolar-capillary membrane
  • Haemoglobin
  • Cardiac output
  • Tissue perfusion
• Oxygen therapy is required for respiratory failure in many conditions like severe asthma, chronic bronchitis, pneumonia, and myocardial infarction, etc.
• Proper supply of oxygen and elimination of carbon dioxide depends on

  • Optimal functioning of chest wall and respiratory muscles
  • Airways and lungs
  • CNS (including medullary respiratory centres)
  • Spinal cord
  • CVS
  • Endocrine system
• A disorder in any portion of these systems can lead to respiratory failure
• Respiratory failure


  A syndrome in which the respiratory system fails in one or both of its gas exchange functions: oxygenation and carbon dioxide elimination
• Characteristics of respiratory failure

  • Inability to keep arterial blood gases at normal level while breathing air at rest at sea level
  • Partial pressure of oxygen usually below 60 mmHg with or without partial pressure of carbon dioxide above 50 mmHg in arterial blood
• Types of respiratory failure

  • Hypoxaemic
  • Hypercapnic
• Hypoxemic respiratory failure (type 1)

  Characterized by an arterial oxygen tension (PaO2) lower than 60 mmHg with a normal or low arterial carbon dioxide tension (PaCO2)
• Conditions associated with type 1 respiratory failure

  • Pulmonary oedema
  • Pneumonia
  • Pulmonary haemorrhage
  • ARDS
  • Pulmonary fibrosis
  • Pneumothorax
  • Pulmonary embolism
  • Pulmonary hypertension
• Hypercapnic respiratory failure (type 2)


  Characterized by a PaCO2 higher than 50mmHg. Arterial oxygen may be normal.
• Causes of type 2 respiratory failure

  • Drug overdose (CNS depression)
  • Neuromuscular disease
  • Chest wall abnormalities
  • Severe airway disorders (e.g. asthma and COPD)
  • Obesity
  • Hypothyroidism
  • Adult respiratory syndrome
• Inadequate gas exchange


  Associated with hypoxaemia with or without hypercarbia (Type-1 respiratory failure or lung failure)
• Inadequate ventilation


  Leads to hypoxaemia with hypercarbia (Type-2 or ventilatory failure)
• Classification of respiratory failure

  • Acute
  • Chronic
• Acute respiratory failure is characterized by life threatening derangements in arterial blood gases and acid-base status
• Presentation of chronic respiratory failure is less dramatic
• Causes of acute respiratory failure

  • Defective ventilation
  • Impaired diffusion and gas exchange
  • Ventilation-perfusion abnormalities
• Causes of defective ventilation

  • Respiratory centre depression
  • Airways obstruction
  • Restrictive defects
• Causes of respiratory centre depression

  • Drugs such as narcotics, anaesthetics, and sedatives
  • Cerebral infarction
  • Cerebral trauma
• Causes of neuromuscular disorders

  • Myasthenia gravis
  • Guillain-Barre syndrome
  • Brain or spinal injuries
  • Polio, porphyria, botulism
• Causes of airways obstruction

  • Chronic obstructive pulmonary disease
  • Acute severe asthma
• Causes of restrictive defects

  • Interstitial lung disease
  • Kyphoscoliosis, ankylosing spondylitis
  • Bilateral diaphragmatic palsy
  • Severe obesity
• Causes of impaired diffusion and gas exchange

  • Pulmonary oedema
  • Acute respiratory distress syndrome
  • Pulmonary thromboembolism
  • Pulmonary fibrosis
• Causes of ventilation-perfusion abnormalities

  • Chronic obstructive pulmonary disease
  • Pulmonary fibrosis
  • Acute respiratory distress syndrome
  • Pulmonary thromboembolism
• Aims of therapy in respiratory failure

  • Achieve and maintain adequate gas exchange
  • Reversal of the precipitating process that led to the failure
• Management of type 1 respiratory failure

  High concentration of oxygen is given to correct hypoxaemia
• Patients with ARDS do not improve with simple oxygen therapy and they need mechanical ventilation (Positive end expiratory pressure - PEEP)
• Management of type 2 respiratory failure

  Ventilatory assistance is needed
• Management of type 2 respiratory failure with previous lung disease

  Controlled oxygen therapy is needed. Mechanical ventilation should be avoided as the weaning from the ventilator is very difficult.
• Hypoxia


  Lack of oxygen at the tissue level
• Hypoxaemia


  A low arterial oxygen tension below the normal expected value (85-100 mmHg)
• General features attributed to hypoxaemia


  • Restlessness
  • Palpitation
  • Sweating
  • Altered consciousness
  • Headache
  • Confusion
  • Cyanosis
• Blood pressure may initially rise but it falls as the severity of hypoxaemia worsens
• Hypercapnia accompanies hypoxaemia whenever there is hypoventilation
• Indications for oxygen therapy

  • Conditions associated with hypoxaemia
  • COPD (low concentration should be used)
  • Acute lung conditions (without underlying chronic lung disease) like pulmonary embolism, pneumonia, tension pneumothorax, acute severe asthma, pulmonary oedema, or myocardial infarction, fibrosing alveolitis (higher concentration of oxygen can be given)
• Maintaining PaO2 above 60 mmHg gives O2 saturation of 90%
• During acute exacerbation of COPD, chemoreceptor drive for ventilation is eliminated which leads to reduced alveolar ventilation
• Hypoxaemia should be reduced immediately by giving oxygen generally in a concentration of 24% to improve oxygenation without losing the respiratory stimulant effect
• Goals of oxygen therapy

  • Relieve hypoxaemia by increasing alveolar tension
  • Reduce the work of breathing
  • Decrease the work of myocardium