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The term respiratory failure, in medicine, is used to describe inadequate gas exchange by the respiratory system, with the result that arterial oxygen and/or carbon dioxide levels cannot be maintained within their normal ranges. A drop in blood oxygenation is known as hypoxemia; a rise in arterial carbon dioxide levels is called hypercapnia. The normal reference values are: oxygen PaO₂ > 60 mmHg, and carbon dioxide PaCO2 < 45 mmHg (values in kPa being PO₂ below 8kPA and PCO₂ above 6.7 kPa). Classification into type I or type II relates to the absence or presence of hypercapnia respectively.

Types

Type 1

Type 1 respiratory failure is defined as hypoxaemia without hypercapnia, and indeed the P_aCO₂ may be normal or low. It is typically caused by a ventilation/perfusion (V/Q) mismatch; the volume of air flowing in and out of the lungs is not matched with the flow of blood to the lungs. The basic defect in type 1 respiratory failure is failure of oxygenation characterized by:

PaO2	low (< 60mmHg)
PaCO2	normal or low (≤ 49mmHg)
PA-aO2	increased

This type of respiratory failure is caused by conditions that affect oxygenation such as:

• Parenchymal disease (V/Q mismatch)
• Diseases of vasculature and shunts: right-to-left shunt, pulmonary embolism
• interstitial lung diseases: ARDS, pneumonia, emphysema.

Type 2

Type 2 respiratory failure is caused by increased airway resistance; both oxygen and carbon dioxide are affected. Defined as the build up of carbon dioxide levels (P_aCO₂) that has been generated by the body. The underlying causes include:

• Reduced breathing effort (in the fatigued patient)
• Increased resistance to breathing (such as in asthma)
• A decrease in the area of the lung available for gas exchange (such as in emphysema).

The basic defect in type 2 respiratory failure is characterized by:

PaO2	decreased
PaCO2	increased
PA-aO2	normal
pH	decreased

Causes

Chest X-ray showing ARDS

• Pulmonary dysfunction
  • Asthma
  • Emphysema
  • Chronic Obstructive Pulmonary Disease
  • Pneumonia
  • Pneumothorax
  • Pulmonary contusion^[1]
  • Hemothorax
  • Acute Respiratory Distress Syndrome (ARDS) is a specific and life-threatening type of respiratory failure.
  • Cystic Fibrosis
• Cardiac dysfunction
  • Pulmonary edema
  • Cerebrovascular Accident
  • Arrhythmia
  • Congestive heart failure
  • Valve pathology
• Other
  • Fatigue due to prolonged tachypnoea in metabolic acidosis
  • Intoxication with drugs (e.g. morphine, benzodiazepines) that suppress respiration.
  • Neurological Disease
  • Toxic Epidermal Necrolysis

Treatment

Mechanical Ventilator

Emergency treatment follows the principles of cardiopulmonary resuscitation. Treatment of the underlying cause is required. Endotracheal intubation and mechanical ventilation may be required. Respiratory stimulants such as doxapram may be used, and if the respiratory failure resulted from an overdose of sedative drugs such as opioids or benzodiazepines, then the appropriate antidote such as naloxone or flumazenil will be given.

See also

• Ventilation/perfusion ratio
• Pulmonary shunt

References