|Classification and external resources|
Respiratory failure is inadequate gas exchange by the respiratory system, with the result that levels of arterial oxygen, carbon dioxide or both 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 more than 80 mmHg (11 kPa), and carbon dioxide PaCO2 lesser than 45 mmHg (6.0 kPa). Classification into type I or type II relates to the absence or presence of hypercapnia respectively.
Type 1 respiratory failure is defined as hypoxemia without hypercapnia, and indeed the PaCO2 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 decreased (< 60 mmHg (8.0 kPa)) PaCO2 normal or decreased (<50 mmHg (6.7 kPa)) PA-aO2 increased
This type of respiratory failure is caused by conditions that affect oxygenation such as:
- Low ambient oxygen (e.g. at high altitude)
- Ventilation-perfusion mismatch (parts of the lung receive oxygen but not enough blood to absorb it, e.g. pulmonary embolism)
- Alveolar hypoventilation (decreased minute volume due to reduced respiratory muscle activity, e.g. in acute neuromuscular disease); this form can also cause type 2 respiratory failure if severe
- Diffusion problem (oxygen cannot enter the capillaries due to parenchymal disease, e.g. in pneumonia or ARDS)
- Shunt (oxygenated blood mixes with non-oxygenated blood from the venous system, e.g. right-to-left shunt)
The basic defect in type 2 respiratory failure is characterized by:
PaO2 decreased (< 60 mmHg (8.0 kPa)) PaCO2 increased (> 50 mmHg (6.7 kPa)) PA-aO2 normal pH decreased
Type 2 respiratory failure is caused by inadequate alveolar ventilation; both oxygen and carbon dioxide are affected. Defined as the build up of carbon dioxide levels (PaCO2) that has been generated by the body but cannot be eliminated. The underlying causes include:
- Increased airways resistance (chronic obstructive pulmonary disease, asthma, suffocation)
- Reduced breathing effort (drug effects, brain stem lesion, extreme obesity)
- A decrease in the area of the lung available for gas exchange (such as in chronic bronchitis)
- Neuromuscular problems (Guillain-Barré syndrome, myasthenia gravis, motor neurone disease)
- Deformed (kyphoscoliosis), rigid (ankylosing spondylitis), or flail chest.
Emergency treatment follows the principles of cardiopulmonary resuscitation. Treatment of the underlying cause is required. Endotracheal intubation and mechanical ventilation is required in severe respiratory failure (PaO2 less than 50 mmHg). Respiratory stimulants such as doxapram are rarely 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.