Respiratory failure is a pathological process in which the external respiratory dysfunction leads to an abnormal decrease of arterial partial pressure of oxygen with or without increased carbon dioxide retention.

PaO₂<60mmHg,PaCO₂>50mmHg,RFI(PaO2/FiO2) ≤300

ETIOLOGY:The dysfunction of external respiration be caused by disorder of ventilation or/and gas exchange.

 PATHOGENESIS

ØDisorder of ventilation

It decreases the alveolar ventilation leaing to hypoventilation. There are two kinds of ventilatory disorder:Blood gas alterations:

PaO₂<60mmHg,PaCO₂>50mmHg.

²Restrictive Ventilatory Disorder:During inspiration the dilation or inflation of alveoli is restricted

²Obstructive Ventilatory Disorder:It is caused by airway obstruction, usual seen in airway inflammation, which may induce swelling of mucous membranes, bronchospasm,obstruction of air way space by sputum or foreign bodies, or the increase of pleural pressure.

   Closure of small airways might be caused by forced expiration in pations with ventilatory disorders.During forced expiration,the intra-thoracic pressure decreases along the airway from small airway to large airway.There must be a point along the airway where the intra-airway pressure equals to the extra-airway pressure.This is called “equal pressure point”.next to this point the intra-airway pressure is lower than the extra-airway pressure,which might lead to the clousure of airway.In healthy people,the equle pressure point is located at the large bronchus with cartilage support,so no closure of the airway occurs during forced expiration.However,in patients with chronic bronchitis or emphysems,the equal pressure point shifts up from the large airways to the small airways without cartilage support,the pressure gradient next to the equal pressure point caused by forced expiration could result in airway closure.    

ØDisorder of gas exchange

 involves:(1)diffusion disorder,(2)Ventilation-perfusion imbalance,(3)Anatomic shunt.

Blood gas alterations:PaO₂<60mmHg,PaCO₂ remains normal or might be lower than normal.or might be even higher than normal.

Ø Respiratory system

Mild hypotonic hypoxemia stimulating peripheral chemoreceptors or hypercapnia stimulating central chemorecepters leads to compensatory hyperventilation.Besides,the respiratory rate and depth in pulmonary diseases may be changed by airway obstruction,altered pulmonary compliance,stimulation of juxtapulmonary-capillary receptors as well.A long-standing increase of respiratory movement may increase the oxygen consumption of respiratory muscles and result in its fatigue,thus reducing its contractility aggravating respiratory failure.

Ø Central Nervous system

Pulmonary encephalopathy is defined as the neuropsychiatria syndrom caused by respiratory failure.It mechanism consists of:the increase of intracranial pressure induced by cerebral vasodiation and edema,which result in headache,weakness,

nausea, vomiting etc.the dysfunction and destruction of neurons caused bu increased production of

γ-aminobutyric acid,decreased resting membrane potential,energy deficiency,release of hydroxylases from microsomes etc.