Respiratory failure Respiratory failure is a pathological process in which the external respiratory dysfunction leads to an abnormal decrease of arterial partial pressure of oxygen (PaO 2 <60mmHg) with or without carbon dioxide retention. If the oxygen fraction of inspired air (FiO 2 ) is not 20%, a respiratory failure index (RFI) lower than 300 is used as a criterion for diagnosis of respiratory failure. RFI=PaO 2 /FiO 2

Classification According to the variation of arterial partial pressure of carbon dioxide, respiratory failures are divided into two types, Type Ⅰ : hypoxemic respiratory failure, no hypercapnia Type Ⅱ : hypercapnic respiratory failure, PaCO 2 >50mmHg

Etiology and pathogenesis Ventilation disorder restrictive ventilation disorder obstructive ventilation disorder Diffusion disorder Ventilation and perfusion imbalance local hypoventilation local hypoperfusion Anatomic shunt

Ventilation disorder Restrictive ventilation disorder Paralysis of respiratory muscle poliomyelitis, hypokalemia Decreased compliance of chest wall deformity of thorax, pleuritis Decreased compliance of lung edema, fibrosis, inflammation, lack of surfactant Hydrothorax and pneumothorax

Ventilation disorder Obstructive ventilation disorder Airway inflammation, bronchospasm, sputum, foreign body, increased pleural pressure. Dyspnea inspiratory dyspnea expiratory dyspnea

Inspiratory dyspnea Inspiratory dyspnea is caused by the obstruction of extrathoracic airway. During inspiration the intraairway pressure is lower than the atmosphere pressure, so the extrathoracic airway is impressed and the obstruction is aggravated.

Expiratory dyspnea Expiratory dyspnea is caused by the obstruction of intrathoracic airway. During expiration the intrathoracic pressure is increased, thus intensifying the pressure on intrathoracic airway and increasing its obstruction.

Equal pressure point During forced expiration, the intrathoracic pressure becomes positive and intra-airway pressure decreases along the airway from small airway to large airway. There must be a point along the airway where the intra- airway pressure is equal to the extra-airway pressure. This is called equal pressure point.

In patients with chronic bronchitis or emphysema, the equal pressure point shifts up from the large airway to the small airway without cartilage support. The pressure gradient next to the equal pressure point could result in airway closure.

Alterations of blood gas PaO 2 ↓, PaCO 2 ↑

Diffusion disorder Surface area of diffusion membrane ↓ Thickness of diffusion membrane↑ and increased blood flow of lung Alterations of blood gas PaO 2 ↓, PaCO 2 N

Ventilation and perfusion imbalance local hypoventilation functional shunt local hypoventilation is caused by ventilation disorder in a part of the lung. The ratio between ventilation and perfusion decreases to less than 0.8. The arterial blood from this part of the lung is hypoxemic.

Ventilation and perfusion imbalance local hypoperfusion dead space like ventilation local hypoperfusion is usually caused by pulmonary embolism. The ratio between ventilation and perfusion increases to more than 0.8 in this part of lung.

Alterations of blood gas PaO 2 ↓, PaCO 2 ↑or↓ or N

Anatomic shunt Blood vessels connecting bronchial veins or pulmonary arteries and pulmonary veins are usually constricted. In patients with pulmonary hypertension, the high blood pressure could dilate these connecting vessels forming anatomic shunt. True shunt

Alterations of metabolism and function Acid-base imbalance Respiratory system Cardiovascular system Central system

Acid-base imbalance Metabolic acidosis Respiratory acidosis Respiratory alkalosis

Respiratory system 30mmHg<PaO 2 <60mmHg stimulating peripheral chemoreceptor 50mmHg<PaCO 2 <80mmHg stimulating central chemoreceptor PaO 2 <30mmHg inhibiting respiratory center PaCO 2 >80mmHg inhibiting respiratory center

Cardiovascular system pulmonary heart disease (cor pulmonale) Pulmonary hypertension formed by hypoxic vasoconstriction, vascular remodeling and increased viscosity. Hypoxia and acidosis impair the myocardial systolic and diastolic function. During dyspnea, change of intrathoracic pressure caused by forced expiration or inspiration may aggravate cardiac diastolic or systolic function.

Central system Pulmonary encephalopathy is defined as the neuropsychiatric syndrome caused by respiratory failure. mechanism The increase of intracranial pressure induced by cerebral vasodilation and edema, which result in headache, weakness, nausea, vomiting,etc. The dysfunction and destruction of neurons caused by increased production of γ-aminobutyric acid, decreased resting membrane potential, energy deficiency, release of hydrolase, etc.

Pathophysiological basis of treatment Treating the causes of respiratory failure Increasing PaO2 Decreasing PaCO2 Treating the consequence of hypoxia and hypercapnia.