respiratory failure
DESCRIPTION
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 2TRANSCRIPT
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 (PaO2<60mmHg) with or without carbon dioxide retention.
If the oxygen fraction of inspired air (FiO2) is not 20%, a respiratory failure index (RFI) lower than 300 is used as a criterion for diagnosis of respiratory failure. RFI=PaO2/FiO2
Classification
According to the variation of arterial partial pressure of carbon dioxide, respiratory failures are divided into two types,
Type : hypoxemic respiratory failure, Ⅰ
no hypercapnia
Type : hypercapnic respiratory failure, Ⅱ
PaCO2>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
PaO2↓, PaCO2↑
Diffusion disorder
Surface area of diffusion membrane ↓
Thickness of diffusion membrane↑
and increased blood flow of lung
Alterations of blood gas
PaO2↓, PaCO2 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
PaO2↓, PaCO2 ↑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<PaO2<60mmHg
stimulating peripheral chemoreceptor
50mmHg<PaCO2<80mmHg
stimulating central chemoreceptor
PaO2<30mmHg
inhibiting respiratory center
PaCO2>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.