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.