Type 1 and Type 2 Respiratory Distress
Hypoxemic respiratory failure (type I) is characterized by an arterial oxygen tension (PaO2) lower
than 60 mm Hg with a normal or low arterial carbon dioxide tension (PaCO 2). This is the most
common form of respiratory failure, and it can be associated with virtually all acute diseases of the
lung, which generally involve fluid filling or collapse of alveolar units. Some examples of type I
respiratory failure are cardiogenic or noncardiogenic pulmonary edema, pneumonia, and pulmonary
hemorrhage.
The pathophysiologic mechanisms that account for the hypoxemia observed in a wide variety of
diseases are V/Q mismatch and shunt.
V/Q mismatch
V/Q mismatch is the most common cause of hypoxemia. Alveolar units may vary from low-V/Q to
high-V/Q in the presence of a disease process. The low-V/Q units contribute to hypoxemia and
hypercapnia, whereas the high-V/Q units waste ventilation but do not affect gas exchange unless the
abnormality is quite severe.
The low V/Q ratio may occur either from a decrease in ventilation secondary to airway or
interstitial lung disease or from overperfusion in the presence of normal ventilation. The
overperfusion may occur in case of pulmonary embolism, where the blood is diverted to normally
ventilated units from regions of lungs that have blood flow obstruction secondary to embolism.
Administration of 100% oxygen eliminates all of the low-V/Q units, thus leading to correction of
hypoxemia. Hypoxemia increases minute ventilation by chemoreceptor stimulation, but the
PaCO2 generally is not affected.
Shunt
Shunt is defined as the persistence of hypoxemia despite 100% oxygen inhalation. The deoxygenated
blood (mixed venous blood) bypasses the ventilated alveoli and mixes with oxygenated blood that
has flowed through the ventilated alveoli, consequently leading to a reduction in arterial blood
content.
Anatomic shunt exists in normal lungs because of the bronchial and thebesian circulations, which
account for 2-3% of shunt. A normal right-to-left shunt may occur from atrial septal defect,
ventricular septal defect, patent ductus arteriosus, or arteriovenous malformation in the lung.
Shunt as a cause of hypoxemia is observed primarily in pneumonia, atelectasis, and severe
pulmonary edema of either cardiac or noncardiac origin. Hypercapnia generally does not develop
unless the shunt is excessive (> 60%). Compared with V/Q mismatch, hypoxemia produced by shunt
is difficult to correct by means of oxygen administration.
Common causes of type I (hypoxemic) respiratory failure include the following:
COPD
Pneumonia
Pulmonary edema
Pulmonary fibrosis
Asthma
Pneumothorax
Pulmonary embolism
Pulmonary arterial hypertension
Granulomatous lung diseases
Cyanotic congenital heart disease
Bronchiectasis
Acute respiratory distress syndrome (ARDS)
Fat embolism syndrome
, Hypercapnic respiratory failure (type II) is characterized by a PaCO2 higher than 50 mm Hg.
Hypoxemia is common in patients with hypercapnic respiratory failure who are breathing room air.
The pH depends on the level of bicarbonate, which, in turn, is dependent on the duration of
hypercapnia. Common etiologies include drug overdose, neuromuscular disease, chest wall
abnormalities, and severe airway disorders (eg, asthma and chronic obstructive pulmonary
disease [COPD]).
A reduction in minute ventilation is observed primarily in the setting of neuromuscular disorders and
CNS depression. In pure hypercapnic respiratory failure, the hypoxemia is easily corrected with
oxygen therapy.
Hypoventilation is an uncommon cause of respiratory failure and usually occurs from depression of
the CNS from drugs or neuromuscular diseases affecting respiratory muscles. Hypoventilation is
characterized by hypercapnia and hypoxemia. Hypoventilation can be differentiated from other
causes of hypoxemia by the presence of a normal alveolar-arterial PO 2 gradient.
Common causes of type II (hypercapnic) respiratory failure include the following:
COPD
Severe asthma
Drug overdose
Poisonings
Myasthenia gravis
Polyneuropathy
Poliomyelitis
Primary muscle disorders
Porphyria
Cervical cordotomy
Head and cervical cord injury
Primary alveolar hypoventilation
Obesity-hypoventilation syndrome
Pulmonary edema
ARDS
Myxedema
Tetanus
Hypoxemic respiratory failure (type I) is characterized by an arterial oxygen tension (PaO2) lower
than 60 mm Hg with a normal or low arterial carbon dioxide tension (PaCO 2). This is the most
common form of respiratory failure, and it can be associated with virtually all acute diseases of the
lung, which generally involve fluid filling or collapse of alveolar units. Some examples of type I
respiratory failure are cardiogenic or noncardiogenic pulmonary edema, pneumonia, and pulmonary
hemorrhage.
The pathophysiologic mechanisms that account for the hypoxemia observed in a wide variety of
diseases are V/Q mismatch and shunt.
V/Q mismatch
V/Q mismatch is the most common cause of hypoxemia. Alveolar units may vary from low-V/Q to
high-V/Q in the presence of a disease process. The low-V/Q units contribute to hypoxemia and
hypercapnia, whereas the high-V/Q units waste ventilation but do not affect gas exchange unless the
abnormality is quite severe.
The low V/Q ratio may occur either from a decrease in ventilation secondary to airway or
interstitial lung disease or from overperfusion in the presence of normal ventilation. The
overperfusion may occur in case of pulmonary embolism, where the blood is diverted to normally
ventilated units from regions of lungs that have blood flow obstruction secondary to embolism.
Administration of 100% oxygen eliminates all of the low-V/Q units, thus leading to correction of
hypoxemia. Hypoxemia increases minute ventilation by chemoreceptor stimulation, but the
PaCO2 generally is not affected.
Shunt
Shunt is defined as the persistence of hypoxemia despite 100% oxygen inhalation. The deoxygenated
blood (mixed venous blood) bypasses the ventilated alveoli and mixes with oxygenated blood that
has flowed through the ventilated alveoli, consequently leading to a reduction in arterial blood
content.
Anatomic shunt exists in normal lungs because of the bronchial and thebesian circulations, which
account for 2-3% of shunt. A normal right-to-left shunt may occur from atrial septal defect,
ventricular septal defect, patent ductus arteriosus, or arteriovenous malformation in the lung.
Shunt as a cause of hypoxemia is observed primarily in pneumonia, atelectasis, and severe
pulmonary edema of either cardiac or noncardiac origin. Hypercapnia generally does not develop
unless the shunt is excessive (> 60%). Compared with V/Q mismatch, hypoxemia produced by shunt
is difficult to correct by means of oxygen administration.
Common causes of type I (hypoxemic) respiratory failure include the following:
COPD
Pneumonia
Pulmonary edema
Pulmonary fibrosis
Asthma
Pneumothorax
Pulmonary embolism
Pulmonary arterial hypertension
Granulomatous lung diseases
Cyanotic congenital heart disease
Bronchiectasis
Acute respiratory distress syndrome (ARDS)
Fat embolism syndrome
, Hypercapnic respiratory failure (type II) is characterized by a PaCO2 higher than 50 mm Hg.
Hypoxemia is common in patients with hypercapnic respiratory failure who are breathing room air.
The pH depends on the level of bicarbonate, which, in turn, is dependent on the duration of
hypercapnia. Common etiologies include drug overdose, neuromuscular disease, chest wall
abnormalities, and severe airway disorders (eg, asthma and chronic obstructive pulmonary
disease [COPD]).
A reduction in minute ventilation is observed primarily in the setting of neuromuscular disorders and
CNS depression. In pure hypercapnic respiratory failure, the hypoxemia is easily corrected with
oxygen therapy.
Hypoventilation is an uncommon cause of respiratory failure and usually occurs from depression of
the CNS from drugs or neuromuscular diseases affecting respiratory muscles. Hypoventilation is
characterized by hypercapnia and hypoxemia. Hypoventilation can be differentiated from other
causes of hypoxemia by the presence of a normal alveolar-arterial PO 2 gradient.
Common causes of type II (hypercapnic) respiratory failure include the following:
COPD
Severe asthma
Drug overdose
Poisonings
Myasthenia gravis
Polyneuropathy
Poliomyelitis
Primary muscle disorders
Porphyria
Cervical cordotomy
Head and cervical cord injury
Primary alveolar hypoventilation
Obesity-hypoventilation syndrome
Pulmonary edema
ARDS
Myxedema
Tetanus
