Hypoxia: types and physiological responses

Definition

Hypoxia is deficiency of O2 at the tissue level.
It occurs when O2 supply to tissue is decreased, or when tissue fails to utilize the O2 that is available.

Type	Basis
Hypoxic hypoxia	Reduced arterial PO2, from decreased inspired O2 or respiratory disease
Anemic hypoxia	Normal arterial PO2, Hb inadequate to carry O2
Stagnant hypoxia	Normal O2 content, decreased blood flow to tissue
Histotoxic hypoxia	Normal O2 delivery, tissue cannot utilize O2 due to a toxic agent

Type

Hypoxic hypoxia

Basis: Reduced arterial PO2, from decreased inspired O2 or respiratory disease

Anemic hypoxia

Stagnant hypoxia

Histotoxic hypoxia

Hypoxic hypoxia causes: low inspired PO2 (high altitude, closed chambers); hypoventilation (airway obstruction, respiratory muscle paralysis, respiratory centre depression, scoliosis).
Diffusion defect (pulmonary edema, fibrosis, lung collapse) and ventilation-perfusion mismatch (large A-V shunts, atelectasis, cyanotic congenital heart disease) also cause hypoxic hypoxia.
Hypoventilation, diffusion defect and V/Q mismatch are together termed Respiratory Hypoxia.
Rapid ascent to 3000 m (10,000 ft) drops alveolar PO2 to about 60 mmHg, producing hypoxia; ascent beyond 5000 m (15,000 ft) produces severe hypoxic symptoms.
Anemic hypoxia causes: anemia (decreased Hb concentration), carbon monoxide poisoning, altered Hb (e.g. methemoglobin).
In CO poisoning Hb content is normal, but Hb is unavailable to bind and deliver O2, so CO poisoning is classified as anemic hypoxia despite normal Hb.
Mild to moderate anemia usually does not severely limit tissue O2 delivery, from a compensatory rise in red cell 2,3-DPG, but such patients develop severe hypoxia on exertion from limited O2-delivery reserve.
Stagnant hypoxia causes: heart failure, shock, vascular obstruction (organ-specific hypoxia); also called hypoperfusion, circulatory or ischemic hypoxia.
It occurs from decreased cardiac output or decreased blood flow secondary to other causes.
In chronic or complete stagnant hypoxia, local acidosis develops from lactate accumulation, which inhibits cellular metabolism.