Hypothesis: Drainage of the peripheral tissue edema by the hyperbaric oxygen therapy because of hyperoxygenation that constricts arterioles and alters the downstream capillary fluid traffic in affected tissues.

Hyperbaric oxygen (HBO) therapy still lacks proper interpretations of its many actions. This hypothesis is based on reports of temporarily elevated peripheral vascular resistance (PVR) during HBO sessions. Besides that, during HBO sessions, hyperoxygenated tissues can reduce their perfusion so much that CO2 can accumulate in them. Tissue perfusion depends on vascular innervation and on the balance between systemic constrictors and local dilators. During an HBO session, increased tissue oxygen levels suppress dilatory mechanisms. Tissue hyperoxygenation increases PVR, suggesting that the HBO action on an edematous tissue may be caused by an oxygen-induced disbalance among Starling capillary forces. The presented hypothesis is that oxygen-caused arteriolar constriction reduces the hydrostatic pressure in downstream capillaries. Thus, more tissue fluid is absorbed in vascular capillaries, under the condition that the plasma colloid osmotic pressure remains unaltered during the HBO session. Among several known mechanisms behind the HBO actions, the vasoconstriction has been listed as a therapeutic modality for the reduction of the tissue edema, for a crush injury, for burns (in an acute phase), and for the compartment syndrome. The Bell's palsy is among often listed indications for the HBO treatment, although evidence is poor and reports of randomized trials are scarce.