Attenuation of Heat-Induced Hypothalamic Ischemia, Inflammation, and Damage by Hyperbaric Oxygen in Rats

Abstract

The present study was attempted to assess the mechanisms underlying the beneficial effects of hyperbaric oxygen (HBO₂; 100% O₂ at 253 kpa) in treating experimental heatstroke. Anesthetized rats were divided into five major groups: normothermic control (NC) rats treated with normobaric air (NBA; 21% O₂ at 101 kpa; NC + NBA); NC rats treated with HBO₂ (NC + HBO₂); heatstroke (HS) rats treated with NBA (HS + NBA); HS rats treated with hyperbaric air (HBA; 21% at 253 kpa; HS + HBA); and HS rats treated with HBO₂ (HS + HBO₂). HS groups were exposed to heat (43°C) for exactly 68 min and then allowed to recover at 26°C. HBA or HBO₂ was adopted 68 or 78 min after the start of heat exposure. Survival time values for (HS + NBA) rats, (HS + HBA) rats at 68 min, (HS + HBA) rats at 78 min, (HS + HBO₂) rats at 68 min, and (HS + HBO₂) rats at 78 min were found to be 90 ± 3, 133 ± 12, 109 ± 9, 240 ± 18, and 170 ± 15 min, respectively. Resuscitation with HBA or HBO₂ at 68 min was superior to those treated at 78 min in prolonging the survival time values. All (HS + NBA) animals displayed hyperthermia, hypotension, and increased cellular levels of ischemia, oxidative stress and damage markers, pro-inflammatory cytokines, and an indicator of polymorphonuclear cell accumulation in their hypothalamus as compared to those of NCs. Heat-induced hyperthermia was not affected by HBA or HBO₂ treatment. However, heat-induced hypotension and hypothalamic ischemia, oxidative stress, neuronal damage, and inflammation were all significantly reduced by HBA or HBO₂ therapy. Compared to those of HBA therapy, HBO₂ therapy had a significantly higher beneficial effect in treating heatstroke. Our results suggested that HBO₂ improved heatstroke outcomes, in part, by restoring normal hypothalamic function. Delaying the onset of HBO₂ therapy reduced the therapeutic efficiency.

Keywords: heat stroke; hyperbaric oxygen; hypothalamus; inflammation; injury; ischemia; oxidative stress.