Hypoxia And Oxygen Therapy

Dr.Niranjan Murthy HL Asst Professor Dept of Physiology SSMC, Tumkur

Hypoxia: lack of oxygen at tissue level Anoxia: complete absence of oxygen in tissues Types: 4. Hypoxic Hypoxia 5. Anemic Hypoxia 6. Stagnant Hypoxia 7. Histotoxic Hypoxia

Hypoxic Hypoxia  Low PaO2  Normal O2 carrying capacity and blood flow Causes:  Low PO2 in inspired air- high altitude  Decreased pulmonary ventilation- airway obstruction, paralysis of respiratory muscle, narcotics  Defect in exchange of gases through the membrane  A-V shunts- cyanotic congenital heart disease

• Characteristic features: PaO2- 40 mm Hg; PvO2- 2 mm Hg %O2 saturation- arterial 75%; venous 45% O2 content- arterial 14 ml/dL; venous 9 ml/dL O2 utilization- 5 ml/dL A-V PO2 difference- 15 mm Hg

Hypoxic hypoxia Via peripheral chemoreceptors

Respiratory center Increased pulmonary ventilation Reduced PaCO2 Shift to left of O2-Hb curve Reduced O2 release from Hb Tissue hypoxia

Anemic Hypoxia • Arterial PO2 is normal • Reduced Hb content • Causes: (iv) Anemia (v) Carbon monoxide poisoning

• Characteristic features:  PaO2- 95 mm Hg; PvO2- 40 mm Hg Reduced % saturation of Hb O2 content of blood is reduced • Hypoxia is not severe at rest because of increase in 2,3 DPG • Severe hypoxia during exercise

Carbon monoxide poisoning • CO combines with Hb at the same point where O2 combines • CO has 250 times more affinity than O2 • Shifts O2-Hb dissociation curve to left • Treated with 100% O2 or O2-CO2 mixture (95% + 5%)

Stagnant Hypoxia •

Normal Hb% and PaO2

• Reduced blood flow to tissues • Causes: (iv) Circulatory failure (v) Hemorrhage

• Characteristic features: PO2- arterial 95 mm Hg; venous 25 mm Hg %O2 saturation- arterial 97%; venous 45% O2 content- arterial 19 ml/dL; venous 9 ml/dL

Histotoxic Hypoxia • Tissue utilization of O2 is hampered • Normal O2 supply to tissues • Cause: cyanide poisoning

• Characteristic features: PO2- arterial 95 mm Hg; venous 90 mm Hg %O2 saturation- arterial 97%; venous 96% O2 content- arterial 19 ml/dL; venous 18.5 ml/dL

Effects of hypoxia • •

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On Respiration: increase ventilation via peripheral chemoreceptors On CNS: drowsiness, disorientation, reduced pain sensibility, emotional outbursts, tremors, cheyne-stoke respiration, death (when %O2 falls below 60) On CVS: increase HR and BP

Treatment of hypoxia Oxygen administration is of importance in hypoxic hypoxia. •

Inhalation of 100% O2 at normal atmospheric pressure

B. Hyperbaric oxygen therapy

Normobaric 100% O2 therapy • It is useful in hypoxic hypoxia. Dangers of inhaling 100% oxygen: Produce nasal congestion, throat pain, cough, substernal discomfort, etc by stimulating irritant receptors. Cause bronchopneumonia if given for more than 24 hrs by inhibiting alveolar macrophages Newborns should not be given more than 40% oxygen

Hyperbaric O2 Therapy • Useful in anemic, stagnant and histotoxic hypoxia • Inhalation of 100% O2 at 1 atm can increase arterial PO2 to 673 mm Hg as it includes PCO2 of 40 mm Hg and PH2O of 47 mm Hg • At 1 atmospheric pressure, oxygen dissolved in plasma is 2ml/dL (i.e 673 x 0.003) • 3 atm of pure O2 will deliver resting O2 needs

• Oxygen toxicity will occur early • Inhibits tissue enzyme activity • Cerebral vasoconstriction • Muscular twitches, tinnitus, jerking respiration, convulsions and coma

CYANOSIS • Bluish discoloration of skin due to presence of 5 gm% of reduced Hb • Sites- lips, undersurface of tongue, ear lobes, nail bed, tip of the nose • Causes- hypoxic hypoxia and stagnant hypoxia • CO poisoning- cherry red discoloration