Hypoxia
HYPOXIA
- State in which tissues suffer from oxygen deficiency.
- I.e., O2 deficiency at tissue level.
TYPES:
- Depending on cause:
- Hypoxic hypoxia.
- Anemic hypoxia.
- Stagnant hypoxia.
- Histotoxic hypoxia.
HYPOXIC HYPOXIA:
- Most common type of hypoxia.
- Hypoxia is due to a decrease in oxygen supply.
- Fall in arterial blood PO2 & Low O2 content.
- Conditions associated:
Seen in hypoventilation:
- E.g., In Restrictive lung diseases, COPD etc.,
- In Reduced oxygen diffusion capacity of lungs:
- In High Altitude.
- Conditions with V/Q mismatch:
- Right to left shunt.
- E.g., Cyanotic CHD or AV malformation.
ANEMIC HYPOXIA:
- Due to decreased oxygen-carrying capacity of blood.
- Since entire oxygen transported by blood combines with hemoglobin.
- Thus “Anemic hypoxia”.
- O2 content of blood is lower despite normal PO2.
- Due to low oxygen saturation of hemoglobin.:
- As PO2, is normal, respiration is not stimulated by anemic hypoxia.
- Since peripheral chemoreceptors are sensitive only to PO2.
- Similar clinical presentation with “Carbon monoxide poisoning”.
- In carbon monoxide poisoning, carbon monoxide uses up the hemoglobin by combining with it.
- Due to “Greater affinity of carbon monoxide for hemoglobin” than oxygen.
- Hence, leaving very little hemoglobin for combining with oxygen.
- This is similar to anemia, where blood Hb level is low.
- Hence, hypoxia produced by CO poisoning also called “Anemic hypoxia”.
STAGNANT HYPOXIA/HYPOPERFUSION HYPOXIA:
- Due to sluggish blood flow.
- E.g., As in CHF or Circulatory shock.
- Both arterial blood PO2 & oxygen content is normal.
- Yet, reduced volume of blood flow in tissues per minute.
- Venous blood contains larger amount of deoxygenated hemoglobin
- Blood stays in capillaries for longer time than normal.
- Due to sluggish blood flow.
- Also due to greater O2 extraction.
- Hence, cyanosis is a prominent feature.
- Arterio-venous O2 difference (A-V O2 difference) is maximum in stagnant hypoxia.
HISTOTOXIC HYPOXIA:
- Due to tissues inability to utilize oxygen delivered by blood.
- Arterial blood PO2 & oxygen content is normal.
- Best example is “Cyanide poisoning”.
- This inhibits cytochrome oxidase.
- Inhibits oxidative metabolism.
- Hence, O2 cannot be utilized by tissues.
- Also, A-V O2 difference markedly reduced/zeroed.
DIFFERENCE BT. HYPOXEMIA vs. HYPOXIA:
- Two similarly appearing terms i.e., Hypoxemia & hypoxia.
HYPOXEMIA:
- Defined as “Low partial pressure of O2 in arterial blood”.
- I.e., Arterial PO2 < 80 mm Hg.
- It does not consider hemoglobin.
- I.e., O2 carrying capacity of hemoglobin or its saturation.
HYPOXIA:
- Defined as “Decreased O2 at tissue level”.
- Caused various factors.
RELATIONSHIP Bt. HYPOXEMIA & HYPOXIA:
- Hypoxemia (low blood partial pressure of O2) can lead to hypoxia.
- Hypoxia caused by hypoxemia referred as “Hypoxemic hypoxia”.
- Nothing but “Hypoxic hypoxia” – I.e., Hypoxia with low PO2.
- Hypoxemia – Most common cause of hypoxia.
CAUSES:
- Hypoxemic hypoxia is caused by,
- Decreased PO2 of inspired air (Decreased FiO2).
- Low barometric pressure (as in high altitude & causes of elevated PCO2).
- Any cause of hypoventilation will cause hypoxemia if PCO2 rises high enough.
- V/Q mismatch.
- Right to left shunt.
- Impairment of diffusion across respiratory membrane.
- Other types of hypoxia (e.g., stagnant, histotoxic & anemic hypoxia) occurs without hypoxemia.
Exam Question
HYPOXIA
HYPOXIC HYPOXIA:
- Most common type of hypoxia.
- Fall in arterial blood PO2 & Low O2 content.
Conditions associated:
- Seen in hypoventilation – E.g., In Restrictive lung diseases, COPD etc.,
- In High Altitude.
- Right to left shunt (e.g., Cyanotic CHD or AV malformation).
ANEMIC HYPOXIA:
- O2 content of blood is lower despite normal PO2.
- Due to low oxygen saturation of hemoglobin.
- As PO2, is normal, respiration is not stimulated by anemic hypoxia.
- Because peripheral chemoreceptors are sensitive only to PO2.
- Seen in “Carbon monoxide poisoning”
- Hypoxia produced by CO poisoning also called “Anemic hypoxia”.
STAGNANT HYPOXIA/HYPOPERFUSION HYPOXIA:
- Due to sluggish blood flow.
- E.g., As in CHF or Circulatory shock.
- Both arterial blood PO2 & oxygen content is normal.
- Blood stays in capillaries for longer time than normal due to sluggish blood flow, because of greater O2 extraction.
- Hence, cyanosis is a prominent feature.
- Arterio-venous O2 difference (A-V O2 difference) is maximum in stagnant hypoxia.
HISTOTOXIC HYPOXIA:
- Best example is “Cyanide poisoning”.
- This inhibits cytochrome oxidase
- Also, A-V O2 difference markedly reduced & it may be zero.
HYPOXEMIA:
- Arterial PO2 < 80 mm Hg.
- It does not consider hemoglobin.
- Most common cause of hypoxia.
CAUSES:
- Decreased PO2 of inspired air (Decreased FiO2).
- Low barometric pressure such as in high altitude & causes of elevated PCO2.
- Any cause of hypoventilation will cause hypoxemia if PCO2 rises high enough.
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