Hypoxia

Physiology of hypoxia

Definition

▪ Reduced oxygen supply to the tissues

Classification

▪ Reduced oxygen tension of arterial blood – Hypoxic hypoxia

▪ Decreased oxygen carrying capacity of the blood – anemic hypoxia

▪ Decreased rate of blood flow to the tissues – stagnant hypoxia

▪ Decreased utilization of oxygen by tissues – histotoxic hypoxia

But first

▪ Arterial Po2

▪ Oxygen carrying capacity of blood (Hb value )

▪ A –V difference of Po2

▪ Percentage saturation of hemoglobin SaO2

Hypoxic hypoxia

▪ Decreased oxygen tension in blood

▪ Decreased O2 saturation of blood

▪ O2 carrying capacity of blood is normal

▪ Blood flow to the tissues normal

▪ Tissue utilization is normal

▪ A –V Po2 difference is low

Causes

▪ Low oxygen tension of inspire air – High altitude

– Closed spaces

– Artificial gas breathing

▪ Hypoventilation – Obstruction to respiratory passages

– Decreased lung compliance

– Paralysis of respiratory muscles

– Respiratory depression

▪ Reduced diffusion of O2 across respiratory membrane – Emphysema , pulmonary edema , fibrosis

Anemic hypoxia

▪ Basic problem is decreased oxygen carrying capacity of blood

▪ Causes – Anemia – bone marrow depression

– CO poisoning

– Met-haemoglobinemia

Characteristic features

▪ Arterial Po2 is normal

▪ O2 carrying capacity of blood is reduced ( SaO2 reduced)

▪ A –V difference is normal

▪ Normal tissue utilization

2,3 DBG significance

▪ Increases O2 delivery to tissues

▪ Hbo2 + 2, 3 DBG Hb 2, 3 DBG + O2

Stagnant hypoxia

▪ Problem – decreased blood flow to tissues (stagnation)

▪ Normal Po2 , SaO2 , Hb , normal tissues

▪ Causes – generalized , localized – Shock , CHF

– Thrombosis , embolism, atherosclerosis

Characteristic Features

▪ Normal pO2

▪ Normal SaO2

▪ A –V pO2 is high than normal

Histotoxic Hypoxia

▪ Problem with tissues

▪ Tissues not able to use the oxygen

▪ Inhibition of cellular respiration (ETC) by poisons

▪ Cyanide , sulphide poisoning

▪ Normal pO2

▪ SaO2 normal

▪ A –V difference is nil

Effects of hypoxia

▪ Hypoxia mainly affects CNS , specially the higher centers

▪ Fulminant hypoxia , acute hypoxia , chronic hypoxia

▪ Effects depends on the severity , duration and type , and compensatory mech of body

Important Symptoms And Signs

▪ Cyanosis

▪ Tachycardia

▪ Tachypnoea

▪ Acute hypoxia resembles alcohol intoxication

▪ Anaerobic glycolysis

▪ Metabolic acidosis with respiratory alkalosis

▪ Angiogenisis

▪ Polycythemia

▪ Pulmonary hypertension