Hypoxia

 

Definition

Hypoxia is Oxygen deficiency, which means inadequate Oxygen supply at the tissue level

 

Respiration requires

-        Ventilation

-        Perfusion – blood supply to the lungs

-        Ventilation / perfusion match to the lungs

-        Gas exchange in lungs (diffusion)

-        Gas transport

-        Gas exchange between cells and interstitial fluid

-        Regulation

Oxygen Cascade

Oxygen - Haemoglobin dissociation curve

Oxygen delivery to tissues

 

-        Amount of Oxygen presented to body cells per minute

-        Oxygen flux is governed by cardiac output * arterial Oxygen content

Major type of Hypoxia

1.    Hypoxic Hypoxia / Hypoxemia

2.    Anaemic Hypoxia

3.    Stagnant Hypoxia/ ischemic Hypoxia

4.    Histotoxic Hypoxia

 

1.    Hypoxic Hypoxia /Hypoxemia

 

·       Hypoxia due to inadequate oxygenation of arterial blood

·       Arterial Oxygen tension is always reduced – Partial pressure of Oxygen is below the normal range

·       <80mmHg in an adult breathing room air at sea level

Common causes

·       Breathing air that leads to low alveolar Oxygen

·       Problems with ventilation (airway resistance, compliance)

-        Air way disorders

Upper - Obstructive sleep apnea

Lower – Asthma

-        Disorders with compliance

Lung – pulmonary fibrosis

Chest wall – pneumothorax, obesity

·       Problems with gas exchange – alveolocapillary membrane (fibrosis, pulmonary oedema)

·       V/Q mismatches (ventilation – perfusion mismatches) – causes, lung collapse

·       Anatomical shunts

-        Venous to arterial shunts ( R to L shunts)

-        Partial pressure of Oxygen reduced

 

·       Breathing air that leads to low alveolar Oxygen

Example – high altitudes – decreased total barometric pressure (while percentage of Oxygen is the same)

 

2.    Anaemic Hypoxia

 

·       Insufficient amount of Hemoglobin to carry Oxygen

·       Reduced carrying Oxygen capacity and consequently reduced content of Oxygen in arterial blood

·       Partial pressure of Oxygen in arterial blood is normal (no hypoxemia) ( because the lungs are normal)

·       Partial pressure of Carbon dioxide in arteries are normal, but partial pressure of Oxygen in veins is reduced (because Oxygen reaching tissue is reduced and therefore more Oxygen is extract from blood to the tissue)

·       Effect of Oxygen therapy, increase only dissolved Oxygen

·       Treatment is correction of Hemoglobin

Carbon monoxide Poisoning

·       In large amount of poisonous

·       Formed by incomplete combustion of Carbon

·       Reacts with hemoglobin and form carbonmonoxy haemoglobin

 

Symptoms

·       Cherry red colour to skin, nail beds, tongue, lips

·       Death if 70-80% of circulating haemoglobin is converted to COHB

Treatment

·       Remove from source

·       Give Oxygen – Hyperbaric Oxygen – 100% Oxygen at high pressure

 

1.    Stagnant Hypoxia

Due to inadequate blood flow

Causes,

·       Peripheral circulatory failure – EG congestive cardiac failure

 

·       Partial pressure of Oxygen is normal or low (left heart failure giving rise to pulmonary oedema)

 

·       Carbon dioxide in arterial blood is normal

 

·       Venous Oxygen tension is markedly reduced

 

2.    Histotoxic Hypoxia

 

·       Inactivation of cytochromic and other enzymes due to poisoning

Example – Cyanide

·       Tissues unable to use Oxygen

·       Partial pressure of Oxygen and Carbon dioxide in arterial blood is normal

·       Oxygen content is normal

·       No stagnation

·       Venous Oxygen tension is increased, because Oxygen is not taken by tissues

Treatment (Cyanide poisoning)

·       Methylene blue or nitrites

·       Act by forming methaemoglobin

·       Which reacts with cyanide to form cyanmethaemoglobin (non toxic)

·       Hyperbaric Oxygen

 

 Effects / symptoms of Hypoxia

Depend on rapidity and severity

1.    Fulminant Hypoxia

·       After exposure of partial pressure of Oxygen < 20mmHg

·       Within 10 – 20 seconds. Unconscious

·       In 4 – 5 mins brain death

2.    Acute hypoxia – high altitude

·       Exposure of partial pressure of Oxygen of 20 – 40 mmHg

·       Headache, nausea, vomiting, incoordination, slow reflexes, over confidence, unconsciousness

·       In mins to hours coma and death

3.    Chronic Hypoxia

·       Exposure of partial pressure of Oxygen 40 – 60mmHg over extended periods of time

·       Severe fatigue, dyspnoea, stroke breathing