Hypoxia

Hypoxia


Hypoxia is defined as an inadequate Oxygen supply to the body tissues.

Normal values : PO2 –  85 -100mm Hg Moderated hypoxia :  PO2 –  < 60mm Hg Serius hypoxia :  PO2 –  < 40mm Hg

Causes of Hypoxia
Hypoxia can be caused by:


 * Inadequate oxygenation of the blood in the lungs: deficiency of O2 in the atmosphere, hypoventilation (neuromuscular disorder);


 * Pulmonary disease: abnormal alveolar ventilation-perfusion ratio;


 * Venous-to-arterial shunts (right-to-left);


 * Inadequate O2 transport to tissues by the blood: anemia / abnormal hemoglobin, generalized/Localized circulatory deficiency;


 * Inadequate tissue ability to use the O2.

Types of Hypoxia
There are four different types of hypoxia:

Hypoxic Hypoxia: In which the partial pressure of Oxygen in arterial blood is reduced. Its causes can be a decrease of atmospheric O2, at high altitudes, but also some Pathophysiological Mechanisms, as for example: Hypoventilation, Diffusion Limitation, Physiological Shunts, Anatomical Shunts.

Anemic Hypoxia: In which the partial pressure of Oxygen in blood is normal, but there is an insufficient or non-functional amount of heamoglobin. Can be caused by: a deficiency of essential nutrients (iron, B12 vitamin), blood loss, hemolytic anemia (cell cycle disease).

Stagnant ( Ischemic) Hypoxia: Is the insufficient O2 that reaches the tissues due to reduced blood flow. Its causes can be: Systemic (entire body) - heart failure; Local area - obstructed artery (arteriosclerosis).

Histotoxic Hypoxia: Is known by the disability of the cell to use O2. Its causes are: inactivation of certain metabolic enzymes, chemical poisons like alcohol and cyanide poisoning by inhibition of oxidative enzymes.

Conclusion
In Hypoxic Hypoxia there is a reduced amount of O2 in the inspired air, so there will be a reduced PO2 in Arterial and Venous blood. On the other hand, in Anemic Hypoxia the amount of O2 in Arterial blood is normal but there is a reduced amount of heamoglobin available to carry O2, so the Arterial PO2 is normal but the Venous PO2 is dereased. In Stagnant Hypoxia there is a normal PO2 and heamoglobin concentration but blood flow to the tissues is so low that adequate O2 is not delivered. In this type of hypoxia the Arterial PO2 is normal but the Venous PO2 is decreased. In Histotoxic Hypoxia there is an adequate delivery of O2 to the tissues but due to the action of a toxic agent, cells can not make use of O2. Due to this fact the Arterial PO2 is normal but the Venous PO2 increases.