HYPOXIA

                                                                                                     HYPOXIA

Hypoxia is defined as reduced availability of oxygen to the tissues. The term anoxia refers to absence of oxygen. In olden days, the term anoxia was in use. Since there is no possibility for total absence of oxygen in living conditions, use of this term is abandoned.

CLASSIFICATION AND CAUSES

OF HYPOXIA

Four important factors which leads to hypoxia are:

1. Oxygen tension in arterial blood

2. Oxygen carrying capacity of blood

3. Velocity of blood flow

4. Utilization of oxygen by the cells.

                                FIGURE 127.2: Effects of hypoventilation

On the basis of above factors, hypoxia is classified into four types:

1. Hypoxic hypoxia

2. Anemic hypoxia

3. Stagnant hypoxia

4. Histotoxic hypoxia.

Each type of hypoxia may be acute or chronic.Simultaneously, two or more types of hypoxia may be

present.

1. Hypoxic Hypoxia

Hypoxic hypoxia means decreased oxygen content in

blood. It is also called arterial hypoxia.

Causes for hypoxic hypoxia

Hypoxic hypoxia is caused by four factors.

i. Low oxygen tension in inspired (atmospheric) air,

which does not provide enough oxygen

ii. Respiratory disorders associated with decreased pulmonary ventilation, which does not allow intake

of enough oxygen

iii.Respiratory disorders associated with inadequate oxygenation in lungs, which does not allow diffusion

of enough oxygen

iv. Cardiac disorders, in which enough blood is not pumped to transport oxygen.

i. Low oxygen tension in inspired air

Oxygen tension in inspired air is reduced in the following conditions:

a. High altitude

b. While breathing air in closed space

c. While breathing gas mixture containing low partial pressure of oxygen (PO2).

Because of these conditions, required quantity of oxygen cannot enter the lungs.

ii. Respiratory disorders associated with decreased pulmonary ventilation Pulmonary ventilation decreases in the following conditions:

a. Obstruction of respiratory passage as in asthma

b. Nervous and mechanical hindrance to respiratory movements as in poliomyelitis

c. Depression of respiratory centers as in brain tumors

d. Pneumothorax.

In these conditions, even though enough oxygen is

available in the atmosphere, it cannot reach the lungs.

iii. Respiratory disorders associated with inadequate oxygenation of blood in lungs

Inadequate oxygenation of blood in lungs occurs in the following conditions:

a. Impaired alveolar diffusion as in emphysema

b. Presence of non-functioning alveoli as in fibrosis

c. Filling of alveoli with fluid as in pulmonary edema,pneumonia, pulmonary hemorrhage

d. Collapse of lungs as in bronchiolar obstruction

e. Lack of surfactant

f. Abnormal pleural cavity such as pneumothorax, hydrothorax, hemothorax and pyothorax

g. Increased venous admixture as in the case of bronchiectasis. In these conditions, in spite of oxygen availability and entrance of oxygen into the alveoli, it cannot diffuse into the blood.

iv. Cardiac disorders

In congestive heart failure, oxygen availability and diffusion are normal, but the blood cannot be pumped

from heart properly.

Characteristic features of hypoxic hypoxia

Hypoxic hypoxia is characterized by reduced oxygen tension in arterial blood. All other features remain normal (Table 127.1).

2. Anemic Hypoxia

Anemic hypoxia is the condition characterized by the inability of blood to carry enough amount of oxygen.

Oxygen availability is normal. But the blood is not able to take up sufficient amount of oxygen due to anemic condition.

Causes for anemic hypoxia

Any condition that causes anemia can cause anemic hypoxia. It is caused by the following conditions:

i. Decreased number of RBCs

ii. Decreased hemoglobin content in the blood

iii. Formation of altered hemoglobin

iv. Combination of hemoglobin with gases other than oxygen and carbon dioxide.

i. Decreased number of RBCs

RBC decreases in conditions like bone marrow diseases, hemorrhage, etc.

ii. Decreased hemoglobin content in the blood Conditions which decrease the RBC count or chang the structure, shape and size of RBC (microcytes, macrocytes, spherocytes, sickle cells, poikilocytes, etc.) can decrease the hemoglobin content in blood.

iii. Formation of altered hemoglobin Poisoning with chlorates, nitrates, ferricyanides,

etc. causes oxidation of iron into ferric form and the hemoglobin is known as methemoglobin. Methemoglobin cannot combine with oxygen. Thus, the quantity of hemoglobin available for oxygen transport is decreased.

iv. Combination of hemoglobin with gases other than oxygen and carbon dioxide When hemoglobin combines with carbon monoxide, hydrogen sulfide or nitrous oxide, it looses the capacity to transport oxygen.

Characteristic features of anemic hypoxia

Anemic hypoxia is characterized by decreased oxygen carrying capacity of blood. All other features remain

normal (Table 127.1).

3. Stagnant Hypoxia

Stagnant hypoxia is the hypoxia caused by decreased velocity of blood flow. It is otherwise called hypokinetic hypoxia.

Causes for stagnant hypoxia

Stagnant hypoxia occurs mainly due to reduction in velocity of blood flow. Velocity of blood flow decreases in the following conditions:

i. Congestive cardiac failure

ii. Hemorrhage

iii. Surgical shock

iv. Vasospasm

v. Thrombosis

vi. Embolism.

Characteristic features of stagnant hypoxia Stagnant hypoxia is characterized by decreased velocity of blood flow. All other features remain normal (Table 127.1).

4. Histotoxic Hypoxia

Histotoxic hypoxia is the type of hypoxia produced by the inability of tissues to utilize oxygen.

Causes for histotoxic hypoxia

Histotoxic hypoxia occurs due to cyanide or sulfide poisoning. These poisonous substances destroy the

TABLE 1: Characteristic features of different types of hypoxia

Features

Hypoxic hypoxia

Anemic hypoxia

Stagnant hypoxia

Histotoxic hypoxia

1.PO2 in arterial blood Reduced	Reduced	Normal	Normal	Normal
2. Oxygen carrying capacity of blood	Normal	Reduced	Normal	Normal
3. Velocity of blood flow	Normal	Normal	Reduced	Normal
4.Utilization of oxygen by tissues	Normal	Normal	Normal	Reduced
5. Efficacy of oxygen therapy	100%	75%	<50%	Not useful

cellular oxidative enzymes and there is a complete paralysis of cytochrome oxidase system. So, even if oxygen is supplied, the tissues are not in a position to utilize it.

Characteristic features of histotoxic hypoxia

Histotoxic hypoxia is characterized by inability of tissues to utilize oxygen even if it is delivered. All other

features remain normal. (Table 1)