Hypoxemia

Refers to an abnormally low level of oxygen in the blood.

More specifically, it is oxygen deficiency in arterial blood.

Has many causes, often respiratory disorders, and can cause tissue hypoxia as the blood is not supplying enough oxygen to the body.

There are 5 mechanisms of hypoxemia: hypoventilation, often occurs in the context of CNS depression or neuromuscular weakness, decreased partial pressure of inspired oxygen, which can result from low ambient oxygen tension at high altitude, and ventilation-perfusion mismatch, in which pulmonary arterial blood flow is directed to poorly ventilated lung segments, and diffusion impairment, with delayed transfer of oxygen from the alveolus to the capillary.

Hypoxemia refers to low oxygen in the blood, and the more general term hypoxia is an abnormally low oxygen content in any tissue or organ, or the body as a whole.

Hypoxemia can cause hypoxia.

Hypoxia can also occur via other mechanisms, such as anemia.

Usually defined in terms of reduced partial pressure of oxygen (mm Hg) in arterial blood.

Also can be expressed in terms of reduced content of oxygen (ml oxygen per dl blood) or percentage saturation of hemoglobin with oxygen, which is either found singly or in combination.

It is caused by five categories of etiologies: hypoventilation, ventilation/perfusion mismatch, right-to-left shunt, diffusion impairment, and low PO2.

Low PO2 and hypoventilation are associated with a normal A-a gradient whereas the other categories are associated with an increased A-a gradient.

The oxygen content of blood is sometimes viewed as a measure of tissue delivery rather than hypoxemia.

Can cause symptoms such as those in respiratory distress-breathlessness, an increased rate of breathing, use of the chest and abdominal muscles to breathe, and lip pursing.

Chronic hypoxemia may be compensated or uncompensated.

In a compensated state, blood vessels supplying less-ventilated areas of the lung may selectively contract, shunting the blood to areas of the lungs which are better ventilated.

If the lungs are not well ventilated chronically, shunting can result in pulmonary hypertension, overloading the right ventricle of the heart and causing cor pulmonale and right sided heart failure.

Polycythemia can also occur with chronic hypoxemia in children, manifesting as delayed growth, neurological development and motor development and decreased sleep quality.

Other symptoms of hypoxemia include cyanosis, and digital clubbing.

Serious hypoxemia occurs (1) when the partial pressure of oxygen in blood is less than 60 mm Hg, or (2) when hemoglobin oxygen saturation is less than 90%.

Severe hypoxia can lead to respiratory failure.

Hypoxemia can result from any cause that impairs the rate or volume of air entering the lungs, or any cause that influences the transfer of air from the lungs to the blood, as well as respiratory or cardiovascular causes such as shunts.

Hypoxemia can result when the proportion of oxygen in the air is low, or when the partial pressure of oxygen has decreased, less oxygen is present in the alveoli of the lungs.

In alveolar hypoventillation not enough oxygen is delivered to the alveoli for the body’s use, and may occur with impaired brainstem control of ventilation the inability to breathe effectively.

Centers in the medulla manage the rate of breathing and the depth of each breath, and is influenced by the blood level of carbon dioxide.

Carbon dioxide levels are determined by central and peripheral chemoreceptors located in the central nervous system and carotid and aortic bodies, respectively.

If the breathing center is dysfunctional hypoxia occurs.

The medullary respiratory centers can be damaged by strokes, epilepsy and cervical neck fractures.

The medullary respiratory centers generate rhythmic impulses which are transmitted along the phrenic nerve to the diaphragm, the muscle that is responsible for breathing.

Metabolic alkalosis can decrease respiratory drive when there is decreased carbon dioxide in the blood

Central sleep apnea can occur during sleep when the breathing centers of the brain pause their activity, leading to prolonged periods of apnea with potentially serious consequences.

Hyperventilation reduces the amount of carbon dioxide in the lungs, and thus the urge to breathe and can result in hypoxemia.

Suffocation, and temporary int2242uption or cessation of breathing as in obstructive sleep apnea may interfere with breathing in infants, a putative cause of SIDS.

Structural deformities of the chest, such as scoliosis and kyphosis, which can restrict breathing and lead to hypoxia.

Muscle weakness of the diaphragm, the primary muscle for drawing new air into lungs, may lead to hypoxemia.

Diaphragmatic weakness may be a result of a congenital disease, such as motor neuron disease, or an acquired condition, such as fatigue in severe cases of COPD.

At altitude the external partial pressure of oxygen decreases, resulting in decreased carriage of oxygen by hemoglobin.

This is seen as a cause of cerebral hypoxia and mountain sickness.

The partial pressure of oxygen at sea level is 150 mmHg, whereas at the peak of Mount Everest, the partial pressure of oxygen is just 43 mmHg

Hypoxia in diving can result from sudden surfacing, as the partial pressure of oxygen is sufficient to maintain good carriage by hemoglobin at depth, but may be insufficient in shallow water.

Hypoxemia can occur with ventilation-perfusion mismatch.

Hypoxemia may develop during intense exercise as a result of preexisting lung diseases.

During exercise, almost half of the hypoxemia is due to diffusion limitations.

An increasingly poor match between ventilation and perfusion is seen with age, as well as a decreased ability to compensate for hypoxic states.

Disease that affect the pulmonary interstium, such as pulmonary fibrosis, can also result in hypoxia, by affecting the ability of oxygen to diffuse into arteries.

Acute or chronic respiratory distress can result in hypoxia.

Cirrhosis can be complicated by refractory hypoxemia due to ventilation-perfusion mismatch.

Such shunting, occurs via the bronchial circulation, which provides blood to the tissues of the lung, and by the smallest cardiac veins, which empty directly into the left ventricle.

Shunting occurs physiologically, due to the effect of gravity.

The highest concentration of blood in the pulmonary circulation occurs in the bases of the pulmonary tree compared to the highest pressure of gas in the apexes of the lungs.

Alveoli may not be ventilated in shallow breathing.

Shunting may also occur with:

Acute lung injury and adult respiratory distress syndrome.

Pathological shunts exist, as well, such as patent ductus arteriosus, patent foramen ovale, and atrial septal defects or ventricular septal defects.

In these shunts the blood from the right side of the heart moves straight to the left side, without first passing through the lungs.

This manifestation is known as a right-to-left shunt, which is often congenital in origin.

Exercise-induced arterial hypoxemia can occur during exercise when a trained individual exhibits an arterial oxygen saturation below 93%.

Dormant capillaries are normally located within deadspace area thus the blood passing through does no become oxygenated resulting in a hypoxemia.

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