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Hypoxemia (blood oxygen deficiency) is a condition in which the partial pressure of oxygen in the blood drops below 60 mmHg. In which conditions does hypoxemia occur? What changes take place in a hypoxic organism? Can complications be life-threatening?

Hypoxemia( blood oxygen deficiency ) occurs when there is too little oxygen in the blood. One of the most important conditions for maintaining homeostasis, i.e. the internal balance of the body, is maintaining proper arterial blood oxygenation. To ensure them, it is necessary to have adequate oxygen content in the atmospheric air, proper functioning of the respiratory system and efficient transport of oxygen from the alveoli to the blood. Disruption of any of these stages may result in hypoxemia.

Hypoxemia and hypoxia

Hypoxia and hypoxemia are states that are similar, but not the same. Hypoxemia is a narrower term, it means reduced oxygenation of arterial blood.

Hypoxia means hypoxia of tissues or the whole organism. The cause of hypoxia may be hypoxemia - then we are talking abouthypoxic hypoxia . The insufficiently oxygenated blood is then unable to supply the tissues with the oxygen they need. However, it is worth realizing that hypoxia and hypoxemia do not always coexist.

Hypoxia can also develop when the blood oxygen level is normal. This may be due to a reduction in the volume of circulating blood or a malfunctioning circulatory system.

One example of such disorders is ischemic stroke. The blood clot blocks the lumen of the vessel, blood (despite its sufficient oxygenation) does not reach the brain, which causes hypoxia.

Hypoxia does not always have to be a consequence of hypoxemia. The decrease in blood oxygenation triggers mechanisms to prevent tissue hypoxia. A good example is the compensatory increase in heart rate (tachycardia). Despite the fact that there is too little oxygen in the blood, faster heartbeat provides the tissues with enough of it.

The definition of hypoxemia in the world of medical publications can be ambiguous. Most authors believe that thepartial pressure drop is the most important criterionblood oxygen below 60 mmHg .

Some people include in this definition a decrease in the percentage of hemoglobin oxygen saturation, i.e.decrease in saturation, below 90% . Others consider this parameter as an indicator of tissue hypoxia.

Physiology of pulmonary circulation

Before explaining the mechanisms of hypoxemia, it is important to understand where oxygen comes from in the blood and how it is transported.

Pulmonary circulation (the so-called small bloodstream) begins in the right ventricle of the heart. Its task is to pump non-oxygenated blood to the pulmonary trunk, which divides into two pulmonary arteries. These arteries gradually branch into vessels of ever smaller caliber. The tiniest of them are called capillaries (capillaries) and form a dense network that wraps around the alveoli.

The capillary wall together with the adjacent alveolar wall form the so-called alveolar-capillary barrier. It is through this barrier that gas exchange takes place - oxygen penetrates from the lumen of the bubble into the blood in the capillary, while carbon dioxide flows in the opposite direction.

The oxygenated blood is then transported to the pulmonary veins, from where it goes to the left atrium of the heart. It is worth paying attention to the fact that in the pulmonary circulation, deoxygenated blood flows in the arteries, and oxygenated blood - in the veins (unlike in the large bloodstream).

Hypoxaemia: causes

Provision of an adequate amount of oxygen in arterial blood requires the fulfillment of 3 basic conditions:

  • enough oxygen in the air we breathe
  • proper air flow with oxygen through the respiratory tract to the alveoli
  • constant blood flow to the pulmonary vessels and the possibility of oxygen infiltration from the inhaled air

The development of hypoxemia may therefore be a consequence of various situations, such as:

  • decrease in the amount of oxygen in the atmospheric air

The most common fall in the content of the inhaled air is experienced at heights. As the altitude increases, the air density decreases and the partial pressure of oxygen decreases. For this reason, being at heights may cause hypoxemia and the development of altitude sickness.

  • hypoventilation, i.e. reducing the air supply to the lungs

Ineffective breathing or its too low frequency result in insufficient inflow of oxygenated air to the alveoli. Slowing down breathing is sometimes the result of metabolic disorders, taking narcotic drugs, andalso overdosing on certain medications (for example anesthetics or anti-epileptics).

Breathing disorders also occur in diseases that disrupt the work of the respiratory muscles - for example in the group of motor neuron diseases (e.g. amyotrophic lateral sclerosis).

The respiratory center that drives the inspiratory-expiratory activity is located in the medulla elongated in the brainstem. Damage to these structures (for example, from ischemia or trauma) can destroy the "control center" of the breath, resulting in subsequent hypoventilation and hypoxemia.

Too low respiratory rate also occurs in obstructive sleep apnea. It is a medical condition characterized by pauses in breathing while sleeping.

  • disturbance of the ventilation / pulmonary flow ratio

Effective blood oxygenation is possible only in the case of its continuous inflow to the capillaries, surrounding the properly ventilated alveoli.

If a part of the lung is poorly ventilated (for example, due to foreign body aspiration into the respiratory tract or inflammation, as in COVID-19), despite the proper blood flow, it will not be saturated with oxygen.

The opposite disorder is also possible: the alveoli are well ventilated and contain the right amount of oxygen, but the blood for some reason does not reach the capillaries.

A typical example of a pulmonary circulatory disorder is a pulmonary embolism, in which the inflow of deoxygenated blood to the pulmonary vessels is blocked by an inherent thrombus.

  • alveolar-capillary barrier dysfunction

The alveolar-capillary barrier enables gas exchange between the lumen of the alveoli and capillaries. Its thickening may make it difficult for oxygen to enter the blood. An example of a medical condition in which barrier function is impaired is spontaneous pulmonary fibrosis.

  • right-left leak

Physiologically, the right half of the heart contains deoxygenated blood which, after passing through the pulmonary circulation, ends up in the left half as oxygenated blood. There are diseases in which deoxygenated blood enters the left ventricle without the oxygenation stage in the lungs. We call such a situation a leak.

Congenital defects in the heart and / or large vessels are the most common causes of right-to-left shunt. The presence of holes in the septum that separates the heart's halves, or the connections between the pulmonary trunk and the aorta, allows non-oxygenated blood to flow straight into the arteries of the large bloodstream.

Examples of congenital heart defects, whichis accompanied by a left-to-right shunt, there are openings in the interventricular or interatrial septum, and a patent ductus arteriosus (leading in utero blood directly from the pulmonary trunk to the aorta).

Hypoxemia and metabolism

Disruption of the supply of oxygen to cells causes an immediate change in their functioning. They limit their activity and switch to the so-called anaerobic metabolism.

Prolonged hypoxia causes the development of progressive metabolic acidosis, leading to irreversible damage to cells and their death. The consequences of hypoxemia can be dramatic - they include multiple organ failure and the patient's death.

Nerve cells are the most sensitive to hypoxia - they lose their function after 1 minute of hypoxia. The cells of the heart muscle are able to survive in such conditions for about 4 minutes, and of skeletal muscles - for up to 2 hours.

Sudden hypoxemia triggers a series of remedial reactions to minimize its effects. The heart rate increases and the blood pressure increases, and the respiratory rate increases.

Additional respiratory muscles are involved in the work, allowing for taking deeper breaths. In the most important organs for survival (brain, heart), the blood vessels widen in order to supply them with as much blood as possible.

In the lungs, the response to hypoxia is reflex vasoconstriction. If a part of the lung is not properly ventilated, vasoconstriction within it allows blood to move to better ventilated areas.

Chronic hypoxemiacan lead to generalized spasm of the pulmonary vessels. Thus, pulmonary hypertension develops, placing an excessive burden on the right ventricle. The overload and failure of the right half of the heart due to changes in the lungs is called the pulmonary heart ( cor pulmonale ).

Another defense mechanism in chronic hypoxemia is the stimulation of erythropoietin production in the kidneys. Erythropoietin (EPO) is a hormone that stimulates the production of red blood cells in the bone marrow. Increasing their number allows the transport of more oxygen.

Hypoxemia: symptoms

The diagnosis of hypoxemia based on clinical symptoms depends on its severity and possible compensation.

Acute hypoxemia is usually manifested by a feeling of shortness of breath, rapid breathing, and increased effort to breathe in. Often times, the heart rate increases to>100 beats per minute.

Because the mostNerve cells are sensitive to hypoxia, the first symptoms of hypoxia may be associated with neurological disorders.

Sudden confusion, disorientation or speech disturbances always require the exclusion of hypoxemia.

Symptoms of chronic hypoxia in the body may include: secondary hyperaemia (increased number of red blood cells), cyanosis and the so-called stick fingers (thickened at the tips). Prolonged hypoxemia in children can cause slow psychomotor development.

The laboratory test that enables the diagnosis of hypoxemia is the measurement of arterial blood gas. It measures the partial pressure of oxygen in the blood. The valid value range for this parameter is 75-100mmHg.

A result of less than 60 mmHg is evidence of hypoxemia. Such a low partial pressure of oxygen usually also corresponds to a decrease in arterial blood saturation below 90%.

Hypoxaemia: treatment

The treatment of hypoxemia depends primarily on the form we deal with: acute or chronic. The diagnosis of hypoxemia always requires the determination of the patient's stability.

Immediate intervention is required in the event of severe dyspnea, increased heart rate, changes in blood pressure or neurological symptoms (confusion, dementia).

Acute hypoxemia may lead to tissue hypoxia and, consequently, multi-organ failure and death.

Increasing the oxygen content in the blood is achieved through oxygen therapy. On the basis of the test results, the doctor selects the oxygen flow appropriate for the patient, which is administered through a special mask or the so-called oxygen mustache.

There are different types of masks that allow you to administer oxygen in various concentrations; the highest concentration is achieved with a mask with a reservoir bag (up to 90% of oxygen in the breathing mixture).

In the most severe cases, it may be necessary to use respiratory support devices by creating positive airway pressure during inhalation. This is calledmechanical ventilation .

In some patients, it is possible to use non-invasive ventilation, in which breathing is supported by means of a mask connected to a ventilator. For the most severely ill,invasive ventilation .

Patient under general anesthesia is intubated, his own breathing is "turned off" and ventilation is taken overrespirator .

All the above-described methods constitute symptomatic treatment. Giving oxygen can help stabilize the patient's condition, but finding is always the keycauses of hypoxia. Oxygen therapy also requires constant monitoring of the patient's condition (regular measurements of saturation, e.g. using a pulse oximeter, gasometry).

In diseases that lead to chronic hypoxemia (most often lung diseases such as COPD, pulmonary fibrosis, severe asthma), long-term oxygen treatment may be necessary.

Oxygen concentrators are currently popular in Poland, allowing for oxygen therapy at home. The patient should breathe through an oxygen mustache / mask connected to a concentrator for at least 15-17 hours a day.

Long-term oxygen therapy prolongs survival and improves the quality of life of patients.

Physical training in hypoxic conditions

The natural response of the body to the reduced oxygen content in the air has been studied for many years in terms of its possible use in training athletes. The advantages of training under hypoxic conditions include an increase in the number of red blood cells and the amount of hemoglobin, and thus - increasing the possibility of oxygen transport through the blood.

Beneficial changes also occur in the metabolism of muscle cells and their reactivity to nerve stimuli.

There have been many different ideas about how to conduct such training, as well as the appropriate level of hypoxia.

Currently, training in high mountain conditions can be replaced with training in hypoxic chambers, simulating the lowering of oxygen in the air at altitudes.

Planning of hypoxic training requires awareness of the risk of side effects (e.g. decrease in physical performance), continuous monitoring of the athlete's he alth, as well as taking into account his individual sensitivity to this type of training.

About the authorKrzysztof BialaziteA medical student at Collegium Medicum in Krakow, slowly entering the world of constant challenges of the doctor's work. She is particularly interested in gynecology and obstetrics, paediatrics and lifestyle medicine. A lover of foreign languages, travel and mountain hiking.

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