Hypoxemia

Introduction Hypoxemia refers to insufficient oxygen in the blood. The arterial partial pressure of oxygen (pao2) is lower than the lower limit of normal age, and the main manifestation is the decrease of blood oxygen partial pressure and blood oxygen saturation. Adult normal arterial oxygen partial pressure (PaO2): 83-108 mmHg. A variety of causes, such as central nervous system disorders, bronchial, pulmonary lesions, etc. caused by ventilation and / or ventilation dysfunction can lead to hypoxia. Due to the degree of hypoxemia, the speed and duration of hypoxia, the impact on the body is also different. Hypoxemia is one of the most common critical illnesses in respiratory diseases and one of the important clinical manifestations of respiratory failure.

Cause

Common causes of hypoxemia are:

1. The partial pressure of oxygen inhalation is too low.

2. Insufficient alveolar ventilation.

3. Dispersion dysfunction.

4. Alveolar ventilation / blood flow imbalance.

5. Right to left shunt

(1) Inspiratory gas partial pressure of oxygen is too low: hypoxia caused by inhalation of low-oxygen partial pressure gas, also known as atmospheric hypoxia.

(2) External respiratory dysfunction: caused by pulmonary ventilation or ventilation dysfunction, called respiratory hypoxia. Common in various respiratory diseases, respiratory center suppression or respiratory muscle paralysis.

(3) Venous blood flow into the artery: more common in congenital heart disease.

an examination

Related inspection

Blood routine urine routine

1. Due to the low oxygen pressure in the arterial blood, PaO2 is lowered, and too low PaO2 can directly lead to a decrease in CaO2 and SaO2.

2. If Hb has no abnormal changes in mass and quantity, CO2max is normal.

3. As PaO2 decreases, 2,3-DPG increases in red blood cells, so blood SaO2 decreases.

4. In hypokale hypoxia, a decrease in PaO2 and blood SaO2 leads to a decrease in CaO2.

5. The arteriovenous oxygen difference is reduced or not changed much. Usually about 5 ml of oxygen is used when 100 ml of blood flows through the tissue, ie A-VdO2 is about 2.23 mmol/L (5 ml/dl). The kinetics of oxygen diffusion from blood to tissue is the difference of oxygen partial pressure between the two. When hypotonic hypoxia, PaO2 is significantly reduced and CaO2 is significantly reduced, the diffusion rate of oxygen is slowed down, and the same amount of blood is diffused to the tissue. The reduction in oxygen eventually leads to a decrease in A-VdO2 and tissue hypoxia. In the case of chronic hypoxia, the A-VdO2 changes may not be evident when the tissue's ability to utilize oxygen is increased.

6. Changes in skin and mucous color

The average concentration of deoxygenated Hb in normal capillaries was 26 g / L (2.6 g / dl). In hypokale hypoxia, the oxygenated Hb concentration of arterial blood and venous blood decreased, the oxygenated Hb in the capillary decreased, and the deoxygenated Hb concentration increased. When the average concentration of deoxygenated Hb in the capillaries increases to 50 g / L (5 g / dl) or more (SaO2 80% ~ 85%), the skin and mucous membranes appear blue-purple, called cyanosis. Purpura is prone to occur in chronic hypotonic hypoxia. Purpura is a manifestation of hypoxia, but patients with hypoxia do not necessarily have purpura. For example, bloody hypoxia caused by anemia may be free of purpura. Similarly, patients with purpura can also be without hypoxia, such as patients with polycythemia vera, because of the abnormal increase in Hb, so that the deoxygenated Hb content in capillaries easily exceeds 50g / L, it is prone to purpura without hypoxia symptoms.

Differential diagnosis

1, hypoxemia, hypoxia and insufficient oxygen supply, these three concepts are mixed up. In scientific research, the concept is unclear and the most deadly. In clinical work, distinguishing these concepts is the basis for our efforts to ensure the safety of patients during surgery.

Hypoxemia refers to the partial pressure of oxygen in the circulatory system is lower than normal, defined as PaO2 is less than 60mmHg, PaO2 is the only indicator to determine the presence or absence of hypoxemia.

The diagnosis of hypoxia is more difficult. In the absence of oxygen, mitochondria cannot undergo aerobic oxidation and undergo anaerobic glycolysis. Since anaerobic glycolysis produces a large amount of lactic acid, the content of lactic acid in the blood is an important indicator for determining whether or not hypoxia is present, and the blood lactic acid concentration exceeding 1.5 mmol/L is hypoxia. However, not all hypoxia has an increase in the concentration of lactic acid in the blood, such as hypoxia in the early stage of hypovolemic shock, but due to vasoconstriction, lactic acid produced in the tissue accumulates in the local tissue without entering the circulatory system, blood The lactic acid in the lactic acid may not rise; likewise, the lactic acid content in the blood does not increase in hypoxia. For example, infusion of a large amount of lactic acid-containing liquid in a patient with cirrhosis may result in an increase in lactic acid content in the blood. Therefore, the determination of the presence or absence of hypoxia should first be clear whether there is a cause of hypoxia. Determination of lactic acid content in the blood is only an auxiliary diagnostic measure. Hypoxia is classified into hypotonic anoxia, anemic anoxia, circulating anoxia, and histogenous anoxia.

2, oxygen supply (Oxygen delivery, DO2) Also known as global oxygen delivery (global oxygen delivery), refers to the total amount of oxygen delivered to the whole body tissue by the circulatory system per unit time. Oxygen supply (DO2) is calculated by the following formula: DO2 = CO × CaO2 where CO is the cardiac output and CaO2 is the arterial blood oxygen content. It can be seen from the above formula that insufficient cardiac output and arterial oxygen content can result in insufficient oxygen supply. Cardiac output is affected by stroke volume and heart rate. Oxygen content is affected by hemoglobin content and oxygen saturation.

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