Causes of hypoxemia

Many people may not be particularly familiar with hypoxemia and do not know what it is. In fact, hypoxemia is simply the lack of oxygen in the human blood. This situation is mostly caused by bronchial or central nervous system diseases. Once hypoxemia is diagnosed, it needs to be actively treated. Don't be careless. Timely treatment can control the disease as soon as possible.

Causes of Hypoxemia

Hypoxemia is mainly caused by lung diseases. If the patient has obstructive ventilation, the vital capacity, forced vital capacity and maximum ventilation will all decrease, while the residual volume and its percentage of the total lung volume will increase, and the diffusion function will be impaired. In severe cases, hypoxemia and/or carbon dioxide retention may occur. The most common causes of hypoxia and hypertension can be summarized into four aspects: alveolar hypoventilation, diffusion dysfunction, ventilation-blood perfusion disorder and increased intrapulmonary shunt volume.

1. Alveolar ventilation deficiency: As mentioned before, alveolar ventilation is the effective gas volume for gas exchange. It is the difference between minute ventilation and dead space ventilation. Due to the regulatory function of the respiratory center, the CO2 production per minute generally does not change much, unless the patient's ventilation is completely controlled by the mechanical ventilator, and due to certain factors such as intravenous nutrition, the respiratory exchange index R is less than 0.8, generally, the alveolar ventilation determines the PaCO2.

2. Diffusion dysfunction: When the pulmonary capillary membrane is damaged or destroyed due to various pathological changes, or the capillary bed area is reduced, or the permeable membrane (including alveolar epithelial tissue and its basement membrane, vascular endothelial cells and their basement membrane, and red blood cell membrane) is thickened, the diffusion function will be impaired. However, the compensatory reserve of human diffusion function is extremely large. Often, when the decline in diffusion function has not yet caused hypoxemia, the lesions in the lungs have already caused significant gas exchange disorders due to other reasons. In patients with diffuse interstitial lung disease, hypoxemia usually does not occur at rest. However, after exercise, the time for blood flow to pass through the alveolar capillary membrane is shortened, making the diffusion dysfunction more significant. Combined with other reasons, hypoxemia may occur. This is one of the reasons why exercise stress test is used in respiratory function measurement.

3. Ventilation-perfusion disorder: It is usually expressed as ventilation disorder, which is the most common cause of hypoxemia. Almost all respiratory diseases can be detected with ventilation disorder components when hypoxemia occurs.

4. Increased intrapulmonary shunt volume: Under pathological conditions, such as when the alveolar cavity is filled with inflammatory exudate or edema fluid or the alveoli are completely collapsed due to atelectasis, the inhaled air cannot enter the alveoli in the diseased area at all. Although blood still flows through this area, gas exchange cannot be carried out. The venous blood containing reduced hemoglobin directly returns to the left heart, as if there is a right-to-left shunt. The sum of the shunt caused by pathological reasons and the anatomical shunt is called intrapulmonary shunt (QS). When intrapulmonary shunt accounts for too large a proportion of cardiac output, hypoxemia will result.

What are the symptoms of hypoxemia?

Characteristics of hypoxemia: The patient's oxygenation status can be monitored by pulse oximetry and arterial blood gas analysis. Both detection methods have their own advantages and disadvantages. Normal blood oxygen partial pressure is above 85-90 mmHg, and normal blood oxygen saturation is >95%. Symptoms of cyanosis and dyspnea lack specificity and are therefore not reliable signs of hypoxemia.

1. The PaO2 decreases due to the low oxygen pressure diffused into the arterial blood. Too low PaO2 can directly lead to a decrease in CaO2 and SaO2.

2. If there is no abnormal change in the quality and quantity of Hb, CO2max is normal.

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

4. In hypotonic hypoxia, PaO2 and blood SaO2 decrease, causing CaO2 to decrease.

5. The arteriovenous oxygen difference decreases or changes little. Normally, when 100 ml of blood flows through tissues, about 5 ml of oxygen is utilized, that is, A-VdO2 is about 2.23 mmol/L (5 ml/dl). The driving force for oxygen diffusion from blood to tissues is the oxygen partial pressure difference between the two. When hypotonic hypoxia occurs, PaO2 and CaO2 are significantly reduced, which slows down the diffusion rate of oxygen. The amount of oxygen diffused to tissues by the same amount of blood is reduced, ultimately leading to a decrease in A-VdO2 and tissue hypoxia. In case of chronic hypoxia, when the tissue's ability to utilize oxygen increases compensatorily, the change in A-VdO2 may not be obvious.

6. Changes in skin and mucous membrane color: The average concentration of deoxygenated Hb in normal capillaries is 26g/L (2.6g/dl). In hypotonic hypoxia, the concentration of oxygenated Hb in both arterial and venous blood decreases, the oxygenated Hb in the capillaries will inevitably decrease, and the concentration of deoxygenated Hb will increase. When the average concentration of deoxygenated Hb in the capillaries increases to above 50g/L (5g/dl) (SaO2≤80%～85%), the skin and mucous membranes may appear bluish purple, which is called cyanosis. Cyanosis is likely to occur in chronic hypoxia. Cyanosis is a manifestation of hypoxia, but not all patients with hypoxia will have cyanosis. Similarly, patients with cyanosis may not have hypoxia. For example, patients with polycythemia vera have an abnormal increase in Hb, which makes the content of deoxygenated Hb in the capillaries easily exceed 50g/L, so cyanosis is easy to occur without symptoms of hypoxia.