What Is Anemic Hypoxia?

Hemichypoxia ^[1] refers to changes in the amount or quality of hemoglobin (Hb), leading to a decrease in CaO2 or Hb-associated oxygen that is not easily released by the tissue. CaO2 is mostly reduced and PaO2 is normal during bloody hypoxia, so it is also called isotonic hypoxemia.

Blood hypoxia

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Blood hypoxia (hemichypoxia) ^[1]

1. Hypoxia caused by a decrease in the amount of Hb in anemia, also known as anemiahypoxia.

2. CO poisoning Hb combines with CO to form carbon oxygen Hb (carboxyhemoglobin, HbCO), thereby losing oxygen transport function. Although the binding rate of CO and Hb is only 1/10 of the binding rate of O2 and Hb, the dissociation rate of HbCO is 1/2100 of the dissociation rate of HbO2. Therefore, the affinity of CO and Hb is 210 times greater than that of O2. When there is 0.1% CO in the inhaled air, 50% of Hb in the blood may be HbCO; CO can also inhibit glycolysis in red blood cells, reduce its 2,3-DPG production, shift the oxygen ionization curve to the left, and HbO2 Oxygen is not easy to release; one Hb molecule can bind to CO and O2 at the same time, and the O2 carried by this Hb is also difficult to release. Therefore, CO poisoning not only prevents the combination of Hb and O2, but also prevents the dissociation of oxygen, resulting in severe hypoxia in the tissue. Under normal circumstances, when red blood cells disintegrate and the pyrrole ring of heme is broken, a small amount of CO can be generated, so normal human blood can have 0.4% HbCO. When inhaling a gas containing 0.5% CO, HbCO in the blood It can reach as high as 70% in only 20-30min, and the poisoned will die from heart and respiratory failure.

3. High iron Hbemia. The divalent iron in Hb can be oxidized into trivalent iron under the action of oxidants to form high iron Hb (methemoglobin, HbFe3 + OH, MHb), also known as denatured Hb or hydroxylated Hb. The trivalent iron loses its oxygen-carrying ability due to the strong binding with the hydroxyl group. In addition, some of the four divalent irons of the Hb molecule are oxidized to the trivalent iron, which can increase the affinity of the remaining Fe2 + and oxygen, leading to the left of the oxygen ionization curve. Move and make the tissue hypoxia. Under physiological conditions, a very small amount of high-iron Hb is continuously formed in the blood, and it is continuously reduced to divalent iron Hb by reducing agents in the blood such as NADH, ascorbic acid, and reduced glutathione, so that the high-iron Hb content in normal blood only accounts for Hb 1% to 2% of the total amount. When nitrite, perchlorate and other oxidants are poisoned, if the Hb content in the blood is increased to 20% to 50%, headaches, weakness, coma, dyspnea, and tachycardia may occur. Speedy symptoms. It is more common that after eating a large amount of nitrate-containing pickles, nitrate is reduced to nitrite by the action of intestinal bacteria, which is absorbed by the latter to cause high iron Hbemia, which is called enterogenous cyanosis.

4. The affinity of Hb and oxygen is abnormally enhanced. For example, if a large amount of stock blood is input, the oxygenated Hb dissociation curve can be shifted to the left due to the low level of 2,3-DPG in the red blood cells in the stock blood. Input a large amount of alkaline liquid to increase the blood pH. In a short time, the affinity of Hb and O2 can also be enhanced by the Bohr effect. In addition, amino acid substitution in the Hb peptide chain (such as arginine at position 92 of the alpha chain is replaced by leucine) can also greatly enhance Hb and O2. The affinity thus makes the tissue hypoxic.

In the case of bloody hypoxia, due to normal external respiratory function, arterial blood oxygen partial pressure and blood oxygen saturation are normal, but the amount of Hb is reduced or the nature is changed, which reduces the blood oxygen capacity and the blood oxygen content.

When CO poisoning occurs, the patient's blood oxygen content decreases. After the blood is taken out and fully saturated with oxygen in vitro, the blood oxygen capacity measured is normal, because at this time O2 has completely replaced CO, and HbCO no longer exists.

The motive force for the diffusion of oxygen to the tissues and cells in the capillary bed is the partial pressure gradient of oxygen between the blood and the tissues and cells. The blood PO2 is high at the end of the capillary arteries, so the O2 diffuses out of the blood vessel quickly. When the blood flows from the arterial end to the venous end, the blood oxygen content gradually decreases, PO2 gradually decreases, and the diffusion rate of oxygen to the tissue gradually decreases. Therefore, the amount of oxygen obtained by the tissue depends on the average oxygen partial pressure in the capillaries and the oxygen in the tissue cells. Partial pressure difference. Although PaO2 is normal in patients with anemia, the average blood oxygen partial pressure in the capillary bed is lower than normal, so the tissue is hypoxic. For example, if an anemia patient has half the normal Hb, their blood oxygen capacity is 10ml / dl, and 100ml of blood flows into the capillary bed. After releasing 1ml of oxygen, SO2 decreases from 95% to 85%, and PO2 decreases from 100mmHg to 53mmHg. However, after 1mlO2 was released from 100ml blood of normal people, SO2 decreased from 95% to 90%, and PO2 decreased to 63mmHg. It can be seen that when the blood of anemia patients flows through the capillaries, the partial pressure of blood oxygen decreases rapidly, and oxygen diffuses into the tissues. The rate also quickly slowed down, causing tissue hypoxia and arterial-venous blood oxygen content differences to be lower than normal.

Hematological hypoxia caused by enhanced affinity between Hb and O2 is special, and its arterial oxygen capacity and oxygen content may not be low, and some are even higher than normal. The tissue hypoxia at this time is due to the greater affinity of Hb and oxygen, and the combined oxygen is not easy to release, and the difference between arterial and venous blood oxygen content is less than normal.

Patients with bloody hypoxia may have no cyanosis. Patients with severe anemia are pale, and even with hypotonic hypoxia, the deoxygenated Hb in the capillaries still does not reach 5g / dl, so cyanosis will not occur; CO poisoning increases the blood HbCO in the blood, so the skin and mucosa are cherry Red, severe hypoxia due to skin blood vessel contraction, skin, mucous membranes are pale; high-speed iron Hb is brown or bluestone, so patients' skin and mucous membranes are brown or similar to cyanosis; pure affinity of Hb and O2 is increased Due to hypoxia, the amount of deoxygenated Hb in capillaries is less than normal, so there is no cyanosis.