Concept of hypoxia
Hypoxia is a standard pathological process resulting from impaired biological oxidation and leading to a disruption of energy metabolism, primarily a decrease in ATP synthesis. Oxygen is the terminal electron acceptor in the mitochondrial respiratory chain, so even a short-term decrease in its availability disrupts cell function, primarily neurons, cardiomyocytes, and renal tubular epithelium.
Hypoxia can result from both external causes (decreased oxygen content in inhaled air) and internal disorders (pathology of respiration, circulation, blood, tissue respiration). Standard gas composition parameters are used for an objective assessment of the body's oxygen supply.
Main parameters of the oxygen regimen
- P atm — atmospheric pressure of the ambient air.
- pO₂ atm — partial pressure of oxygen in the atmosphere.
- pAO₂ — partial pressure of oxygen in alveolar air; always lower than atmospheric due to dilution by water vapor and CO₂, as well as diffusion into the blood.
- paO₂ — partial pressure of oxygen in arterial blood (normal is about 80–100 mmHg).
- SaO₂ — arterial blood hemoglobin saturation, the ratio of oxyhemoglobin to total hemoglobin; normally 96–98 %, in venous blood — 60–75 %.
- VO₂ — oxygen content volume (mL of O₂ per 100 mL of blood).
Fundamental difference: VO₂ depends on both the quantity and quality of hemoglobin, whereas SaO₂ reflects only quality — the degree of saturation of available hemoglobin with oxygen, but not its absolute amount. Therefore, in anemia, saturation may remain normal while the oxygen-carrying capacity of blood is sharply reduced.
Classification of hypoxias
According to their origin, hypoxias are divided into two large groups:
- Exogenous — caused by changes in oxygen in the inhaled air:
- hypobaric — decrease in barometric pressure (high altitude, depressurization);
- normobaric — decrease in O₂ content at normal pressure (confined spaces, mines).
- Endogenous — associated with the pathology of internal systems:
- respiratory — ventilation, diffusion, or perfusion disorders;
- circulatory — circulatory failure;
- hemic — anemias, hemoglobin inactivation (carboxy-, methemoglobinemia);
- tissue — blockade of respiratory chain enzymes (e.g., by cyanides);
- overload — mismatch between oxygen delivery and sharply increased tissue demand.
Changes in gas composition in exogenous hypoxia
In the hypobaric form, the determining factor is the drop in atmospheric pO₂. This triggers a predictable chain of shifts:
- decrease in pAO₂ — reduction of oxygen tension in the alveoli;
- decrease in paO₂ — arterial hypoxemia;
- decrease in SaO₂ — drop in saturation;
- decrease in pvO₂ — reduction of oxygen in venous blood;
- compensatory hyperventilation, leading to a decrease in paCO₂ and the development of gaseous alkalosis.
Thus, exogenous hypoxia is characterized by a simultaneous decrease in all parameters of oxygen content and tension along with hypocapnia.
Changes in gas composition in endogenous hypoxias
In endogenous forms, the initial atmospheric pO₂ and frequently pAO₂ remain normal, while shifts begin at deeper stages of oxygen transport:
- Respiratory hypoxia: pAO₂ is decreased or normal, paO₂ and SaO₂ are decreased, paCO₂ is increased (hypercapnia, gaseous acidosis).
- Circulatory hypoxia: paO₂ and SaO₂ are normal, but pvO₂ and SvO₂ are decreased; the arteriovenous oxygen difference is increased due to slowed blood flow.
- Hemic hypoxia: paO₂ is normal, SaO₂ is decreased (or normal in anemia), VO₂ is sharply reduced due to a drop in the blood's oxygen-carrying capacity.
- Tissue hypoxia: paO₂, SaO₂ and pvO₂ are increased, the arteriovenous oxygen difference is decreased — tissues are unable to utilize the incoming O₂.
Key points
- Hypoxia is a disruption of biological oxidation with ATP deficiency.
- SaO₂ reflects hemoglobin quality, while VO₂ reflects both quality and quantity.
- Exogenous hypoxia begins with a drop in atmospheric pO₂ and is accompanied by hypocapnia.
- Endogenous hypoxias: respiratory, circulatory, hemic, tissue, overload.
- In tissue hypoxia, venous blood remains "arterialized".
- In the hemic form, saturation may be normal despite reduced blood oxygen-carrying capacity.
- The arteriovenous O₂ difference is an important differential criterion for the type of hypoxia.