Classification of Hypoxia

Etiological Classification

The fundamental criterion for differentiating hypoxic conditions is their underlying cause. Based on this, two main groups are distinguished.

1. Exogenous Hypoxia Develops as a result of reduced oxygen intake into the body. This occurs due to a low partial pressure (pO2) or insufficient oxygen content in the inspired air. It includes two variants:

• Normobaric. Occurs at normal atmospheric pressure but with reduced pO2 (e.g., in confined spaces).
• Hypobaric. Associated with a general decrease in atmospheric pressure (a typical example is ascending to high altitudes).

2. Endogenous Hypoxia Caused by pathological processes within the body itself. It includes the following forms:

• Respiratory;
• Cardiovascular (circulatory);
• Hemic (blood);
• Tissue;
• Substrate;
• Overload.

Features of Specific Forms

Some types of hypoxia have specific developmental mechanisms not directly related to impaired oxygen transport by the blood.

• Tissue Hypoxia. Occurs when oxygen utilization during biological oxidation is impaired directly within the cells. Meanwhile, oxygen delivery to tissues remains normal.
• Overload Hypoxia. Represents an imbalance between demand and supply. It develops when the demand of tissues and organs for oxygen or metabolic substrates increases significantly. This demand cannot be met even with maximum activation of transport mechanisms (functional overload occurs).
• Substrate Hypoxia. Forms against the background of normal blood oxygen levels. The main cause is a deficiency of oxidation substrates (predominantly lipids and carbohydrates).

> Important: Exogenous and respiratory hypoxia are characterized by the development of arterial hypoxemia. Therefore, these two types are sometimes combined under the common term "hypoxemic hypoxia".

Classification by Rate of Development and Duration

The timeframe over which the pathological picture unfolds determines the clinical course:

1. Fulminant (hyperacute). Development time is a matter of seconds. A severe condition arises instantly, often leading to death. Examples: aircraft depressurization at altitudes above 9,000–11,000 meters, rapid massive blood loss.
2. Acute. Develops within several minutes (typically within 1 hour) after exposure to the causative factor. Characteristic of acute blood loss and acute respiratory failure (RF).
3. Subacute. Forms over several hours (within the first 24 hours). Occurs in venous hemorrhage, slowly progressive respiratory or heart failure, and upon exposure to methemoglobin formers (nitrates, nitrogen oxides, benzene).
4. Chronic. Lasts for more than a few days (weeks, months, years). Typical of anemias, chronic heart failure, or chronic respiratory failure.

Classification by Severity of Disorders

According to the severity of functional impairment, hypoxia is divided into four categories:

• Mild;
• Moderate;
• Severe;
• Critical (lethal).

Comprehensive criteria are used for precise severity assessment. First, the degree of impairment in the patient's neurological and mental activity is analyzed. Concurrently, the severity of disorders in vital organ systems—cardiovascular and respiratory—is evaluated. Objective laboratory confirmation of severity is provided by deviations in blood gas composition and acid-base balance (ABB).