What is the difference between hypoxia and hypoxemia?

From the Guidelines

Hypoxia and hypoxemia are distinct medical conditions, with hypoxemia referring to low oxygen levels in the blood and hypoxia describing inadequate oxygen supply at the tissue level. The key difference between hypoxia and hypoxemia lies in their definitions and the levels at which they affect the body. Hypoxemia is specifically characterized by low oxygen levels in the blood, typically defined as an arterial oxygen partial pressure (PaO2) below 80 mmHg or oxygen saturation (SaO2) less than 95% 1. On the other hand, hypoxia is a broader term that encompasses inadequate oxygen supply at the tissue level, resulting in cells being unable to maintain normal function. This can occur due to various reasons, including but not limited to, low blood oxygen levels (hypoxemia), anemia, stagnant blood flow, or histotoxic causes where tissues cannot utilize available oxygen effectively.

Causes and Mechanisms

• Hypoxemia can be caused by several mechanisms, including ventilation-perfusion mismatch, shunting, and hypoventilation, often seen in conditions like pneumonia, chronic obstructive pulmonary disease (COPD), and at high altitudes 1.
• Hypoxia, while often a consequence of hypoxemia, can also occur independently due to issues with oxygen delivery (e.g., anemia, carbon monoxide poisoning) or utilization (e.g., cyanide poisoning) by tissues.

Clinical Manifestations and Treatment

Clinical manifestations of both conditions may include shortness of breath, confusion, rapid heart rate, and bluish discoloration of the skin (cyanosis), though symptoms vary depending on severity and onset speed. Treatment focuses on addressing the underlying cause while providing supplemental oxygen as needed. For hypoxemia, especially in acute settings, supplemental oxygen is crucial to increase blood oxygen levels. In cases of hypoxia not due to hypoxemia, treatment targets the specific cause, such as increasing red blood cell production in anemia or removing toxic substances.

Guidelines and Recommendations

Recent guidelines, such as those from the British Thoracic Society 1 and the European Respiratory Society 1, emphasize the importance of maintaining adequate oxygen saturation levels, typically above 90%, in acutely ill patients, with specific targets that may vary based on the patient's condition and comorbidities. For patients with chronic conditions like COPD, long-term oxygen therapy (LTOT) is recommended for those with severe hypoxemia, as it has been shown to improve survival.

In summary, understanding the distinction between hypoxia and hypoxemia is crucial for appropriate diagnosis and management, focusing on correcting the underlying cause of oxygen deficiency and ensuring adequate oxygen supply to tissues. The management of both conditions prioritizes improving oxygen delivery and utilization, with supplemental oxygen being a cornerstone of treatment for hypoxemia, and targeted therapies for the specific causes of hypoxia.

From the Research

Definition and Difference

• Hypoxia refers to a condition in which oxygen delivery to the tissues does not meet their metabolic needs, causing damage to any tissue in the human body 2.
• Hypoxemia, on the other hand, refers to a reduction in PO2 below the normal range, regardless of whether gas exchange is impaired in the lung, CaO2 is adequate, or tissue hypoxia exists 3.

Causes and Effects

• Hypoxemia can be caused by various factors, including V/Q mismatching, alveolar hypoventilation, and anemia, and can lead to tissue hypoxia if left uncorrected 3.
• Tissue hypoxia can cause organ failure and life-threatening conditions, and inadequate effort to correct hypoxia can cause either disturbance of the adaptation or undesirable tissue hyperoxia 4.

Diagnosis and Treatment

• The degree of tissue oxygenation is estimated by measurement of oxygen blood level, and the lack of oxygen on cellular and tissue level can lead to organ failure and life-threatening conditions 4.
• The treatment of hypoxemia and hypoxia involves oxygen therapy, and recent literature supports rather restrictive strategy of oxygen therapy in critical care 4.

Types of Hypoxia

• There are different types of hypoxia, including hypoxic, anemic, and cardiac-related hypoxemia, and identical counterregulations occur in the cell metabolism, regardless of the cause of hypoxemia 5.
• The clinical assessment and indication for treatment vary widely depending on the cause of hypoxemia, and restrictive and generally accepted rules are specified in the transfusion guidelines for anemic hypoxemia 5.