From the Guidelines
Hypoxemia, or low oxygen saturation, is primarily caused by impaired oxygen delivery to the body's tissues.
Causes of Hypoxemia
The main causes of hypoxemia can be categorized into several key areas:
- V/Q mismatch: This occurs when there is poor aeration of areas of the lung or abnormalities of gas exchange within the lung, often seen in serious illnesses such as pneumonia 1.
- Low oxygen-carrying capacity: Conditions like anaemia reduce the amount of oxygen that can be carried by the blood, leading to hypoxemia 1.
- Toxic substances: Substances like carbon monoxide can block oxygen binding to haemoglobin, despite normal oxygen levels in the lungs and blood 1.
- Low cardiac output: A state where the heart is unable to pump enough blood to meet the body's oxygen demands, leading to stagnant hypoxia 1.
- Impaired oxygen metabolism: Certain conditions, such as severe sepsis or specific types of poisoning, can lead to histotoxic hypoxia, where the body is unable to metabolize oxygen at the mitochondrial level 1.
Key Factors
It's essential to note that the brain is the most sensitive organ to the adverse effects of hypoxia, and sudden exposure to low SaO2 levels (below about 80%) can cause impaired mental functioning even in healthy individuals 1. Most experts emphasize the importance of keeping the SaO2 above 90% for most acutely ill patients 1.
From the Research
Causes of Hypoxemia
The causes of hypoxemia, or low oxygen saturation, can be attributed to several factors, including:
- Ventilation-perfusion inequality, which is the most common cause of impaired gas exchange in patients with lung disease 2
- Alveolar hypoventilation, ventilation/perfusion mismatches, diffusion disorders, true shunts, and a decrease in the partial pressure of inspired oxygen 3
- Low oxygen affinity hemoglobin variants, which can cause asymptomatic patients to present with cyanosis and/or low O2 saturation without signs of chronic hypoxia 4
- Intra- and extrapulmonary shunt, which can be augmented by certain medications such as sodium nitroprusside 5
- Relative or absolute shunts, which can be determined using the alveolar-arterial oxygen tension difference and the diffusion capacity for carbon monoxide 6
Pathophysiological Mechanisms
The pathophysiological mechanisms that contribute to hypoxemia include:
- Ventilation-perfusion inequality, which always causes hypoxemia and can also cause hypercapnia in patients with chronic obstructive pulmonary disease (COPD) 2
- Diffusion disorders, which can be caused by conditions such as pneumonia or pulmonary edema 3
- True shunts, which can be caused by conditions such as atrial septal defects or pulmonary arteriovenous malformations 3
- Decrease in the partial pressure of inspired oxygen, which can be caused by conditions such as high altitude or respiratory failure 3
Diagnostic Approach
A systematic approach to diagnosing hypoxemia includes:
- Measuring the alveolar-arterial oxygen tension difference and the diffusion capacity for carbon monoxide 6
- Administering 100% oxygen to correct hypoxemia in cases of relative shunt or diffusion disorder 6
- Using arterial blood gas analysis and determining the partial pressure of O2 at 50% (p50) to diagnose low oxygen affinity hemoglobin variants 4
- Assessing the patient's medical history, physical examination, and laboratory results to determine the underlying cause of hypoxemia 3