From the Guidelines
Hypoxia with normal diffusion capacity can occur due to several mechanisms that impair oxygen delivery despite intact alveolar-capillary gas exchange, including ventilation-perfusion mismatch, hypoventilation, right-to-left shunting, and low inspired oxygen concentration. The main causes of hypoxia with normal diffusion capacity include:
- Ventilation-perfusion mismatch, which occurs in conditions like asthma, COPD, pneumonia, and pulmonary embolism, where some areas of the lung receive adequate blood flow but insufficient ventilation 1
- Hypoventilation, which results from central respiratory depression (opioid overdose, brain injury), neuromuscular disorders (myasthenia gravis, Guillain-Barré syndrome), or chest wall restrictions (obesity, kyphoscoliosis) 1
- Right-to-left shunting, which happens when blood bypasses ventilated alveoli through anatomic pathways like intracardiac defects or arteriovenous malformations, or through physiologic shunting in atelectasis or severe pneumonia 1
- Low inspired oxygen concentration, which occurs at high altitudes or in enclosed spaces with poor ventilation 1 These mechanisms cause hypoxemia despite normal diffusion capacity because the problem lies not in the transfer of oxygen across the alveolar-capillary membrane but in other aspects of the oxygen delivery pathway, as noted in the BTS guideline for oxygen use in adults in healthcare and emergency settings 1. It is essential to identify and address the underlying cause of hypoxia to improve oxygen delivery to the tissues and prevent tissue hypoxia, which can be subdivided into four main causes: hypoxaemic, anaemic, stagnant or histotoxic 1.
From the Research
Causes of Hypoxia with Normal Diffusion Capacity
- Hypoxia can occur due to various factors, including alveolar hypoventilation, ventilation/perfusion mismatches, true shunts, and a decrease in the partial pressure of inspired oxygen 2
- Anemia, which can be caused by iron deficiency or chronic disease, can lead to hypoxia despite normal diffusion capacity 3, 4, 5
- Cardiovascular medications, such as beta blockers, angiotensin-converting enzyme inhibitors, and calcium-channel blockers, can impact iron metabolism and contribute to anemia 4
- Pulmonary hypertension is also linked to iron homeostasis and anemia, and regular iron status assessment and supplementation strategies are recommended in these patients 5
- In critically ill patients, hypoxaemia can be caused by a range of factors, including respiratory pathophysiology, and requires prompt diagnostic approach and management 2, 6