From the Guidelines
Hypoxemia is present on arterial blood gas (ABG) when the oxygen saturation (SaO2) is less than 90% on room air, or in cases where the partial pressure of oxygen (PaO2) is less than 60 mmHg 1.
Causes of Hypoxemia
- Respiratory failure, where it may be less than 90% on room air
- Carbon monoxide poisoning, where carboxyhemoglobin levels may be elevated
- Certain medications such as opioids, including morphine, or benzodiazepines, such as midazolam, can also lead to decreased O2 saturation due to their respiratory depressant effects
- Chronic obstructive pulmonary disease (COPD) or pneumonia, where O2 saturation may be low, often less than 92% on supplemental oxygen, due to impaired gas exchange 1
Key Points
- The brain is the most sensitive organ to the adverse effects of hypoxia, but other organs in patients with critical illness may be vulnerable to the risk of hypoxic tissue injury at oxygen levels above this range 1
- Most experts emphasise the importance of keeping the SaO2 above 90% for the majority of acutely ill patients 1
- The present guideline suggests a desirable target saturation range of 94-98% 1
- Sudden exposure to low SaO2 levels (below about 80%) can cause impaired mental functioning even in healthy participants 1
- Oxygen therapy is less effective in other causes of tissue hypoxia including anaemia where there is a low carrying capacity or where the carrying capacity of haemoglobin has been reduced by a toxic substance 1
From the Research
Definition of Hypoxemia
Hypoxemia is defined as an arterial oxygen tension (PaO2) of less than 60 mmHg or a PaCO2 of greater than 50 mmHg, or both, whilst breathing room air 2.
Arterial Blood Gas (ABG) Analysis
ABG analysis is used to assess and monitor the metabolic and oxygen status of patients with acute exacerbation of chronic obstructive pulmonary disease (COPD) 3.
- The analysis measures parameters such as pH, partial pressure of carbon dioxide (PCO2), partial oxygen pressure (PO2), bicarbonate (HCO3), and venous and arterial blood oxygen saturation (SO2).
- The results of ABG analysis can indicate the presence of hypoxemia.
Presence of Hypoxemia on ABG
Hypoxemia is present on ABG when the PaO2 is less than 60 mmHg 2.
- This can occur in various conditions, including acute respiratory failure, COPD, pneumonia, and COVID-19 2, 4.
- The presence of hypoxemia on ABG can be an indicator of respiratory failure and requires prompt attention and treatment.
Factors Affecting Hypoxemia Detection
Several factors can affect the detection of hypoxemia, including:
- The use of supplemental oxygen, which can take time to equilibrate after cessation 5.
- The difference between pulse oximeter and arterial blood gas levels, which can be significant in some cases 6, 4.
- The presence of underlying conditions, such as neurological or mental illnesses, which can increase the risk of silent hypoxemia 4.