What does a partial pressure of oxygen (PO2) level of 155 mmHg indicate?

Understanding a PO2 of 155 mmHg

A partial pressure of oxygen (PO2) of 155 mmHg indicates hyperoxemia, which means there is an excess of oxygen in the blood beyond the normal physiological range. 1

Normal PO2 Values and Context

• The normal range for partial pressure of arterial oxygen (PaO2) in healthy adults is 80-110 mmHg (10.9-14.8 kPa), with variations based on age 1
• For young adults (18-24 years), the normal range is 90-111 mmHg (12.0-14.8 kPa) 1
• PaO2 naturally decreases with age, with more pronounced decline after age 55 1
• For adults over 64 years, the normal range is 68-111 mmHg (9.0-14.8 kPa) 1

Clinical Significance of a PO2 of 155 mmHg

• A PO2 of 155 mmHg falls into the category of hyperoxemia 2
• Hyperoxemia is defined as a high PO2 in the blood, with severe hyperoxemia typically considered as PaO2 greater than 200 mmHg and mild hyperoxemia as PaO2 between 120-200 mmHg 3
• A PO2 of 155 mmHg therefore represents mild hyperoxemia 3
• This level is often seen when a patient is receiving supplemental oxygen 2

Potential Implications and Concerns

• Hyperoxemia can lead to increased oxidative stress and potential organ damage after reperfusion 2
• Studies have shown that severe hyperoxemia is associated with higher mortality rates and fewer ventilator-free days compared to both mild hyperoxemia and normoxia 3
• Time spent in hyperoxemia shows a linear and positive relationship with hospital mortality 3
• Excessive oxygen exposure can cause oxygen toxicity affecting organs such as the heart, lungs, eyes, and nervous system 4

Management Considerations

• Current guidelines suggest targeting a normal oxygen saturation range of 94-98% to avoid both hypoxemia and hyperoxemia 2, 4
• For patients with ROSC after cardiac arrest, guidelines recommend avoiding hyperoxemia (weak recommendation, low-certainty evidence) 2
• When administering supplemental oxygen, it should be titrated to maintain PaO2 within the physiological range 5
• In patients receiving supplemental oxygen, an SpO2 target of 95% maximizes the likelihood of maintaining PaO2 in the physiological range 5
• After discontinuation of supplemental oxygen in patients without pulmonary disease, arterial PO2 values typically return to baseline within 5-7 minutes 6

Clinical Approach to a Patient with PO2 of 155 mmHg

• Assess whether the patient is receiving supplemental oxygen and consider titrating down if appropriate 2
• Monitor for signs of oxygen toxicity, particularly in patients receiving prolonged oxygen therapy 4
• Consider the clinical context - a transient elevation in PO2 may be less concerning than sustained hyperoxemia 3
• Remember that a PO2 of 155 mmHg alone does not determine tissue oxygenation, which also depends on hemoglobin concentration, cardiac output, and tissue oxygen extraction 1
• If the patient is critically ill, aim to limit PaO2 levels within a safe range, as with other physiologic variables 3

Caveats and Pitfalls

• Interpreting PO2 values requires consideration of the clinical context, including the patient's age, underlying conditions, and whether they are receiving supplemental oxygen 1
• A single elevated PO2 reading should be interpreted cautiously and repeated if necessary 3
• Avoid abrupt withdrawal of supplemental oxygen in patients who may have developed physiologic dependence 2
• Remember that both hypoxemia and hyperoxemia can be harmful - aim for normoxemia when possible 2, 3