From the Guidelines
Only a small amount of oxygen dissolves in blood because oxygen has low solubility in plasma, but the partial pressure of oxygen (PaO₂) increases quickly due to the physical relationship between gas pressure and concentration. When we breathe in oxygen, only about 1.5% of oxygen in arterial blood is dissolved in plasma, while the majority (98.5%) binds to hemoglobin 1. According to Henry's Law, the amount of gas dissolved in a liquid is proportional to its partial pressure above the liquid. As we inhale oxygen-rich air, the partial pressure gradient drives oxygen into the blood rapidly, causing PaO₂ to rise quickly. However, the actual amount dissolved remains small due to oxygen's poor solubility in water. This rapid increase in PaO₂ is crucial because it facilitates oxygen loading onto hemoglobin, which is the primary oxygen transport mechanism in blood. The hemoglobin-oxygen binding follows a sigmoid curve, meaning that small increases in PaO₂ can significantly increase oxygen saturation, especially in the steep portion of the curve between 20-60 mmHg.
Some key points to consider:
- The amount of oxygen carried in the blood is often expressed in terms of how saturated circulating haemoglobin is with oxygen (SO2) 1.
- Sudden exposure to low SaO2 levels (below about 80%) can cause impaired mental functioning even in healthy participants 1.
- 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.
- PaO2 should be maintained within a normal range (e.g., between 70 and 90 mmHg) or SaO2 between 92 and 97% 1.
In terms of oxygen therapy, high-flow nasal cannula (HFNC) oxygen can generate low levels of PEEP in the upper airways, decrease work of breathing and reduce dead space 1. HFNC can be an attractive technique as a first-line therapy to avoid intubation, especially in patients with PaO2/FiO2 ≤ 200 mmHg at enrollment 1. However, the results need confirmation, and patients should be monitored closely, as deterioration can occur abruptly 1.
From the Research
Oxygen Dissolution and PaO₂ Increase
- The amount of oxygen that dissolves in the blood is limited, but the partial pressure of arterial oxygen (PaO₂) can increase quickly due to several factors 2.
- The haemoglobin-oxygen dissociation curve plays a crucial role in understanding the relationship between oxygen saturation and partial pressure, which helps explain why PaO₂ increases quickly 3.
- The alveolar-arterial gradient and the PaO₂/FiO₂ ratio are useful measures in assessing pulmonary dysfunction, and changes in these values can reflect changes in PaO₂ 4.
Factors Affecting Oxygen Availability
- Several factors, such as anaesthesia, hypoxia, and stress, can affect the regulation of atmospheric, alveolar, arterial, capillary, and tissue partial pressure of oxygen (PO₂) 2.
- Supplemental oxygen therapy and noninvasive ventilation can be used to treat hypoxemic and hypercapnic respiratory failure in patients with chronic obstructive pulmonary disease (COPD), which can help increase PaO₂ 5.
Clinical Implications
- Maintaining an appropriate range of PaO₂ is critical for patients with traumatic brain injury (TBI), and values below 60 mmHg within 6 hours of admission have been associated with poor outcomes 6.
- Understanding the factors that affect oxygen availability and the relationship between oxygen saturation and partial pressure is essential for clinical practice and patient care 3, 2.