From the Guidelines
The human brain can tolerate hypoxia due to diminished peripheral capillary oxygen saturation (SPO2) for only a few minutes before tissue hypoxia and cell death occur, especially in the brain.
Key Factors Influencing Tolerance
- The brain appears to be the most vulnerable organ during profound hypoxaemia, with brain malfunction being the first symptom of hypoxia and brain injury being the most common long-term complication in survivors of cardiac arrests and other episodes of profound hypoxaemia 1.
- Sudden exposure to low SaO2 below about 80% can cause altered consciousness even in healthy participants 1.
- Mental functioning becomes impaired if the PaO2 falls rapidly to <6 kPa (45 mm Hg, SaO2 <80%) and consciousness is lost at <4 kPa (30 mm Hg, SaO2 <56%) in normal participants 2.
Target Oxygen Saturation Levels
- Most experts emphasize the importance of keeping the SaO2 above 90% for most acutely ill patients 1.
- The BTS guideline recommends a target SaO2 (and SpO2) ≥94% for most hypoxaemic patients to ensure that the actual oxygen level remains above 90% for most of the time with a 4% margin of safety to allow for variability in oxygen saturation levels, their recordings, and oximeter error 2.
- The guideline development committee believes that a target range of 94-98% will achieve normal or near-normal SpO2 for most adults in the UK and will avoid any risk of clinically significant hypoxaemia 3.
Clinical Considerations
- Preventing hypoxic episodes is considered more important than avoiding any potential risk of hyperoxia 4.
- Titrate the inspired oxygen concentration to maintain the arterial blood oxygen saturation in the range of 94-98% and avoid hypoxaemia, which is also harmful 5.
From the Research
Duration of Human Brain Tolerance to Hypoxia
The human brain's tolerance to hypoxia due to diminished peripheral capillary oxygen saturation (SPO2) is a complex topic. Research studies have investigated the effects of hypoxia on cognitive performance, cerebral oxygenation, and peripheral oxygen saturation.
- The study by 6 found that cognitive performance, particularly in complex central executive tasks, was reduced at FIO2 0.12, and was strongly correlated with reductions in SpO2 and cerebral oxygenation.
- Another study by 7 discussed the brain's ability to respond to mild hypoxia with acute and chronic adaptive mechanisms, involving systemic and central metabolic and vascular processes mediated by hypoxia-inducible factor (HIF)-1.
- The study by 8 examined the effects of continuous hypoxia on flow-mediated dilation in the cerebral and systemic circulation, finding that cerebral flow-mediated dilation remained preserved despite a reduction in internal carotid artery antegrade shear rate and cerebral O2 delivery.
- Research by 9 investigated the effect of added inspired CO2 during artificially induced hypoxic normobaric hypoxia on complex task performance, finding that brain oxygenation levels were significantly higher during isocapnic hypoxia than poikilocapnic hypoxia.
- A more recent study by 10 compared the suitability of cerebral oximetry using near-infrared spectroscopy (NIRS) and traditional pulse oximetry for measuring cerebral oxygen saturation during hypoxia testing, finding that NIRS is a reliable method for detecting cerebral oxygen saturation, offering significant advantages over traditional pulse oximetry in stability and reliability.
Key Findings
- Cognitive performance is reduced at FIO2 0.12, and is strongly correlated with reductions in SpO2 and cerebral oxygenation 6.
- The brain can respond to mild hypoxia with acute and chronic adaptive mechanisms, involving systemic and central metabolic and vascular processes mediated by HIF-1 7.
- Cerebral flow-mediated dilation remains preserved despite a reduction in internal carotid artery antegrade shear rate and cerebral O2 delivery during continuous hypoxia 8.
- Added inspired CO2 during hypoxia can improve performance by preventing hypocapnia-induced vasoconstriction of brain blood vessels 9.
- NIRS is a reliable method for detecting cerebral oxygen saturation, offering significant advantages over traditional pulse oximetry in stability and reliability during hypoxia testing 10.