Oxygen Therapy

Administering extra oxygen is easy, but there are some more things we could and should think about. Once 30% or more of the blood in the pulmonary circulation passes through an area of low V/Q, the hypoxia cannot be corrected by simply increasing the oxygen content of the inspired gas.

Hypoxic Drive
Oxygen induced hypercapnea is likely to include:
- Worsened ventilation-perfusion mismatch (due to pulmonary vasoconstriction)
- Decreased binding affinity of haemoglobin for carbon dioxide
- Reduced minute ventilation.
So giving extra oxygen is unlikely to be a problem but we need to be aware of it, and monitor it.

Alveolar Gas Equation 
PAO2 = 95 x FIO2(%) – 1.25 x PaCO2
Under normal circumstances the difference between this and the arterial O2 tension (PaO2) measured by the ABG machine is 2-4 kPa. This difference is known as the A-a gradient. Its calculation can help to distinguish between types of hypoxia.


References
http://learning.bmj.com/learning/module-intro/oxygen-therapy.html?moduleId=10053774&searchTerm=%E2%80%9Coxygen%E2%80%9D&page=1&locale=en_GB

http://www.rcemlearning.co.uk/modules/oxygen-in-human-physiology/

http://www.rcemfoamed.co.uk/portfolio/hypoxic-drive-fact-or-fiction/

http://emcrit.org/wp-content/uploads/2015/06/nejmoa1503326.pdf

http://www.annemergmed.com/article/S0196-0644(15)01500-0/abstract