Tuesday, 29 March 2016
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.
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.