Blood Gases - Physiology

Physiology

·        For acid-base disturbances, see Acid-Base balance(Topic)

Blood Gases

·        Normal Values: 

o   PaO₂: 75 – 100 mmHg, 10 – 13.3 KPa. Dependent on ventilation/perfusion balance (A-a gradient) and inspired O2 concentration

o   O2 saturation: 95 – 100%

o   PaCO2: 36 – 46 mmHg. If high then hypoventilation, if low then hyperventilation. Measured with a capnograph. pH falls by 0.1 for every rise of 10 CO₂

o   Plasma HCO₃ (arterial): 22 – 26 mmol/L

o   To convert mmHg to kPa divide by 7.5 (multiply by 0.133)

·        PAO2 is lower than inspired PO₂ because:

o   It becomes saturated with water vapour

o   It is diluted by expired CO₂

o   O₂ is absorbed into the blood

·        Factors affecting the A-a gradient (normally 5 mmHg at FIO2 of 21%, may be up to 100 at 100%):

o   Ventilation/Perfusion balance (V/Q): most common cause of a fall.  Responds well to O₂ therapy

o   Diffusion

o   Shunts (pathological or anatomical)

·        Calculating the A-a gradient:

o   PIO₂ = (PB – PH₂O) * FIO₂ = (760-47) * 21% = 150 mmHg

o   PAO₂ = PIO₂ – PaCO₂/R

o   A-a gradient = PAO₂ – PaO₂. Normal is 5 – 15

·        Saturation = % of haemoglobin that is fully bound. Determined by PO₂ and shape of dissociation curve

·        Oxyhaemoglobin dissociation curve 

o   Shifted right by ­blood temp, CO₂, H+, 2,3BPG

o   Shifted left by ¯blood temp, CO₂, H+, 2,3BPG

·        O2 available to tissues (Oxygen flux) depends on:

o   O₂ saturation

o   Hb concentration

o   Blood flow

o   Normal flux is 1000 ml/min.  If flux falls below 250 then hypoxia

·        Cyanosis:

o   Peripheral cyanosis: capillary de-oxy Hb > 50 g/litre.  Eg due to being cold and vasoconstricted 

o   Central cyanosis: due to ¯saturation and de-oxy Hb > 50 g/litre eg in mouth and tongue