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Components of ABG for interpretation

  1. Acid Base status
  2. Alveolar Ventilation
  3. Oxygenation status
  4. Lung's ability in O2 transport
  5. Carboxyhemoglobin


Approach to interpreting Arterial blood gases


Mechanisms of hypoxia

Low FIO2
V/Q mismatch
Diffusion barrier
Anatomical shunt

Oxygen dissociation curve


A-a gradient


A-a gradient = (Alveolar PO2 - Arterial PO2)

Alveolar PO2 = ((Barometric pressure - Water vapor pressure) x FIO2) - (PaCO2/Respiratory quotient)

                          ((760-47) x 21) - (40/0.8)

Barometric pressure: Varies:  760 for Chicago

Water vapor pressure at body temp: 47

FIO2 : 21% when it is room air

Alveolar PCO2 is same as arterial PCO2 : 40 mm Hg

Respiratory quotient : 0.8



Widened A-a gradient

Normal range: 5-20 mm Hg.

FIO2 of 1.0 : upto 100 mm Hg

Increases with age.

Widened A- a gradient indicates that there is something wrong in Lungs with oxygen transfer.

Even in patients with diffusion barrier the widened A-a oxygen difference is probably due to V/Q mismatch.


Anatomical shunt


Partial pressure of Oxygen in arterial blood (PaO2)

Dissolved oxygen in the plasma is the determinant for PaO2.

Oxygen carried in hemoglobin has no influence on PaO2.

Thus low hemoglobin, Carbon monoxide, Methhemoglobinemia which profoundly affect Oxygen content do not affect PaO2.

A normal PaO2 does not necessarily mean normal oxygen content.

Oxygen Carrying capacity/delivery 

1 gram of Hemoglobin can hold about 1.34 ml of oxygen when 100% saturated.

Dissolved in plasma = .003 ml O2/dl plasma/mm Hg PaO2

The formula used to calculate  Oxygen content  (CaO2) is : (Oxygen carried in Hemoglobin + Oxygen dissolved in plasma)

(1.34 x Hemoglobin x Oxygen saturation) + .003(PaO2)

(1.34 x 15 x 0.98) + 0.003x 100)

(19.7 +0.3) = 20 O2 dl

Hemoglobin : 15 grams/dl

Cardiac output: 5 litters/min

Oxygen delivery is oxygen content x cardiac output = 1000 ml/minute

Oxygen consumption per minute = 250 ml per minute




Normal:  <2%

Smoker: <9%

Coma: 50%

Death: 80%


In Non-Smokers with elevated Carboxyhemoglobin consider



Carbonmonoxide poisoning

CO does not affect PaO2

CO affects SaO2 and O2 content

Calculated SaO2 and pulse oximeter should not be relied for measurement of SaO2 in cases suspected of CO poisoning.

Measured SaO2 should be obtained

Half life of CO breathing room air is 6 hours


Oxygen saturation (SaO2)

4 binding sites for oxygen in hemoglobin. SaO2 is percentage of heme binding sites saturated with Oxygen. Depends on the concentration of dissolved Oxygen.

Calculated saturation: Will be wrong in the presence of COHb.

Measured saturation: utilizes 4 wavelengths of light and can separate out oxyhemoglobin from others.

Oximeters do not differentiate hemoglobin bound to CO from hemoglobin bound to O2. Hence misleading saturation results when there is COHb and MetHb.