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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

• Follow the steps outlined in the lesson on Acid base in assessing the acid base status
• Decide whether the PO2 is normal and note FIO2
• Calculate A-a gradient to determine whether there is anything wrong with the lungs
• Decide on the mechanism of hypoxemia, if present
• Evaluate CO2 and Bicarbonate levels
• Decide whether it is acute or chronic if there is CO2 retention or hyperventilation
• Check on Carboxyhemoglobin level

Mechanisms of hypoxia

Low FIO₂
Hypoventilation
V/Q mismatch
Diffusion barrier
Anatomical shunt

Oxygen dissociation curve

• Steep part of the curve is between 20-60 mm Hg

  • in venous blood this characteristic helps deliver oxygen to tissues

• Flat portion of curve is between 60-100 mm Hg

  • CaO₂ is not significantly different between PaO₂ of 60-100 mm of Hg. Useful feature on arterial side.

A-a gradient

Equation

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)

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.

• V/Q mismatch

• Anatomical shunt

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

Anatomical shunt

• Cardiac causes
  • All right to left shunts
  • Tetrology of Fallot
  • Eisenmenger's physiology
• Pulmonary causes
  • ARDS
  • AV fistula
  • Acute atelectasis

Partial pressure of Oxygen in arterial blood (PaO₂)

• reflects oxygen dissolved in plasma
• cannot give information about adequacy of oxygen content (need Hg values and saturation) or Oxygen delivery (need cardiac output)
• primarily useful in calculating A-a oxygen difference (Is lung transferring oxygen appropriately?)

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 

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

Carboxyhemoglobin

Normal:  <2%

Smoker: <9%

Coma: 50%

Death: 80%

In Non-Smokers with elevated Carboxyhemoglobin consider

• Auto exhaust

• Gas leak in house

• Exposure to 2nd hand smoke

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.