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

Step 1: Asess acid base status

Follow the steps outlined in the lesson on Acid base.

Step 2: Assess alveolar ventilation

Step 3: Assess oxygenation status

Step 4: Assess oxygen delivery sytem

Step 5: Evaluate Carboxyhemoglobin status

 

 

 

 

 

 

 

 

 

 

Mechanisms of hypoxia

Low FIO2
Hypoventilation
V/Q mismatch
Diffusion barrier
Anatomical shunt

 

 

 

 

 

 

 

 

 

 

 

Oxygen dissociation curve

 

 

 

 

 

 

 

 

 

 

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)

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

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

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.

 

 

 

 

 

 

 

 

 

 

 

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

Oxygen delivery is oxygen content x cardiac output

 

 

 

 

 

 

 

 

 

 

Normal values for Oxygen carrying capacity, delivery and utilization

 

Hemoglobin : !5 grams/dl

Cardiac output: 5 litters/min

 

 

 

 

 

 

 

 

 

 

 

Carboxyhemoglobin

Normal:  <2%
Smoker: <9%
Coma: 50%
Death: 80%

Non-Smokers with elevated Carboxyhemoglobin 

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.