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Components of ABG for interpretation
Approach to interpreting Arterial blood gases
Mechanisms of hypoxia
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 = (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.
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
In Non-Smokers with elevated Carboxyhemoglobin consider
Gas leak in house
Exposure to 2nd hand smoke
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.