A NEW CONCEPT TO ASSESS THE ALVEOLAR DIFFUSION DAMAGE, A NEW WAY FOR ANALYSIS ARTERIAL BLOOD GAS ANALYSIS

1. ASSESMENT OF LUNG
PARENCHYMAL DAMAGE WITH
ARTERIAL BLOOD GAS ANALYSIS
(ABG)
PROF.N.K.AGRAWAL

2. OBJECTIVE
The objective of the study was to
assess the lung damage at alveoli
level with arterial blood gas analysis
(ABG)

3. The major task of the lung is to oxygenate the blood
and remove carbon dioxide, this happens by exchange
of gas between alveoli and blood capillary. Oxygen
diffuses passively from the alveoli to plasma. The
diffusion is affected by-
Surface
area
available
Thickness
of
membrane
Pressure
difference
of the gas
across the
barrier
Molecular
weight of
the gas
Solubility of
gas.

4. The alveoli are main respiratory surface in human being, they
are small sacs of branching of bronchioles, and they are one cell
thick and provide large area for gas exchange
If all other factors are kept constant the main factor are surface
area and thickness of alveoli. The gas exchange is hence
inversely proportionate to thickness, the more is the thickness
less will be diffusion. Patients with chronic bronchitis, COPD,
fibrosis pneumonia, ARDS etc have changes in thickness and
surface area.2

5. RESPIRATORY EQUATIONS
PATM = PN 2+Po2+Pco2+PH2O
P760 = P514 + P159 + P40 + P47
PI02 = FIO2 X ( PB - -- H2 0 )
PAO2 = FiO2 x ( Pb- 47 ) - 1.25 x PCO2

6. The normal P ( A-a) O2 difference at patient breathing air is 5
to 20 mm of Hg while at Fio2 of 1 it may go up to 110 mm of
Hg. The A-a gradient increases by5-7 mm of Hg with every
10% rise of FIO2
Expected Pao2 = PAo2 X ( 1- k ) (k = 0.05 at 20-40 %, 0.1 at 41-
60%,0.15 at 61-80%,0.2 at 81-100 % FIO2 )

7. • The expected Pao2 will be EPao2 = PAO2 X (1-k)
(Below equation is copyright of author)
Epa02– Pa02
Lung Damage = ------------------- x100
EPao2

8. Case No. 1
A patient in ICU had Pao2 78 mm of hg with Fio2 =1
PAo2 = 1x (760 - 47) – 1.25x32
= 1x 713 - 40
= 673 mm of Hg
EPao2= PAo2x 1-0.2
= 0.8 x 673
= 536 mm Hg
536 - 78
LD = -------------------
536
= 0.85x 100
= 85 %

9. CASE NO.2
A patient posted for thoracotomy the ABG showed Pao2=57
PAo2 = 0.2 x (760-47) – 1.25x32
=147-40
=107
EPao2 = 0.95x PAo2
= 0.95 x 107
=101mm of Hg
101 - 57
LD = ----------------- x 100
101
= 44 %

10. CONCLUSION
Here we conclude that whenever the ABG reports
are read in one must take in account the Fio2 and
atmospheric pressure. Using the above equation we
will be able to assess the lung damage correctly to
manage the patients who are appearing normal as
per Pao2 available.