This document presents a study assessing lung damage at the alveolar level using arterial blood gas (ABG) analysis. It describes how gas exchange occurs between alveoli and blood capillaries, noting factors like surface area, membrane thickness, and gas pressure that affect diffusion. Patients with lung conditions have changes in these factors. The document introduces equations incorporating FiO2 and atmospheric pressure to calculate the expected versus measured PaO2 from ABG results. This is used to quantify lung damage in two patient cases in intensive care and pre-operation. The study concludes ABG reports should account for FiO2 and pressure to assess lung function and prognosis.

1. ASSESMENT OF LUNG
PARENCHYMAL DAMAGE
WITH ARTERIAL BLOOD
GAS ANALYSIS
PROF.N.K.AGRWAL
JNMC, SAWANGI,WARDHA

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 SMALL SACS OF
BRONCHIOLES
If all other factors are kept constant the
main factor are surface area and
thickness of alveoli. The gas exchange is
hence inversely proportional to thickness
of membrane, the more is the thickness
less will be diffusion.

6. Patients with chronic
bronchitis, COPD, fibrosis,
pneumonia, ARDS etc
have changes in thickness
and surface area.2

7. RESPIRATORY EQUATIONS
PATM = PN 2+Po2+Pco2+PH2O NO.1
P760 = P514 + P159 + P40 + P47 NO. 2
PI02 = FIO2 X ( PB - -- H2 0 ) NO .3
PAO2 = FiO2 x ( Pb- 47 ) - 1.25 x PCO2 NO 4

8. PAO2 = FiO2 x ( 760- 47 ) - 1.25 x 32
 = FiO2 x 713 -40
 = FiO2 x 673

9. The normal P ( A-a) O2 at room air
- 5 to 20 mm of Hg
Fio2 - 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

10. 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 % )

11.  The expected PaO2 will be EPaO2 = PAO2 X (1-k)
(equation is copyright of author )
EpaO2– PaO2 (ABG)
Lung Damage = ------------------- x100
EPaO2

12. Case No. 1
A patient in ICU had PaO2 78 mm of hg with Fio2 =1
EPaO2= PAO2x ( 1-0.2 )
= 673 x 0.8
= 536 mm Hg ( using respiratory eq. no 4 )
536 - 78
LD = -------------------
536
= 0.85x 100
= 85 %

13. CASE NO.2
A patient posted for thoracotomy the ABG showed PaO2=57
at room air FIO2= 0.2
EPaO2 = PAO2 x (1- 0.05)
= 107 x 0.95
=101mm of Hg
101 - 57
LD = ----------------- x 100
101
= 44 %

14. 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 may be able
to calculate the lung damage , in ICU as
well as in pre operative phase, to assess the
prognosis.

15. REFERENCES :
 1.West JB: Respiratory physiology-- The essentials, 4th ed. Baltimore, William & Wilkins 1990
 2.respiratory physiology 7th edition wyeli & Churchill page 168-169
 3. Nunn JF: Alveolar air equation Anesthesiology 1996; 85:946
 4.Mansoor,R. Murray and Nadel's Text book of respiratory medicine,5 the edition,philadelphia,PA 2010
 5.7th edition Wylie and Churchill Davidson page 168-169

 6. Gas exchange in human retrieved 19 march 2013