To assess the lung damage at alveoli level with the help of arterial blood gas analysis. We have studied ABG of few cases randomly to assess the lung damage at the level of parenchyma,it is possible, if we have Etco2, PAo2, Pao2 , Fio2 values . The diffusion is affected by Surface area ,Thickness of membrane, Pressure gradient ,Molecular weight of the gas, Solubility of gas. If all other factors are kept constant more is the thickness less will be diffusion. Patients with chronic bronchitis, COPD, fibrosis pneumonia, ARDS etc have changes in thickness and surface area. P760 = P514+P159+ P40+P47

The normal P ( A-a) O2 difference at patient breathing air is 5-20 mm Hg ,the A-a gradient increases by 5-7 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 ) PAo2 = (Pb-47) × Fio2 – 1.25 x Etco2 EPao2-Pao2 Ld = ----------------- x 100 (new equation ) EPao2 A twelve year boy posted for laparatomy for intussusceptions under general anesthesia, during intubation the boy aspirated. In SICU Fio2 -1, Etco2- 32 ,Pao2- 78 ,the PAo2 expected is 673 EPao2 = 0.8 x 673 = 536 536-78 Ld = ---------------- 536 = 85 %

Here we conclude that the ABG reports are helpful to assess the lung damage in intensive care unit as well in pre operative period.