Context: There exists a deficiency in literature regarding the impact of
chest radiographic findings on PaCO2 ETCO2 gradient. Aim: To evaluate PaCO2  ETCO2 gradient and it’s correlation with chest radiographic findings, in mechanically ventilated patients. Materials and Methods: A prospective observational study was undertaken on mechanically ventilated adult patients, at an Indian Emergency department. Minimum sample size was 151, derived through a pilot study, with a power of 80% and 95% confidence. Arterial blood gas samples were collected, one each, from selected patients, analyzed and PaCO2 recorded. Highest ETCO2 values and chest radiographic
findings, also recorded simultaneously. Mean ETCO2 and PaCO2 values were compared with chest radiographic findings for radiopaque and radiolucent CXR groups. ETCO2 PaCO2 gradient was analyzed using unpaired Student test, and ETCO2 PaCO2 correlation using Pearson’s correlation coefficient. p value <0.05 was accepted as statistically significant. Results: Mean age of study group was 54.15 ± 17.714 years. Mean PaCO2was significantly elevated in the radiopaque CXR group (55.03 ± 25.70)mmHg , than radiolucent CXR group (39.46 ± 14.77)mmHg, p <0.001. Mean ETCO2 showed no significant variations between the two groups (p 0.261). ETCO2 PaCO2 gradient in
radiolucent CXR and radiopaque CXR respectively were (0.741 ± 5.77,
p<0.001) and (13.28 ± 17.93; p < 0.001). A more statistically significant and promising correlation between arterial PaCO2 and ETCO2 values were obtained in radiolucent CXR (r= 0.928, p< 0.001 ) as compared to radiopaque CXR (r=0.718, p<0.001). Conclusion: ETCO2 values can reliably predict PaCO2, in mechanically ventilated patients with radiolucent chest radiographs.