As acute breathlessness has been recognized as an important medical problem, multiple attempts has been made to improve the management of such a condition. Several researches have been done to show the correlation between ETCO2 and PaCO2 especially in intubated patients. For more than 40 years since it was first used, several attempts has been made to recognize the utility and accuracy of capnometry use in patients with respiratory conditions. Microstream capnometer has a potential to predict PaCO2 level especially in the case of pure acidosis and hypocapnia. It can be a potential form of non invasive cardiopulmonary monitoring in non-intubated acute breathlessness patient. However the use of ETCO2 to predict PaCO2 should be done with caution especially in cases that involve pulmonary disorders and acid base imbalance.