A patient undergoing surgery suddenly develops hypotension. The monitor shows that the end tidal CO, has decreased abruptly by 155 mm Hg. What is the probable diagnosis -

Ans is 'b' i.e. pulmonary embolism First what is end tidal CO2 - The air exhaled during normal respiration initially consists of the dead space. Then a mixture of dead space air plus alveolar air comes out. The air coming out at the end consists purely of alveolar air. - Measurement of concentration of gases in this alveolar air is useful for analysis. The CO2 concentration in the last 10m1. of tidal volume is known as end tidal CO2 In pulmonary embolism the blood supply to ceain pas of the lung is cut off due to the emboli in circulation. This reduces the exchange of gases between the alveoli and pulmonary capillaries, as a result the conc. of CO2 in alveolar air decreases (CO2 is carried from tissues to the lung by blood and is transferred to the alveoli in exchange of 02). The decrease in alveolar CO2 will lead to decrease in end tidal CO2. Other conditions in which end tidal CO decreases (i) Increase in dead space (decrease in end tidal CO2 in pulmonary embolism can be explained by this also. Pulmonary embolism causes increase in physiological dead space hence decreases the end tidal CO2) (ii) Hyperventilation (iii) Malignant hypeherinia