In the immediate post operative period the common cause of respiratory insufficiency could be because of the following, except -

Ans is 'c' i.e. Mild hypovalemia The first two options are quite obvious causes of respiratory insufficiency (see the table at the end of this answer) The confusion is between Mild hypovolemia and myocardial infarction About mild hypovolemia Bailey writes that "Healthy volunteers could have 10-15 percent of their blood volume removed with no significant change in hea rate, blood pressure, cardiac output or blood flow to the splanchnic bed". - In mild hypovolemia, the systemic blood circulation is maintained by shifting the blood from splanchnic circulation to the systemic circulation. About Myocardial infarction - CSDT writes "Approx. 0.4% of all pts undergoing an op. in USA develop post operative MI. The incidence increases to 5 to 12% pts. undergoing operation for manifestation of atherosclerosis. Over half of post-operative MI are asymptomatic." The inference we could draw is Even though MI has a very low post op. incidence it can cause respiratory insufficiency. Mild hypervolemia on the other hand is well compensated by body and remains asymptomatic, thus we are in our of mild hypovolemia.