Choice A reason: Administering heparin to the client is an important intervention, but it is not the first action the nurse should take. Heparin is an anticoagulant that prevents further clot formation, but it does not dissolve existing clots or improve oxygenation.

Choice B reason: Encouraging the client to cough and deep breathe is a helpful measure to promote lung expansion and expectoration of secretions, but it is not the first action the nurse should take. Coughing and deep breathing may also increase the client's dyspnea and anxiety.

Choice C reason: Obtaining the client's vital signs is a necessary step to monitor the client's condition, but it is not the first action the nurse should take. Vital signs may indicate hypoxia, tachycardia, hypotension, or other signs of shock, but they do not address the immediate problem of impaired gas exchange.

Choice D reason: Placing the client in high-Fowler's position is the first action the nurse should take. This position allows for maximum lung expansion and improves oxygenation. It also reduces venous return and cardiac workload, which may decrease pulmonary artery pressure and right ventricular strain.