Failed weaning attempt

A 76-year-old man with a history of chronic obstructive pulmonary disease has been on ventilatory support for 4 days, since he had an acute myocardial infarction. The ventilator settings are VT$$700\mathrm{mL};\mathrm{SIMV}$$rate 8 breaths/min;FiO2​0.5;PEEP5 cmH2​O. ABG results on these settings are:pH7.37,PaCO2​36 mmHg,PaO2​78,SpO293%. The patient currently meets all criteria for weaning and is placed on a t-piece. Within 10 minutes he develops restlessness, tachycardia, rapid, shallow breathing patterns, and diaphoresis. TheSpO2​drops from$$93\%$$to$$90\%$$, and the pulmonary artery occlusion pressure rises from 12 to$$17\mathrm{mm}\mathrm{Hg}$$. The patient does not complain of chest pain and has no dysrhythmias. What do you think is responsible for the failed weaning attempt? What changes would you make for the next attempt at weaning?