The city added a second ambulance to East Boston after wait times for top-priority calls there spiked to 8 minutes, a full 2 minutes more than the department’s target for critical emergencies.Boston EMS chief Jim Hooley said the new ambulance has been staffed day and evenings since March 12.Call times in East Boston for reports of heart attacks, difficulty breathing, “man down” or other critical incidents had jumped to 8 minutes, while the goal for the top-priority incidents is 6 minutes, Hooley said.