Jim Sherman's resignation came as a surprise to the employees of Los Robles Hospital and Medical Center, spokesperson Kris Carraway-Bowman said.

Sherman, 48, was the president and CEO of Los Robles Hospital for 4½ years. Under his leadership the hospital added a $120-million expansion.

On Feb. 25, employees received an email from HCA Far West Division president Bryan Rogers with the subject, "Change in leadership." The communication told them Sherman had resigned and "will be exploring other opportunities."

Natalie Mussi, the current chief operation officer, was named as the interim CEO until a replacement is found.

"We lost the captain of the ship, but we're in good hands with Natalie Mussi, who is a remarkable person," Carraway-Bowman said.

Mussi probably won't be wearing two hats for long.

"We're searching very aggressively for a new CEO and hope to name one soon," CarrawayBowman said.

The new CEO will oversee another hospital expansion that, according to Carraway-Bowman, is necessary due to a state law that requires hospitals to seismically retrofit inpatient buildings by Jan. 1, 2013, so they can better withstand earthquakes.

"We were going to retrofit the original 1968 building but found that we must instead build a replacement building," CarrawayBowen said.

The city has accepted an application from the hospital to add a 200,000-square-foot building, a fourth story on the new wing and a new parking structure.

The original hospital area will be used for expanding outpatient services such as radiology and labs, Carraway-Bowman said.

Those departments have been using the same amount of space for 40 years, she said.