Rafael Frühbeck de Burgos, the 80-year-old Spanish-born maestro who was scheduled to conduct the National Symphony Orchestra for three performances of Rachmaninoff and Respighi, withdrew from Saturday night’s performance because of health concerns.

National Symphony Orchestra spokeswoman Patricia O’Kelly said the maestro has a fever and symptoms of pneumonia and is under a doctor’s care. NSO assistant conductor Ankush Kumar Bahl led the orchestra in his place on Saturday.

Frühbeck’s poor health caused worry during Friday night’s performance when he “seemed to totter, alarming the musicians, some of whom stopped playing,” during Respighi’s “Pines of Rome,” according to Washington Post critic Philip Kennicott. Kennicott noted that the maestro, who served as principal guest conductor of the NSO from 1980 to 1988, looked more frail than in other recent appearances with the NSO.

The musicians helped Frühbeck to a sitting position on the podium so that he could finish conducting the concert. Though visibly unwell, he stood for the final moments of the concert and acknowledged a heartfelt standing ovation.

The Kennedy Center noted that Bahl was in orchestra practices all week acting as “a pair of ears” in the house. Had Frühbeck needed to leave the stage during Friday’s performance, Bahl was prepared to finish the program.

Rafael Frühbeck de Burgos (Morten Abrahamsen)