Rafael Frühbeck de Burgos, a beloved 80-year-old conductor with a long relationship with the National Symphony Orchestra, made it through Friday night’s concert, but just barely and by dint of superhuman effort. Near the end of a long and taxing program, the Spanish-born maestro seemed to totter, alarming the musicians, some of whom stopped playing. He was helped to a sitting position on the podium and continued to keep beating time and giving cues, his head hanging low and his arms barely visible above the first-stand musicians.

Frühbeck looked far more frail than he has during recent appearances with the NSO, for which he served as principal guest conductor from 1980 to 1988. He walked slowly onto and off the stage, and he moved cautiously up and down the steps of the conductor’s podium.

But there was no indication that he was ill until principal violist Daniel Foster rushed from the stage to a side door during the third movement of Respighi’s “Pines of Rome.” Shortly afterward, a violinist left the stage, and it was clear the musicians were concerned about the conductor.

“Pines of Rome” is a colorful showpiece for a large orchestra that lasts a little over 20 minutes. The last movement is a vigorous march that begins, according to Respighi’s program note, in the “misty dawn on the Appian Way.” It builds slowly but relentlessly to a thunderous, cymbal-crashing, organ-growling, brassy finale. Audiences adore its bombast and spectacle, and it always brings the house down.

It had been an evening of large, late 19th-century and early 20th-century music, made longer by the decision of a dynamic young pianist, Daniil Trifonov, to play an encore after his fiery performance of Rachmaninoff’s “Rhapsody on a Theme of Paganini.” That extended the concert past the usual 10 p.m. mark, but it also set a festive tone for the evening and the packed house.

Frühbeck was obviously determined not to deny the audience the rousing pleasure of Respighi’s potboiler. He grabbed at the metal rail behind him and stood even as the orchestra began to trail off, not sure whether he was well enough to continue. He obviously wasn’t, but he continued anyway, cuing the percussion and brass with a few sharp gestures above his head as he sat, slumped, on the podium. A few moments before the end of the piece, he stood up and conducted its last moments.

The audience response was thunderous. Frühbeck turned and faced them, gave a wan smile, then left the stage. When he returned to acknowledge the ovation, many of the musicians, half or a third his age, were in tears.

A spokeswoman for the NSO was not able to provide details about Frühbeck’s health but said that he planned to conduct the same program Saturday night.