An elderly giant panda at the San Diego Zoo has been diagnosed with an incurable heart problem.

The zoo says Gao Gao (Gow Gow) had a medical checkup Tuesday and was found to have pulmonic stenosis, a narrowing of the pulmonary heart valve.

Gao Gao was put on medication and is responding well to the treatment, but the condition is progressive. It's most commonly the result of a birth defect.

The zoo says symptoms can include a and fatigue. Gao was diagnosed with a heart murmur three years ago.

The panda, who's about 26 years old, also has .

Gao Gao and two other pandas are on loan to the zoo from China. Only 1,600 are believed to remain in the wild.