It is the latest in a string of health alerts for the former leader, who was jailed for massacres committed while he was in power in the 1990s.
His personal doctor Alejandro Aguinaga told AFP that Fujimori collapsed in prison in Lima and was rushed to a clinic.
"He suffered a severe rapid heart rate, a marked palpitation. His life was in danger but he is now stabilizing and the possibility that he suffered a heart attack has been ruled out."
Fujimori is serving a 25-year sentence for human rights abuses and corruption.
He was convicted in 2007 for his role in killings by a death squad targeting supposed guerrillas.
He has since been in and out of hospital with heart, back and stomach trouble and growths on his tongue, which has been operated on several times for cancer.
The courts have rejected efforts by his family to have him freed from prison.
Fujimori was president from 1990 to 2000.
Last year his daughter Keiko Fujimori narrowly lost a presidential election to Pedro Pablo Kuczynski.
Keiko Fujimori's populist Popular Force party nevertheless controls the single-chamber legislature.
Kuczynski has suggested he might consider a pardon for Fujimori, but denied any backroom deal with Popular Force.
Eyewitness? Submit your stories now via social or:
Email: news@pulselive.co.ke
