Amr El-Kahky, Al Jazeera's correspondent in Tripoli, said: "This is the final verdict, it cannot not be appealed.
"The only way out is a pardon from the Libyan government and it is expected this will happen later today."
Tarek Bazley, Al Jazeera's correspondent in Sofia, said: "This was no surprise. The family's hopes have been repeatedly dashed."
The six were convicted of infecting 438 children with HIV-tainted blood at a hospital in the Mediterranean city of Benghazi.
Fifty-six of the children have since died.
All six have maintained their innocence and say they confessed under duress.
Foreign health experts have cited poor hygiene as the probable cause of the epidemic in Benghazi, Libya's second city.
The families of the five nurses had demanded that the women be acquitted, saying that a new death sentence followed by an expected pardon later would not be justice for them.
A representative of the victims' families has said that a compromise deal would see the death penalty commuted to jail terms, which could be served in the medics' country of origin, as Libya and Bulgaria have an extradition treaty.
The doctor was recently granted a Bulgarian passport, meaning he could also benefit from such an arrangement.
George Bush, the US president, had urged Kadhafi in a letter delivered on Monday to help in the dispute over the fate of the medics, the White House said on Tuesday.
Bush told the Libyan leader that the case and lingering issues tied to the 1989 Lockerbie bombing needed his attention.