He was sentenced in 2008 to seven years in prison for attempted murder.
Investigators have exhumed 130 bodies for tests as part of the inquiry into the crimes.
Hoegel, 41, has already spent almost a decade in prison for other patient deaths, and is accused of intentionally administering medical overdoses to victims so he could bring them back to life at the last moment. "We will do our utmost to learn the truth", Bührmann said.
"I didn't expect it to happen today", he said.
It was then that Hoegel confessed to his psychiatrist at least 30 more murders were committed in Delmenhorst, prompting investigators look at suspicious deaths in Oldenburg.
Each time, the nurse followed a similar procedure; he would inject his patient with a medication triggering a cardiac arrest, which would follow an attempted resuscitation.
Police are said to believe the true number of his victims could be as high as 300.
Killing in itself was never his aim, according to one psychologist who evaluated him.
Fuerza Popular: "Ahora solos hemos blindado a Hinostroza"
Esta decisión fue aprobada con 18 votos de legisladores fujimoristas y de Juan Sheput , quien al final terminó cambiando su voto. " Ahora solos hemos blindado a Hinostroza ", escribió.
The choice of victim appears to have been entirely random, with their ages ranging from 34 to 96.
When he managed to revive a patient, he was sated, but only for a few days, the expert said, adding: "For him, it was like a drug".
Around 126 relatives of the victims are co-plaintiffs in the trial, which is expected to run until May next year, a court spokeswoman told CNN.
Additional convictions could make it harder for Hoegel to get parole. Known as Dr Death, Shipman was sentenced to 15 life terms in 2000; he died prison in 2004, apparently a suicide.
Oldenburg police chief Johann Kuehme past year said other medical workers at Oldenburg were aware of an elevated number of resuscitations, and initial indications of possible wrongdoing by the nurse in Delmenhorst emerged as early as April 2003.
Asked by the judge why he didn't admit to the remaining murders before now, Hoegel said it was "out of shame" and because it took him a long time to face up to what he had done at the first hospital.
Authorities are pursuing criminal cases against former staff at the two medical facilities.