A former nurse who liked to put patients into cardiac arrest because he enjoyed the feeling of being able to resuscitate them has been convicted of 85 counts of murder in a court in northwestern Germany.

The Oldenburg court on Thursday sentenced 42-year-old Niels Hoegel to life in prison and noted the "particular seriousness of the crimes" to ensure he serves beyond the standard 15-year term, the dpa news agency reported.

The killings took place while Hoegel worked as a nurse at two hospitals in northwestern Germany from 1999 to 2005.

Prosecutors said he attacked patients in order to impress colleagues by subsequently trying to revive them.

A former colleague told a German newspaper that Hoegel's nickname was "Resuscitation Rambo" because of the way he "pushed everyone else aside" when patients needed to be resuscitated.

In all Hoegel was charged with 100 counts of murder in the northern cities of Delmenhorst and Oldenburg. Police believe he may have killed far more but the cremation of bodies had destroyed any possible evidence.

Hoegel had confessed to 55 murders and the court in Oldenburg convicted him of 85.

He is believed to be the most prolific killer in Germany's modern history.

Judge Buehrmann expressed his regret that the court had not been able to "lift the fog" for many grieving relatives when he delivered the sentence.

In a closing statement Wednesday, Hoegel expressed regret, saying he realized how much pain and suffering he'd caused with his "terrible deeds."

Hoegel has already spent a decade in prison following a previous life sentence he received for six other murders.

In 2005, Hoegel was caught in the act of administering unprescribed medication to a patient and he was sentenced to seven years for the attempted murder in 2008, but the families of his other suspected victims pressed for a further investigation.

At a second trial that ended in 2015 he was jailed for life for two murders and two attempted murders.

However, during that trial he confessed to a psychiatrist that he had killed up to 30 people.

Investigators then widened the investigation, exhuming 130 former patients and looking for evidence of medication that could have triggered cardiac arrest. They also pored over records in the hospitals where he worked.

Records at the Oldenburg hospital showed rates of deaths and resuscitations had more than doubled when Hoegel was on shift, German media reported.

The case has shocked Germany, not least because senior staff at the two hospitals are accused of having turned a blind eye to unusually high mortality rates.

The exhumation and autopsy of more than 130 bodies were necessary to build the case for the prosecution.

Police suspect that Hoegel's final death toll may be more than 200.

But the court was unable to say for sure because of gaps in Hoegel's memory and many likely victims were cremated before autopsies could be performed.

(With input from agencies)