A new trial has been ordered for a man serving a life sentence for ramming his truck into a crowd outside an Alberta bar, killing one teenager and badly injuring another.

Jeffrey Leinen admitted to driving the truck that ran over and killed Nicholas Baier, 18, in Olds in October 2010. His lawyer argued that he stepped on the gas pedal in an involuntary panic response.

But Leinen was found guilty of second-degree murder and aggravated assault and sentenced to life in prison with no chance of parole for 14 years.

In a decision released Monday, two out of three judges on an Alberta Court of Appeal panel agreed to grant a new trial based on the lower court judge's error in failing to properly instruct the jury on the legal significance of the panic attack defence.

"I guess I'm not surprised. I'm glad to hear that the Court of Appeal found there was some basis for it and we'll see what happens at a new trial," said Andre Ouellette, who was Leinen's lawyer during his trial, but wasn't involved in the appeal.

Started with bar fight

According to the appeal documents, Leinen was drinking in the Texas Mickey Bar when he got into a fight. Bouncers took him outside, where he got into a further scuffle.

"He then got into his truck with a passenger. Someone hit the back of the truck from outside and uttered obscenities or threats. The appellant accelerated the truck toward a crowd that had gathered outside the bar. He hit two people, killing one and injuring the other," the judges wrote.

"His defence was that his acceleration was an involuntary panic response. Alternatively, he did not intend to kill or injure anyone, and what happened was an accident."

During the trial, the jury heard from a neuropsychologist and a forensic psychiatrist, who both testified about the nature of panic responses and how the human brain responds to fearful stimuli.

"We are not persuaded that we can classify as harmless the failure to point out the legal implications of the panic response defence. The appellant was entitled to a properly instructed jury," the judges wrote. "The matters on which the charge was deficient went to each of the offences on the indictment. This error may have had an impact on the verdict."