They have performed a handful executions using nitrogen gas over the last few years in Mississippi and Alabama. From what I understand, the people have all shown signs of distress and oxygen hunger during the executions, and the autopsies show signs of distress.

I think the pathologist that reviewed the first such execution had said that it would likely have gone better if a sedative were administered beforehand. However, I'm pretty sure that nitrogen hypoxia executions were being used because pharmaceutical companies were unwilling to provide medications for use in legal injections, so that would likely extend to sedatives for use during executions. But in the absence of sedatives, the process is panic inducing, which causes people to resist inhaling the nitrogen, which in turn means that they are not exhaling as much carbon dioxide and thus experience the panic associated with suffocation.