shallow breathing as a result of rapid heart rate is to he expected, but that wouldn't cause a sense of depleted oxygen. The cause of death is hypoxia, but in theory one should los consciousness due to low o2 saturation well before respiratory depression begins. The feeling of suffocation would come from carbon dioxide build up, which is not what is happening with SN. Your body still thinks it can breathe till it really can't, but your brain needs a lot of oxygen to keep you conscious.