A respiratory series was a treatment for individuals who had undergone severe damage to their lungs. In 2371, while trying to restore Neelix's breathing after his lungs were stolen by the Vidiians, The Doctor asked his medical assistant Lieutenant Tom Paris if he had been taught how to perform a respiratory series during his two semesters of biochemistry at Starfleet Academy. Paris replied that he hadn't, so The Doctor did it on Neelix himself. (VOY: "Phage")