Ibsen became involved in the 1952 poliomyelitis outbreak in Denmark, where 2,722 patients developed the illness in a 6-month period[2] with 316 suffering respiratory or airway paralysis.

After detecting high levels of CO2 in blood samples and inside a little boy's lung,[3] Ibsen changed management directly.

He instituted protracted positive pressure ventilation by means of intubation into the trachea, and enlisting 200 medical students to manually pump oxygen and air into the patients lungs.

In 1953, Ibsen set up the world's first medical/surgical intensive care unit in a converted student nurse classroom in Kommunehospitalet (The Municipal Hospital) in Copenhagen, and provided one of the first accounts of the management of tetanus with muscle relaxants and controlled ventilation.

He jointly authored the first known account of ICU management principles in Nordisk Medicin, 18 September 1958: 'Arbejdet på en Anæsthesiologisk Observationsafdeling' ("The Work in an Anaesthesiologic Observation Unit") with Tone Dahl Kvittingen from Norway.