highly significantp < 0.001
LTOT induced a systematic increase of arterial PaO 2 in all patients, and changes proved highly significant already within the first year of LTOT (from 52.1 mm Hg ± 6.6 sd to 65.1 mm Hg ± 8.7 sd, p < 0.001); then PaO 2 values remained unchanged over the remaining two years.