Respiratory system compliance and prone positioning. Using the change in Pao2/Fio2 ratio pre- and post-prone positioning ≥20% as the response criteria, 26 subjects met the criteria in the first prone positioning session (n=36) whereas 13 and 11 subjects responded in the second (n=27) and third (n=20) prone positioning sessions, respectively. Responders’ respiratory system compliance before each prone positioning in the three sessions was similar to non-responders. Cst, compliance of respiratory system.