Fig. 1

Schematic diagram of the research hypothesis. CVP1 and CVP2 increased from baseline values when PEEP was elevated from initial PEEP; however, ICP1 and ICP2 remained unchanged in the beginning. Thus, GAP1 and GAP2 were narrowed gradually until they disappeared when PEEP reached PEEP1 and PEEP2 (dark grey and light grey shades). PEEP1 and PEEP2 were critical pressure values (GAP1 = 0 and GAP2 = 0) for patient A and patient B, respectively; thereafter, CVP1 at adjusted-PEEP1 would exceed baseline ICP, which would contribute to an elevation of ICP1, and likewise for CVP2 at adjusted-PEEP2. CVP and ICP at initial PEEP were termed baseline ICP and baseline CVP, respectively. ICP1 and CVP1 represented intracranial pressure and central venous pressure at different levels of PEEP for patient A. ICP2 and CVP2 referred to intracranial pressure and central venous pressure for patient B. GAP1 and GAP2 were calculated by ICP1-CVP1 and ICP2-CVP2. Abbreviations: CVP, central venous pressure; PEEP, positive end-expiratory pressure; ICP: intracranial pressure; P_ICGap: Gap between baseline ICP and baseline CVP