Breathing pattern and perception at different levels of volume assist and pressure support in volunteers

Objective: Volume assist (VA) amplifies the breathing effort whereas pressu re support ventilation (PSV) provides a fixed, effort-independent ventilato ry support. According to the concept of VA, its level should compensate for the pathologically increased (additional) elastance (E-add), However, it i s unclear whether breathing subjects prefer an exact compensation of E-add and whether they are able to adjust the support level by themselves. Design: Prospective, interventional study. Setting: Laboratory. Subjects: Twelve healthy volunteers, nine females, three males, aged 21-33 yrs. Interventions: Increased E-add was generated by banding of the thorax and a bdomen. Volunteers breathed via a mouthpiece with VA or PSV using a positiv e end-expiratory pressure of 5 cm H2O (0.5 kPa), The study was subdivided i nto two parts. In part I, volunteers were instructed to adjust the level of VA and PSV themselves starting from three different, randomly applied leve ls in each mode (2, 8, 14 cm H2O or cm H2O/L; 0.2, 0.8, 1.4 kPa[/L]). In pa rt II, 20 levels of VA and PSV (1-20 cm H2O or cm H2O/L, 0.1-2 kPa[/L]) wer e randomly selected by an investigator and estimated by the volunteers usin g a visual analog scale. Additionally, the breathing pattern was characteri zed. Measurements and Main Results: E-add (7.1 +/- 1,5 cm H2O/L [0.7 +/- 0.2 kPa /L], mean a so) corresponded almost exactly to the "self-adjusted" VA level of part I (7.0 +/- 3.3 cm H2O/L [0.7 +/- 0.3 kPa/L]) and to the adequate l evel of part II (8-9 cm H2O/L [0.8-0.9 kPa/L]). The accordant PSV levels we re 5.7 +/- 2.6 cm H2O (0.6 +/- 0.3 kPa) and 6-7 cm H2O (0.6-0.7 kPa), The b reathing pattern was less influenced by changes of the support level with V A compared with PSV, which may explain in part the greater comfort of VA. Conclusions: We confirmed the theoretical assumption that VA should be adap ted to E-add. Furthermore, we demonstrated that conscious subjects are able to adjust the level of VA and PSV themselves.