Prone position reduces lung stress and strain in severe acute respiratory distress syndrome

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Abstract
The present authors hypothesised that in severe acute respiratory distress syndrome (ARDS), pronation may reduce ventilator-induced overall stress (i.e.transpulmonary pressure (PL)) and strain of lung parenchyma (i.e.tidal volume (VT)/end-expiratory lung volume (EELV) ratio), which constitute major ventilator-induced lung injury determinants. The authors sought to determine whether potential pronation benefits are maintained in post-prone semirecumbent (SRPP) posture under pressure-volume curve-dependent optimisation of positive end-expiratory pressure (PEEP).A total of 10 anesthetised/paralysed, mechanically ventilated (VT = 9.0±0.9 mL·kg−1predicted body weight; flow = 0.91±0.04 L·s−1; PEEP = 9.4±1.3 cmH2O) patients with early/severe ARDS were studied in pre-prone semirecumbent (SRBAS), prone, and SRPPpositions. Partitioned respiratory mechanics were determined during iso-flow (0.91 L·s−1) experiments (VTvaried within 0.2–1.0 L), along with haemodynamics, gas exchange, and EELV.Compared with SRBAS, pronation/SRPPresulted in reduced peak/plateauPLatVTs≥0.6 L; static lung elastance and additional lung resistance decreased and chest wall elastance (in prone position) increased; EELV increased (23–33%);VT/EELV decreased (27–33%); arterial oxygen tension/inspiratory oxygen fraction and arterial carbon dioxide tension improved (21–43/10–14%, respectively), and shunt fraction/physiological dead space decreased (21–50/20–47%, respectively).In early/severe acute respiratory distress syndrome, pronation under positive end-expiratory pressure optimisation may reduce ventilator-induced lung injury risk. Pronation benefits may be maintained in post-prone semirecumbent position.