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

¹ University of Athens Medical School, Dept of Intensive Care Medicine, Attikon University Hospital, and ² University of Athens Medical School, Dept of Intensive Care Medicine, Evaggelismos General Hospital, Athens, Greece

CORRESPONDENCE: S. D. Mentzelopoulos, 12 Ioustinianou Street, GR-11473, Athens, Greece. Fax: 30 2103218493. E-mail: sdm@hol.gr

Keywords: Acute respiratory distress syndrome, gas exchange, lung recruitment, mechanical ventilation, mechanics of the respiratory system, prone position

Received: September 9, 2004
Accepted November 19, 2004

The present authors hypothesised that in severe acute respiratory distress syndrome (ARDS), pronation may reduce ventilator-induced overall stress (i.e. transpulmonary pressure (P_L)) and strain of lung parenchyma (i.e. tidal volume (V_T)/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 (SR_PP) posture under pressure-volume curve-dependent optimisation of positive end-expiratory pressure (PEEP).

A total of 10 anesthetised/paralysed, mechanically ventilated (V_T = 9.0±0.9 mL·kg^–1 predicted 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 (SR_BAS), prone, and SR_PP positions. Partitioned respiratory mechanics were determined during iso-flow (0.91 L·s^–1) experiments (V_T varied within 0.2–1.0 L), along with haemodynamics, gas exchange, and EELV.

Compared with SR_BAS, pronation/SR_PP resulted in reduced peak/plateau P_L at V_Ts0.6 L; static lung elastance and additional lung resistance decreased and chest wall elastance (in prone position) increased; EELV increased (23–33%); V_T/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.

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