Paradoxical response to chest wall loading predicts a favorable mechanical response to reduction in tidal volume or PEEP

Abstract

AbstractBackgroundChest wall loading has been shown to paradoxically improve respiratory system compliance (CRS) in patients with moderate to severe acute respiratory distress syndrome (ARDS). The most likely, albeit unconfirmed, mechanism is relief of end-tidal overdistension in ‘baby lungs’ of low-capacity. The purpose of this study was to define how small changes of tidal volume (VT) and positive end-expiratory pressure (PEEP) affect CRS(and its associated airway pressures) in patients with ARDS who demonstrate a paradoxical response to chest wall loading. We hypothesized that small reductions of VTor PEEP would alleviate overdistension and favorably affect CRSand conversely, that small increases of VTor PEEP would worsen CRS.MethodsProspective, multi-center physiologic study of seventeen patients with moderate to severe ARDS who demonstrated paradoxical responses to chest wall loading. All patients received mechanical ventilation in volume control mode and were passively ventilated. Airway pressures were measured before and after decreasing/increasing VTby 1 ml/kg predicted body weight and decreasing/increasing PEEP by 2.5 cmH2O.ResultsDecreasing either VTor PEEP improved CRSin all patients. Driving pressure (DP) decreased by a mean of 4.9 cmH2O (supine) and by 4.3 cmH2O (prone) after decreasing VT, and by a mean of 2.9 cmH2O (supine) and 2.2 cmH2O (prone) after decreasing PEEP. CRSincreased by a mean of 3.1 ml/cmH2O (supine) and by 2.5 ml/cmH2O (prone) after decreasing VT.CRSincreased by a mean of 5.2 ml/cmH2O (supine) and 3.6 ml/cmH2O (prone) after decreasing PEEP (P < 0.01 for all). Small increments of either VTor PEEP worsened CRSin the majority of patients.ConclusionPatients with a paradoxical response to chest wall loading demonstrate uniform improvement in both DP and CRSfollowing a reduction in either VTor PEEP, findings in keeping with prior evidence suggesting its presence is a sign of end-tidal overdistension. The presence of ‘paradox’ should prompt re-evaluation of modifiable determinants of end-tidal overdistension, including VT, PEEP, and body position.