Longitudinal PEEP Responses Differ Between Children With ARDS and at Risk for ARDS

It is unknown whether lung mechanics differ between patients with pediatric ARDS and at risk for ARDS. We aimed to examine the hypothesis that, compared to ARDS, subjects at risk of ARDS are characterized by higher end-expiratory lung volume (EELV) or respiratory system compliance (C_RS) and lower distending pressure (stress) applied on the lung or parenchymal deformation (strain) during mechanical ventilation.

Consecutively admitted subjects fulfilling the PALICC ARDS criteria were considered eligible for inclusion in this study. A ventilator with an integrated gas exchange module was used to calculate EELV, C_RS, strain, and stress after a steady state had been achieved based on nitrogen washout/washin technique. All subjects were subjected to incremental PEEP trials at 0, 6, 12, 24, 48, and 72 h.

A total of 896 measurements were longitudinally calculated in 32 mechanically ventilated subjects (n = 15 subjects with ARDS; n = 17 subjects at risk for ARDS). EELV correlated positively with strain or stress in the ARDS group (r = 0.30, P < .001) and the at risk group (r = 0.60, P < .001). C_RS correlated with strain (r = 0.40, P < .001) only in subjects at risk for ARDS. EELV increased over time as PEEP rose from 4 to 10 cm H₂O in subjects with ARDS (P = .001). In the at risk group, EELV only increased at 48 h (P = .001). Longitudinally, C_RS (P = .001) and EELV (P = .002) were lower and strain and stress were higher in subjects with ARDS compared to those at risk for ARDS (P = .002), remaining within safe limits. Strain and stress increased by 24 h but declined by 72 h in subjects with ARDS at a PEEP of 4 cm H₂O (P = .02). In the at risk group, strain and stress declined from 6 h to 72 h at a PEEP of 10 cm H₂O (P = .001).

Longitudinally, C_RS and EELV were lower and strain and stress were higher in subjects with ARDS compared to subjects at risk for ARDS. These parameters behaved differently over time at PEEP values of 4 or 10 cm H₂O. At these PEEP levels, strain and stress remained within safe limits in both groups.