The effect of lung expansion and positive end-expiratory pressure on anesthetized children

BACKGROUND: Imaging studies have shown that general anesthesia in children results in atelectasis. Lung recruitment total lung capacity (TLC) maneuvers plus positive end-expiratory pressure (PEEP) are effective in preventing atelectasis. However, physiological changes in children during general anesthesia have not been elucidated. METHODS: In eight anesthetized and mechanically ventilated children (median age: 3.5 years; range: 2.3-6.5), we measured static respiratory system elastance (E,,), flow resistance (R-int), and elastance and resistance components resulting from tissue viscoelasticity (Delta E and Delta R, respectively) using the constant inflow, end-inspiratory occlusion method preceded by TLC maneuvers, both with zero PEEP (ZEEP) and PEEP (5 cm H,C) for comparison. RESULTS: With constant inspiratory flow (VI) and ZEEP, increases in end-inspiratory lung volume above relaxation volume (tidal volume V) from 8 to 20 mL.kg(-1),, resulted in decreases in E,, from 1.06 to 0.82 cm H2O.mL(-1).kg, Delta E from 0.16 to 0.09, and Rint from 0.13 to 0.11 cm H2O.mL(-1).s.kg, whereas Delta R increased from 0.08 to 0.12 (P < 0.05). Similar relationships were found with PEEP. Increases in V, (8 to 26 mL.s(-1).kg) with constant V-T and ZEEP resulted in decreases in E-st from 1.09 to 0.9 and Delta R from 0.17 to 0.06 (P < 0.01), whereas Delta E and R-int did not change. There was a similar flow and volume dependence of elastance and resistance with PEEP. CONCLUSIONS: The observed steady decreases in E-st with increasing V, (up to 16 mL/kg with PEEP) indicate marked reductions in end-expiratory relaxation volume (functional residual capacity) even with PEEP. Similarity in results with ZEEP and PEEP suggests that TLC-maneuvers and O-2-N-2 ventilation prevented airway closure throughout the study