Regional tidal ventilation and compliance during a stepwise vital capacity manoeuvre

Abstract

To determine whether, during mechanical ventilation, an optimal positive end-expiratory pressure (PEEP) can be identified by measurement of regional tidal volume and compliance [V (T(reg)), C (RS(reg))]. Sixteen anaesthetized intubated neonatal piglets underwent a stepwise vital capacity manoeuvre performed during pressure control ventilation, with 5 cmH₂O PEEP increments to 25 cmH₂O, and decrements to 0 cmH₂O. Peak inflating pressure was 10 cmH₂O above PEEP throughout. The manoeuvre was performed in the normal lung, after repeated saline lavage and after surfactant therapy. Global V (T) and C (RS) were measured at the airway opening; V (T(reg)) and C (RS(reg)) were measured in the ventral, medial and dorsal lung using electrical impedance tomography (EIT). Most uniform distribution of regional tidal ventilation was noted during PEEP decrements after lung recruitment, at varying PEEP levels. In the lavaged and surfactant-treated lung the PEEP optimal for ventilation distribution was also associated with highest mean V (T(reg)) [lavaged: 95 ± 9.3% of maximum, mean ± standard deviation (SD); surfactant-treated: 92 ± 17%] and global V (T) (96 ± 10%; 96 ± 15%). Regional C (RS) plots clearly demonstrated co-existent ventral overdistension and dorsal recruitment, particularly during PEEP increments; whereas during PEEP decrements, peak C (RS(reg)) values showed considerable interregional concordance [e.g. peak C (RS(reg)) in the lavaged left lung; ventral: 0.017 ± 0.0036; medial: 0.016 ± 0.0054; dorsal: 0.017 ± 0.0073 cmH₂O⁻¹; P = 0.98, analysis of variance (ANOVA)]. After lung recruitment, a PEEP level can be identified by EIT at which tidal ventilation is uniformly distributed, with associated concordance in compliance between lung regions. Bedside monitoring of regional tidal ventilation and compliance using EIT may thus aid in PEEP selection.