Regional tidal ventilation and compliance during a stepwise vital capacity manoeuvre
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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.
KeywordsTidal volume Ventilation distribution Lung compliance Electrical impedance tomography
We thank Dr. Denis Morel, Anesthesiological Investigation Unit, University Hospital of Geneva for his contribution to these studies. This work was supported by a restricted research grant by Viasys Healthcare, and the surfactant (Curosurf) was generously provided by Nycomed.
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