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With adaptive support ventilation (ASV), a microprocessor-controlled mode of mechanical ventilation, the ventilator adapts tidal volume (VT) size based on the Otis least work of breathing formula. In recent studies in patients with ALI/ARDS, ASV applied VT of 7.3 (6.7 to 8.8) ml/kg ideal body weight (IBW). It is unclear whether an open-lung approach was used in these studies. Lung recruitment improves lung compliance, and as a consequence may allow the ventilator to apply too large a VT with ASV.
Ten consecutive patients with ALI/ARDS, ventilated in accordance with our local protocol dictating frequent recruitment maneuvers, were observed while the ventilator was switched from pressure control ventilation (PC) to ASV. Thereafter, all patients were subjected to an additional standard recruitment procedure. The primary endpoint was VT before and after switch of the ventilator, and after standard recruitment.
Four patients suffered from ALI, six patients from ARDS. Seven patients had an extrapulmonary cause for ALI/ARDS. VT increased from 6.5 ± 0.8 ml/kg IBW to 9.0 ± 1.6 ml/kg IBW (P < 0.01) after switch from PC to ASV. Additional recruitment after switch of the ventilator did not affect VT size (9.3 ± 1.4 ml/kg IBW, P > 0.05). In seven patients ASV applied VT >8 ml/kg IBW, in one patient VT even increased to >12 ml/kg IBW.
Patients with ALI/ARDS may be ventilated with too large a VT when subjected to ASV. Our results contrast findings of previous studies on ASV in patients with ALI/ARDS, probably because we frequently use recruitment maneuvers.