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Effects of inspiratory pause on CO₂ elimination and arterial PCO₂ in acute lung injury

¹Medical Intensive Care Unit, AP-HP, INSERM Unit 841, Centre Hospitalier Albert Chenevier-Henri Mondor, Créteil, France; ²Department of Clinical Physiology, University Hospital, Lund, Sweden

Submitted 22 May 2008 ; accepted in final form 17 September 2008

A high respiratory rate associated with the use of small tidal volumes, recommended for acute lung injury (ALI), shortens time for gas diffusion in the alveoli. This may decrease CO₂ elimination. We hypothesized that a postinspiratory pause could enhance CO₂ elimination and reduce Pa_CO2 by reducing dead space in ALI. In 15 mechanically ventilated patients with ALI and hypercapnia, a 20% postinspiratory pause (Tp20) was applied during a period of 30 min between two ventilation periods without postinspiratory pause (Tp0). Other parameters were kept unchanged. The single breath test for CO₂ was recorded every 5 min to measure tidal CO₂ elimination (VtCO₂), airway dead space (V_Daw), and slope of the alveolar plateau. Pa_O2, Pa_CO2, and physiological and alveolar dead space (V_Dphys, V_Dalv) were determined at the end of each 30-min period. The postinspiratory pause, 0.7 ± 0.2 s, induced on average <0.5 cmH₂O of intrinsic positive end-expiratory pressure (PEEP). During Tp20, VtCO₂ increased immediately by 28 ± 10% (14 ± 5 ml per breath compared with 11 ± 4 for Tp0) and then decreased without reaching the initial value within 30 min. The addition of a postinspiratory pause significantly decreased V_Daw by 14% and V_Dphys by 11% with no change in V_Dalv. During Tp20, the slope of the alveolar plateau initially fell to 65 ± 10% of baseline value and continued to decrease. Tp20 induced a 10 ± 3% decrease in Pa_CO2 at 30 min (from 55 ± 10 to 49 ± 9 mmHg, P < 0.001) with no significant variation in Pa_O2. Postinspiratory pause has a significant influence on CO₂ elimination when small tidal volumes are used during mechanical ventilation for ALI.

gas exchange; dead space; mechanical ventilation; ARDS