Submitted on May 22, 2008
Revised on August 13, 2008
Accepted on September 17, 2008
Effects of inspiratory pause on CO2 elimination and arterial PCO2 in acute lung injury
Jérôme Devaquet1, Bjorn Jonson2*, Lisbet Niklason3, Anne-Gaëlle Si Larbi1, Leif Uttman3, Jérôme Aboab1, and Laurent J. Brochard4
1 INSERM Unit 841, Université Paris 12
2 Lund Univerity
3 University Hospital
4 Hopital Henri Mondor
* To whom correspondence should be addressed. E-mail: bjorn.jonson{at}med.lu.se.
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 post-inspiratory pause could enhance CO2 elimination and reduce PaCO2 by reducing dead space in ALI. In 15 mechanically ventilated patients with ALI and hypercapnia, a 20% post-inspiratory pause (Tp20) was introduced between two ventilation periods without post-inspiratory pause (Tp0). Other parameters were kept unchanged. The single breath test for CO2 was recorded every 5 minutes to measure tidal CO2 elimination (VtCO2), airway dead space (VDaw) and slope of the alveolar plateau. PaO2, PaCO2, physiological and alveolar dead space (VDphys, VDalv) were determined at the end of each 30 minute period. The post-inspiratory pause, 0.7±0.2 s, induced on average less than 0.5 cm H2O of intrinsic PEEP. During Tp20, VtCO2 increased immediately by 28±10% (14±5 ml per breath compared to 11±4 for Tp0) and then decreased without reaching the initial value within 30 minutes. The addition of a post-inspiratory pause decreased significantly VDaw by 18±6% and VDphys by 11±4% with no change in VDalv. During Tp20, the slope of alveolar plateau initially fell to 65±10 % of baseline value and continued to decrease. Tp20 induced a 10±3% decrease in PaCO2 at 30 minutes (from 55±10 to 49±9 mmHg, p<0.001) with no significant variation in PaO2. Post-inspiratory pause has a significant influence on CO2 elimination when small tidal volumes are used during mechanical ventilation for ALI.
