The Science Journal of the American Association for Respiratory Care

2012 OPEN FORUM Abstracts


Lara Brewer, Kyle M. Burk, Simon A. Rodriguez, Joseph A. Orr; University of Utah Health Sciences Center, Salt Lake City, UT

Background: The ratio of physiologic dead space (Vd) to tidal volume (Vt), also called dead space fraction (Vd/Vt) is calculated using the Bohr-Enghoff equation: Vd/Vt = (PaCO2-PeCO2)/PaCO2, where PaCO2 is arterial partial pressure of CO2 and PeCO2 is mixed expired CO2. Since high Vd/Vt has been identified as a pulmonary-specific predictor of mortality for patients with early ARDS, it would be valuable to know whether Vd/Vt has changed without requiring a new arterial blood gas (ABG) measurement for every Vd/Vt estimation. In prior work, we found evidence that in healthy lungs, the Vd/Vt can be updated based on breath-by-breath tidal volume (Vt) and airway deadspace (Vdaw) measurements for up to an hour. The aim of this study was to evaluate whether Vd/Vt can also be updated without new ABG information during an acute lung injury which models ARDS. Methods: Five male swine (27-30 kg) were ventilated with FiO2 of > 0.4, Vt of 12 mL/kg, and I:E time ratio of 1:2; RR was adjusted to maintain etCO2 of 35-40 mmHg. An arterial cannula provided continuous BP and periodic ABG samples. The NM3 volumetric capnometry monitor (Philips Respironics, Wallingford, CT) recorded PeCO2, Vdalv, Vtalv, and Vd/Vt. An acute lung injury was induced by intravenous injection of 0.8 mL/kg oleic acid. Two Vt and two RR settings were used to create four ventilation levels during which ABG was sampled. ABG was sampled at 5, 15, and 30 minutes after each ventilator settings change. Breath-by-breath updates to Vd/Vt were calculated as: Vd/Vt = (Vdalv_old + Vdaw)/ Vt, where Vdalv_old = (Vdalv/Vtalv)old * Vtalv_current. The (Vdalv/Vtalv)old was calculated using the old PaCO2. Vd/Vt updated from old ABG information was compared to Vd/Vt from current ABGs. Results: During lung injury Vd/Vt calculated using old ABG information and current ventilation parameters was accurate compared to Vd/Vt calculated from current ABG. Linear regression analysis of the Vd/Vts gave r2 of 0.84. The difference between the two measurements was -0.02 ± 0.03 (-3.7% ± 4.8%). For both healthy and injured lungs, the difference between the two measurements was -0.007 ± 0.03 (-1.1% ± 5.3%). Conclusion: This study provides additional evidence that updating the displayed Vd/Vt based on old ABG and current Vt and Vdaw may be preferable to fixing the value until the next ABG sample, even during lung injury. We plan future evaluation of breath-by-breath updates to Vd/Vt for longer periods. Sponsored Research - Philips Medical