The Science Journal of the American Association for Respiratory Care

2010 OPEN FORUM Abstracts


Lara Brewer, Matthias Görges, Joseph Orr; Bioengineering, University of Utah, Salt Lake City, UT

Background: The ratio of physiologic dead space (Vd) to tidal volume (Vt), Vd/Vt, has been identified as a pulmonary-specific predictor of mortality for patients with early Acute Respiratory Distress Syndrome. 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 the mixed expired CO2. The two components of Vd are airway dead space (Vdaw) and alveolar dead space (Vdalv). When Vt is altered, the measured Vd/Vt changes the ratio of airway dead space to Vt, which alters PeCO2. However, it is not likely the fraction of unperfused alveoli, termed alveolar dead space fraction (Vdalv/Vtalv), changes when tidal volume is adjusted. It may be possible to update Vd/Vt and Vdalv calculations based on each newly measured Vt as long as lung perfusion status has not changed significantly. The first aim of this study was to compare the changes measured in Vd/Vt and Vdalv/Vtalv subsequent to a change in Vt. The second aim was to test whether Vd/Vt could be accurately updated with each Vt. Methods: Five swine (30-33.8 kg) were ventilated with FiO2 of > 0.5, 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 draws. The NM3 volumetric capnometry monitor (Philips Respironics, Wallingford, CT) recorded PeCO2, Vdalv, Vtalv, and Vd/Vt. Two Vt settings were used. Each time the Vt was altered, RR was adjusted to maintain the original alveolar minute ventilation. Stabilization was allowed for at least an hour, followed by ABG draw. Breath-by-breath updates to Vd/Vt were calculated as: Vd/Vt = (Vdalvold + Vdaw)/Vt, where Vdalvold = (Vdalv/Vtalv)old * Vtalvcurrent. Vdalv/Vtalvold was calculated using original PaCO2. Results: Subsequent to Vt reduction, ABG-based Vd/Vt increased by 0.1 ± 0.1 (n=9). Vdalv/Vtalv increased slightly by 0.008 ± 0.037. Linear regression analysis of the breath-by-breath Vd/Vt and the ABG-based Vd/Vt after the Vt reduction gave r2 of 0.88. The difference between the two measurements was 0.003 ± 0.028 (0.48% ± 5.46%). Conclusions: As expected, a change in Vt led to observed changes in Vd/Vt but not to significant changes in Vdalv/Vtalv. This is because without gross disturbances in pulmonary perfusion, the main determinant of the changes in Vd/Vt is Vt. Updating the displayed Vd/Vt based on current Vt and Vdaw may be preferable to fixing the value until the next ABG draw. Sponsored Research - Philips-Respironics