Comparison of the pulmonary dead-space fraction derived from ventilator volumetric capnography and a validated equation in the survival prediction of patients with acute respiratory distress syndrome

摘要

Purpose:This prospective observational study aims to evaluate the accuracy of dead-space fraction derived from the ventilator volumetric capnography (volumetric CO2) or a prediction equation to predict the survival of mechanically ventilated patients with acute respiratory distress syndrome (ARDS).Methods:Consecutive VD/VT measurements were obtained based upon a prediction equation validated by Frankenfield et al for dead-space ventilation fraction:VD/VT 0.320 + 0.0106 (PaCO2-ETCO2) + 0.003 (RR) + 0.0015 (age) in adult patients who had infection-related severe pneumonia and were confirmed as having ARDS.Here PaCO2 is the arterial partial pressure of carbon dioxide in mmHg;ETCO2,the endtidal carbon dioxide measurement in mmHg;RR,respiratory rate per minute;and age in years.Once the patient had intubation,positive end expiratory pressure was adjusted and after Phigh reached a steady state,VD/VT was measured and recorded as the data for the first day.VD/VT measurement was repeated on days 2,3,4,5 and 6.Meanwhile we collected dead-space fraction directly from the ventilator volumetric CO2 and recorded it as Vd/Vt.We analyzed the changes in VD/VT and Vd/Vt over the 6-day period to determine their accuracy in predicting the survival of ARDS patients.Results:Overall,46 patients with ARDS met the inclusion criteria and 24 of them died.During the first 6 days of intubation,VD/VT was significantly higher in nonsurvivors on day 4 (0.70 ± 0.01 vs 0.57 ± 0.01),day 5 (0.73 ± 0.01 vs.0.54 ± 0.01).and day 6 (0.73 ± 0.02 vs.0.54 ± 0.01) (all p =0.000).Vd/Vt showed no significant difference on days 1-4 but it was much higher in nonsurvivors on day 5 (0.45 ± 0.04 vs.0.41 ± 0.06) and day 6 (0.47 ± 0.05 vs.0.40 ± 0.03) (both p =0.008).VD/VT on the fourth day was more accurate to predict survival than Vd/Vt.The area under the receiver-operating characteristic curve for VD/VT and Vd/Vt in evaluating ARDS patients survival was day 4 (0.974 ± 0.093 vs.0.701 ± 0.023,p 0.0024) with the 95％ confidence interval being 0.857-0.999 vs.0.525-0.841.Conclusion:Compared with Vd/Vt derived from ventilator volumetric CO2,VD/VT on day 4 calculated by Frankenfield et al's equation can more accurately predict the survival of ARDS patients.