| Objective The aim of this prospective observational study was to evaluate the accuracy of dead-space fraction derived from Ventilator Volumetric Capnography(volumetric CO2) or prediction equation to predict survival in mechanically ventilated patients with acute respiratory distress syndrome(ARDS)Calculated intrapulmonary shunt is considered the best index of oxygenation,we aim to investigate the reference value of Qs/Qt in ARDS patients.Methods Serial VD/VT measurements were obtained based upon a prediction equation validated by Frankenfield DC et al[1] for dead space ventilation fraction: VD/VT= 0.320 + 0.0106(Pa CO2‐ETCO2) + 0.003(RR) + 0.0015(age)(where Pa CO2 is the arterial partial pressure of carbon dioxide in mm Hg;ETCO2,end-tidal carbon dioxide measurement in mm Hg; and RR, respiratory rate per minute; age in years)in adult patients who were with severe pneumonia resulted from infection and diagnosed with ARDS eventually. We calculated VD/VT with in 1 day of the point at which the patient with intubation after adjusting PEEP and Phigh then reaching a steady state,we repeated the VD/VT measurement on ARDS days 2,3, 4,5 and 6, and meanwhile collected dead-space fraction, denoted as Vd/Vt, from ventilator derived volumetric CO2. We analyzed the changes in VD/VT and Vd/Vt over the 6-day period to determine their accuracy of the prognosis about ARDS patients’ outcome.Daily Qs/Qt measurements were gained based upon a equation : Qs/Qt=(Cc O2-Ca O2)/(Cc O2-Cv O2)(where Cc O2 is End-capillary Oxygen Content, Ca O2, Arterial Oxygen Content;and Cv O2, Venous Oxygen Content)in adult patiens who were diagnosed with ARDS resulted from various reasons eventually,then we repeated the Qs/Qt measurement on ARDS days 2,3, 4,5,6 and 7,and analyzed the changes in Qs/Qt over the one week period to determine their accuracy of the prognosis about ARDS patients’ outcome.ResultsOverall, 24 of 46 patients with ARDS met enrollment criteria died. During the first 6 days of ARDS,VD/VT was significantly higher in non-survivors on day 4(0.70±0.01 vs 0.57±0.01,p=0.000), day 5( 0.73±0.01 vs 0.54±0.01, p=0.000), day 6(0.73±0.02 vs 0.54±0.01, p=0.000). Vd/Vt was not significantly higher in non-survivors on day 1(0.40±0.04 vs 0.39±0.06,p=0.075),day 2(0.43±0.03 vs 0.41±0.03, p=0.13), day 3(0.42±0.02 vs 0.41±0.03, p=0.291), day 4(0.41±0.03 vs 0.39±0.03, p=0.093) and significantly higher in non-survivors on day 5(0.45±0.04 vs 0.41±0.06, p=0.008) and day 6(0.47±0.05 vs 0.40±0.03, p=0.008).VD/VT has higher accuracy to predict survival than Vd/Vt on the fourth day,the area under the receiver-operating characteristic curve for VD/VT and Vd/Vt for evaluating the prognosis was day 4(0.974±0.093 vs 0.701±0.023, p=0.0024),95 percent confidence interval was day4(0.857~0.999 vs 0.525~0.841)(2) 20 of 46 patients with ARDS met enrollment criteria died. During the first 7 days of ARDS,the mean Qs/Qt in survivors and non-survivors were on 0h(0.23±0.06 VS 0.28±0.07,p=0.199)、day2(0.27±0.06 VS 0.30±0.03,p=0.09)、day3(0.28±0.09 VS 0.33±0.06,p=0.169)、day4(0.27±0.05 VS 0.33±0.11,p=0.065)、day5(0.25±0.04 VS 0.34±0.05,p=0.000)、day6(0.19±0.04 VS 0.33±0.06,p=0.000)、day7(0.16±0.02 VS 0.35±0.06,p=0.000),and Qs/Qt was significantly higher in non-survivors from the fifth day. The area under ROC curve(AUC) for Qs/Qt evaluating the prognosis on 5 day was 0.958, 95% confidence interval(95%CI) was 0.777~0.999(p<0.0001,Z=13.13),the cutoff value is 0.28。Conclusion(1)In ARDS, compared with Vd/Vt derived from Ventilator volumetric CO2, VD/VT calculated by a prediction equation constructed by Frankenfield DC et al1 estimated outcomes of patients with a higher prognostic value on day 4.(2)Clinically,Dynamic continuous Qs/Qt can help bedside physician to analyze the changes of the patient’s condition. Qs/Qt can be used as a key indicator of prognosis in patients with ARDS. |
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