Closing Capacity and Gas Exchange in Chronic Heart Failure: Statistical Analysis

Closing Capacity and Gas Exchange in Chronic Heart Failure: Statistical AnalysisBreathing pattern and mouth occlusion pressure 100 ms after onset of inspiratory effort (P0.1), maximal inspiratory pressure (Plmax), CV, CC, and alveolar plateau slope (AN2) were measured (VMAX 229; SensorMedics), as previously described. The Pimax was measured at RV according to American Thoracic Society/ERS criteria with predicted values obtained from Black and Hyatt.
The CV and AN2 were measured in triplicate by a single-breath N2 test with the mean taken as the final value. The CV was expressed in liters or as the percentage of VC measured during the single-breath exhalation. By adding RV to CV, the CC (in liters) was obtained and was also expressed as the percentage of total lung capacity (TLC) [ie, CC/TLC ratio]. Predicted CV/VC and CC/TLC ratios were obtained from Buist and Ross. From these predicted ratios, the predicted values of CV and CC (in liters) were computed using predicted values of VC and TLC, respectively. other
Tidal FL was assessed with the negative expiratory pressure (NEP) technique. A NEP of —5 cm H2O was applied (Direc/ NEP System 200A; Raytech Instruments; Vancouver, BC, Canada) 0.2 s after the onset of expiration. Flow-volume curves obtained without and with NEP were superimposed; patients in whom the expiratory flow with NEP was the same as the reference flow during part of the whole expiration were considered to have FL.
The alveolar oxygen tension used to compute the alveolar-arterial oxygen pressure difference (P[A-a]O2) was estimated using the following equation: alveolar oxygen tension = ([PB — 47) X Fio2]) — Paco2/ R, where PB is barometric pressure, Fio2 is the fractional O2 concentration of inspired air, and R is the respiratory quotient, which was assumed to be 0.8. The data are presented as mean ± SD. Correlation coefficients were obtained with the Spearman (p) nonparametric test for MRC score and the Pearson test (r) for all other parameters. In a stepwise multivariate analysis, we also used the Pearson multiple correlation for determining the MRC score because the Poisson fit was almost identical to that obtained with the Pearson correlation. Where appropriate, paired and unpaired Student t tests were used. Statistical analysis was performed using a statistical software package (SPSS Statistical Package; SPSS; Chicago, IL).

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