MODEL FITTING OF VOLUMETRIC CAPNOGRAMS IMPROVES CALCULATIONS OF AIRWAY DEAD SPACE AND SLOPE OF PHASE III

Background. This study assessed the performance of a Functional Approximation based on a Levenberg-Marquardt Algorithm (FA-LMA) to calculate airway dead space (VDaw) and the slope of phase III (S-III) from capnograms. Methods. We performed mathematical simulations to test the effect of noises on the calculation of VDaw and S-III. Data from ten mechanically ventilated patients at 0, 5 and 10 cmH(2)O of PEEP were also studied. FA-LMA was compared with the traditional Fowler's method (FM). Results. Simulations showed that: (1) The FM determined VDaw with accuracy only if the capnogram approximated a symmetrical curve (S-III = 0). When capnograms became asymmetrical (S-III > 0), the FM underestimated VDaw (-3.1% to -0.9%). (2) When adding noises on 800 capnograms, VDaw was underestimated whenever the FM was used thereby creating a bias between-5.54 and -1.28 ml at standard deviations (SD) of 0.1-1.8 ml (P < 0.0001). FA-LMA calculations of VDaw were close to the simulated values with the bias ranging from -0.21 to 0.16 ml at SD from 0.1 to 0.4 ml. The FM overestimated S-III and showed more bias (0.0041-0.0078 mmHg/ml, P < 0.0001) than the FA-LMA (0.0002-0.0030 mmHg/ml). When calculating VDaw from patients, variability was less with the FA-LMA leading to mean variation coefficients of 0.0102, 0.0111 and 0.0123 compared to the FM (0.0243, 0.0247 and 0.0262, P < 0.001) for 0, 5 and 10 cmH(2)O of PEEP, respectively. The FA-LMA also showed less variability in S-III with mean variation coefficients of 0.0739, 0.0662 and 0.0730 compared to the FM (0.1379, 0.1208 and 0.1246, P < 0.001) for 0, 5 and 10 cmH(2)O of PEEP, respectively. Conclusions. The Functional Approximation based on a Levenberg-Marquardt Algorithm showed less bias and dispersion compared to the traditional Fowler's method when calculating VDaw and S-III.