Tidal volume monitoring with electrical impedance tomography (EIT) on COPD patients : relationship between EIT and diffusion lung transfer (DL,CO)

Abstract

Equations used to estimate ventilation out of EIT images, validated on healthy volunteers show a significant bias and a larger variance when applied to COPD patients. The differences in estimation values were found to be highly correlated to anthropometric parameters and CO exchange (DL,CO) in a previous group of COPD patients. A new group of 13 COPD male patients (FEV1/FVC<70% and FVC ≥ 80%) was used in this study, with stricter inclusion criteria than in the previous one. Anthropometric parameters (age, weight, height, skinfolds) and values of several pulmonary function tests, such as spirometry (FVC, FEV1 and FEV1/FVC), static volumes (RV and TLC) and pulmonary diffusion (DL,CO and DL,CO /VA) were recorded. The EIT system (TIE4sys) and a pneumotach were simultaneously connected to monitor tidal volume. The anthropometric main values of COPD patients were: Age 67±9 years; height 1.65±0.05 m; weight 72±11 kg; BMI 26.4±3.3 kg/m2; Subscapular skinfold 23±9 mm. The mean values of pulmonary function test were: Spyrometry FVC 83±13, FEV1 47±15% of reference value and FEV1/FVC 40±9%. Static Volume test RV 168±43% and TLC 119±12% of reference value. Pulmonary diffusion test DL,CO 56±15%, DL,CO,ADJ 57±15% and DL,CO /VA 68±13% of reference value. The mean tidal volumes estimated with TIE4sys and the pneumotach were: 0.544±0.196 L and 0.763±0.168 L r = 0.861 (p<0.01). The mean difference was 0.218±0.100 L (p<0.01). The differences show a strong correlation with the subscapular and rear skinfold; and the pulmonary diffusion values. The correction equation was: Difference=0.09-0.005 ⋆ (DL,CO,ADJ) +0.028 ⋆ (rear_skinfold)-0.01 ⋆ (subscapular skinfold) (R2=0.899; p<0.01). The differences of determinations estimated with pneumotach and TIE4sys can be attributed to changes in lung internal anatomy (DL,CO, DL,CO,adj Hb, DL,CO/VA) and also anthropometric characteristics like subscapular skinfold.