To investigate the influence of VT size on respiratory mechanics during mechanical ventlation (MV) in COPD patients.
Airway pressure (Pao) and flow (V’) were recorded from 8 pts without respiratory disorder (WRD) and 8 pts with COPD exacerbation. They were under MV, sedation and muscle relaxation during measurements, performed at 6 levels of VT (300-800 ml) and 0 hPa of PEEPe. Data were analysed according to: Pao=EEP + E1.V+E2.V2 + k1.V’ + k2.V’.|V’| + k3.V.V’, where E1 & k1 are the linear coefficients of elastance and resistance, k2 the V’ dependence of resistance coefficient, E2 & k3 the volume dependence of elastance & resistance, respectively and EEP the end-expiratory pressure. Root Mean Square Difference (RMSD) of the regression was also calculated. All coefficients were compared between the two groups of pts and between the 6 levels of VT with the aid of two ways ANOVA with replication (p=0.05).
-Significant higher values for all calculated coefficients (except E1) between WRD and COPD patients at all VT levels (k3 more negative in the COPD group). -k2 negative at all VT levels in the COPD group. -k3 negative at all VT levels in both groups. -E1 showed a tendency to increase on both groups. -E2 decreased significantly with VT increase in both groups. -k1 decreased non significantly with VT increase in both groups. -k2 decreased non significantly in the WRD group, while corresponding changes in the COPD group were not homogenous. -k3 turned significantly to more positive values with increasing VT. -EEP increased non-significantly with increasing VT in both groups. -RMSD increased with increasing VT in both groups, non-significantly in the COPD group.
The present results suggest that tidal volume size influences strongly respiratory mechanics in mechanically ventilated COPD patients, specifically their dependencies on flow and volume.
Careful monitoring, with the aid of the proposed non-linear model may help for better adjustment of the tidal volume in COPD patients under MV.
P.K. Behrakis, None.