Influence of inertance on linearity of respiratory mechanics during mechanical ventilation in ARDS

Aim

The aim of the study is to investigate the influence of respiratory system inertance (Irs) on the linearity of respiratory mechanics during mechanical ventilation (MV) under conventional frequencies.

Methods

Airway pressure (Pao), flow (V') and volume (V) were recorded from 11 ARDS patients under MV at 10 and 20 c/min. Data were analyzed according to: Pao = EEP + Ers.V + Rrs.V' and Pao = EEP + Ers.V + Rrs.V' + Irs.V", where Ers and Rrs are the respiratory system elastance and resistance, and EEP is the end-expiratory pressure. The fitness of data to models was evaluated by the standard error of the regressions (RMSD). Comparisons between all coefficients were done at 10 and at 20 c/min with the aid of the Wilcoxon rank test (P < 0.05).

Results and conclusion

The mean values ± SD of all coefficients are presented in Table 1. Irs did not differ significantly between 10 and 20 c/min. The use of the inertive term results in a significantly higher Ers, lower Rrs and lower RMSD (P < 0.001) at both frequencies, but these differences are clinically irrelevant. We conclude that Irs is not negligible during MV in ARDS. Respiratory pathophysiology and the ventilator characteristics may contribute to the important role of Irs during MV.

Table 1