Time constant/volume relationship of passive expiration in mechanically ventilated ARDS patients

Abstract

T Ti im me e c co on ns st ta an nt t/ /v vo ol lu um me e r re el la at ti io on ns sh hi ip p o of f p pa as ss si iv ve e e ex xp pi ir ra at ti io on n i in n m me ec ch ha an ni ic ca al ll ly y v ve en nt ti il la at te ed d A AR RD DS S p pa at ti ie en nt ts s The mean values of τE for each volume slice did not differ significantly throughout expiration, averaging 690±218 ms (mean±SD of five slices and 12 patients). We show that the flow-dependent resistance of the endotracheal tube (RETT) is mainly responsible for the observed time constant homogeneity.We conclude that in ARDS patients during uninterrupted mechanical ventilation the time constants of passive expiration are markedly modified by the flow-dependent resistance of the endotracheal tube (RETT), and also by the external resistance of tubing and ventilator (REX). RETT and REX render τE about three times larger than the time constant of the patient's respiratory system alone.

Lung emptying in patients with acute respiratory distress syndrome: effects of positive end-expiratory pressure

“…GUTTMANN et al [6] observed that in patients with ARDS who were mechanically ventilated on PEEP, consecutive volume (V) portions were exhaled with nearly identical time constants (te), as indicated by a relatively constant V/flow (V9) relationship throughout expiration. This observation was attributed to to the presence of the endotracheal tube.…”

“…Indeed, taking into account the resistance of the endotracheal tube and ventilator circuit, GUTTMAN et al [6] recalculated the te of the pure respiratory system (ters) and observed a progressive increase toward the end of expiration. However, in this study [6] the ters was calculated with the assumption that the respiratory system compliance did not differ between inspiration and expiration. The possibility of hysteresis [7] renders this assumption questionable.…”

Expiratory Time Constant and Sleep Apnea Severity in the Overlap Syndrome

“…16 However, there are significant differences in the time constant when it is measured at different lung volumes in subjects with lung disease. [22][23][24] Specifically, in patients with the acute respiratory distress syndrome and those with COPD, the time constant increases as expiration proceeds. 22,23 Notably, a single compartment cannot accurately describe passive expiration in intubated and paralyzed patients.…”

“…[22][23][24] Specifically, in patients with the acute respiratory distress syndrome and those with COPD, the time constant increases as expiration proceeds. 22,23 Notably, a single compartment cannot accurately describe passive expiration in intubated and paralyzed patients. 25 However, our patients were in a stable state in the outpatient setting, as opposed to intubated under conditions of respiratory failure, 14,23 or paralysis.…”

“…14,22,23 This model consists of a single flow unit with a constant resistance, in series with a single elastic unit of constant compliance. 16 However, there are significant differences in the time constant when it is measured at different lung volumes in subjects with lung disease.…”