Assessment of expiratory flow limitation in chronic obstructive pulmonary disease: a new approach

Abstract

Patients with severe chronic obstructive pulmonary disease (COPD) usually experience expiratory flow limitation (EFL) during spontaneous breathing at rest [1]. As EFL reduces the effectiveness of expiration, it results in dynamic hyperinflation with consequent dyspnoea, which is one of the major complaints of patients with COPD. In these patients, the consequences of EFL are markedly increased during exercise [2]. Bearing in mind that EFL is a good predictor of dyspnoea in COPD patients [3,4], simple methods for detecting EFL without perturbing normal breathing are of clinical interest. In 1995 KOULOURIS et al. [5] proposed a method to detect EFL based on the external application of a negative pressure at the mouth during tidal expiration (negative expiratory pressure (NEP)). EFL is detected by comparing the magnitude of expiratory flow before and after application of NEP: in case of EFL, the expiratory flow does not rise when increasing the driving pressure. More recently, NINANE et al.[6] proposed the estimation of EFL during spontaneous breathing with a similar approach. To increase the driving pressure during expiration, these authors applied a positive abdominal pressure by manual compression of the patient9s abdominal wall. When compared with the NEP technique, this approach has the advantage of instrumental simplicity, but suffers from the drawbacks of applying abdominal pressure of uncontrolled magnitude and lack of automatisation. Both procedures can be applied during breathing at rest as well as during exercise [7,8].In this issue of the European Respiratory Journal (ERJ), DELLACÀ et al.[9] present interesting novel data showing that the forced oscillation technique (FOT) can be useful for noninvasively detecting EFL during spontaneous breathing. The FOT, which was proposed in the 1950s [10], is based on applying a small-amplitude oscillation pressure at the mouth. Using the FOT the patient9s respiratory mechanics can be determined by simply recording the oscillatory pressure and flow signals at the mouth, provided that the oscillation frequency is much higher than the breathing rate. The relationship between the pressure and the flow oscillation is the impedance of the respiratory system, which has two components: respiratory resistance (Rrs) and reactance (Xrs). In a simple interpretation, Rrs is attributed to airways and tissue resistances, whereas Xrs is determined by the inertial and compliant properties of the respiratory system. The rationale of using FOT for assessing EFL is that under this flow regime the forced oscillation applied at the mouth cannot reach the alveoli given the fact that the choke point shifts upstream of the lung during EFL. Therefore, the effective impedance viewed from the FOT device corresponds to only a fraction of the respiratory system: from the mouth to the choke point. According to this hypothesis, the associated decrease in the effective respiratory compliance results in a reduction of the measured Xrs during expiration when compared with the non flow-limited inspi...

Airway dynamics in COPD patients by within-breath impedance tracking: effects of continuous positive airway pressure

“…However, it is also well established that inhomogeneous obstruction in the lung periphery results in rapid decreases in effective compliance with increasing breathing rate [33,34] or oscillation frequency [5,22,28,29,35]. This may explain why the Xrs values are more negative in the COPD subjects, in accordance with the majority of earlier findings [3,5,7,22,25,[35][36][37][38].…”

Breath-by-breath quantification of progressive airflow limitation during exercise in COPD: A new method

“…The forced oscillation technique requires complex instrumentation and special maneuvers. 15 The NEP method does not allow assessment of breath-by-breath changes in EFL because the method necessitates comparison of the flow profiles of expirations with and without the application of NEP 14,15 usually taken several breaths apart.…”

Non-interventional monitoring of expiratory flow limitation during experimental mechanical ventilation

“…We firstly proposed (Vassiliou et al, Eur Respir J 1996) that strongly negative values of expiratory reactance, recognized with the aid of the forced oscillation technique (FOT) method suggest the presence of EFL with high sensitivity and specificity during experimental MV [13]. Since then, newer investigations confirmed the validity and usefulness of the proposed technique, which is easier to apply during spontaneous breathing than during mechanically assisted ventilation [13][14][15][16][17][18][19][20][21][22][23].…”