A A s si im mp pl le e m me et th ho od d t to o d de et te ec ct t e ex xp pi ir ra at to or ry y f fl lo ow w l li im mi it ta at ti io on n d du ur ri in ng g s sp po on nt ta an ne eo ou us s b br re ea at th hi in ng g N.G. Koulouris*, P. Valta*, A. Lavoie*, C. Corbeil**, M. Chassé**, J. Braidy**, J. Milic-Emili*A simple method to detect expiratory flow limitation during spontaneous breathing. N.G. Koulouris, P. Valta, A. Lavoie, C. Corbeil, M. Chassé, J. Braidy, J. Milic-Emili. ERS Journals Ltd 1995. ABSTRACT: Patients with severe chronic obstructive pulmonary disease (COPD) often exhale along the same flow-volume curve during quiet breathing as during a forced expiratory vital capacity manoeuvre, and this has been taken as indicating flow limitation at rest. To obtain such curves, a body plethysmograph and the patient's co-operation are required. We propose a simple technique which does not entail these requirements. It consists in applying negative pressure at the mouth during a tidal expiration (NEP). Patients in whom NEP elicits an increase in flow throughout the expiration are not flow-limited. In contrast, patients in whom application of NEP does not elicit an increase in flow during most or part of the tidal expiration are considered as flowlimited. Using this technique, 26 stable COPD patients were studied sitting and supine.Eleven patients were flow-limited both seated and supine, eight were flow-limited only when supine, and seven were not flow-limited either seated or supine. Only 5 of 19 patients who were flow-limited seated and/or supine had severe ventilatory impairment (forced expiratory volume in one second (FEV 1 ) <40% predicted).We conclude that the NEP technique provides a simple, rapid, and reliable method for detection of expiratory flow limitation in spontaneously breathing subjects, which does not require the patient's co-operation, and can be applied in different body positions both at rest and during muscular exercise. Our results also indicate a high prevalence of flow limitation in COPD patients at rest, particularly when supine. Eur Respir J., 1995, 8, 306-313 It has long been suggested that patients with severe chronic obstructive pulmonary disease (COPD) may exhibit expiratory flow limitation at rest, as reflected by the fact that they breathe tidally along or above their maximum expiratory flow-volume curves [1][2][3]. The effects of expiratory flow limitation may be partly compensated by breathing at lung volumes higher than the relaxation volume of the respiratory system [3]. The latter condition, which is termed dynamic pulmonary hyperinflation, is associated with intrinsic positive endexpiratory pressure (PEEPi) [4]. The combined effects of increased flow resistance, dynamic hyperinflation and PEEPi place a severe burden on the inspiratory muscles of COPD patients [5][6][7], and may also contribute to dyspnoea [8].Though dynamic hyperinflation is the hallmark of expiratory flow limitation, the prevalence and clinical significance of this phenomenon have not been adequa...

Two new methods, application of negative pressure at the airway opening during expiration (NEP) and reduction of flow resistance by bypassing the expiratory line of the ventilator by exhaling into the atmosphere (ATM), were used to detect expiratory flow limitation in 12 semirecumbent (45 degree) mechanically ventilated patients, seven with chronic airway obstruction (CAO). An increase of expiratory flow with NEP or ATM, relative to the preceding control breath, was taken as indicating absence of expiratory flow limitation. By contrast, the portion of the tidal expiration over which there was no change in flow with NEP or ATM was considered as flow-limited. With NEP, nine patients exhibited flow limitation, six (all with CAO) were flow-limited over most of the tidal expiration (> 70% VT), and three at < 60% VT. Although the results with NEP and ATM were in general in good agreement, in the three non-flow-limited patients the ATM method gave erroneous results. Six patients were also studied supine, including two who were not flow-limited when semirecumbent: both became flow-limited when supine. We conclude that NEP provides a simple method to detect flow limitation in mechanically ventilated patients. The supine position enhances flow limitation.

The effects of different levels of positive end-expiratory pressure (PEEP) (zero to 15 cm H2O) on the static inflation volume-pressure (V-P) curve of the respiratory system and on gas exchange were studied in eight patients with the adult respiratory distress syndrome (ARDS). Alveolar recruitment with PEEP was quantified in terms of recruited volume, i.e., as difference in lung volume between PEEP and zero end-expiratory pressure (ZEEP) for the same static inflation pressure (20 cm H2O) from the V-P curves obtained at the different PEEP levels. In addition, static compliance of the respiratory system at fixed tidal volume (0.7 L) was determined at the different PEEP levels. The results suggest that: (1) in some patients with ARDS the V-P curves determined on ZEEP exhibit an upward concavity reflecting progressive alveolar recruitment with increasing inflation volume, and PEEP results in alveolar recruitment (range of recruited volume at 15 cm H2O of PEEP: 0.11 to 0.36 L); (2) in other patients with ARDS the V-P curves on ZEEP are characterized by an upward convexity, and PEEP results in a volume displacement along this curve without alveolar recruitment and with enhanced risk of barotrauma; (3) the PEEP-induced increase in arterial oxygenation is significantly correlated to the recruited volume but not to the changes in static compliance. The shape of the static inflation V-P curves on ZEEP allows the prediction of alveolar recruitment with PEEP.

By use of the technique of rapid airway occlusion, the effects of inspiratory flow, volume, and time on lung and chest wall mechanics were investigated in 10 chronic obstructive pulmonary disease (COPD) patients mechanically ventilated for acute respiratory failure. We measured the interrupter resistance (Rint), which in humans reflects airway resistance; the additional resistances due to time constant inequality and viscoelastic pressure dissipations within the lungs (delta RL) and the chest wall; and the static and dynamic elastances of lung and chest wall. We observed that 1) static elastances of lung and chest wall in COPD patients were similar to those of normal subjects; 2) Rint of the lung was markedly increased and flow dependent in COPD patients, whereas Rint of the chest wall was negligible as in normal subjects; and 3) in COPD patients, delta RL was markedly increased at all inflation flows and volumes, reflecting increased time constant inequalities within the lungs and/or altered viscoelastic behavior. The results imply increased dynamic work due to Rint and delta RL and marked time dependency of pulmonary resistance and elastance in COPD patients.

The effects of inspiratory flow (V) and inflation volume (delta V) on the mechanical properties of the respiratory system in eight ARDS patients were investigated using the technique of rapid airway occlusion during constant-flow inflation. We measured interrupter resistance (Rint,rs), which in humans represents airway resistance, the additional resistance (delta Rrs) due to viscoelastic pressure dissipations and time constant inequalities, and static (Est,rs) and dynamic (Edyn,rs) elastance. The results were compared with a previous study on 16 normal anesthetized paralyzed humans (D'Angelo et al. J. Appl. Physiol. 67: 2556-2564, 1989). We observed that 1) resistance and elastance were higher in ARDS patients; 2) with increasing V, Rint,rs and Est,rs did not change, delta Rrs decreased progressively, and Edyn,rs increased progressively; 3) with increasing delta V, Rint,rs decreased slightly, delta Rrs increased progressively, and Est,rs and Edyn,rs showed an initial decrease followed by a secondary increase noted only in the ARDS patients. The above findings could be explained in terms of a model incorporating a standard resistance in parallel with a standard elastance and a series spring-and-dashpot body that represents the stress adaptation units within the tissues of the respiratory system.