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The Best Peak Expiratory Flow Is Flow-Limited and Effort ...

The Best Peak Expiratory Flow Is Flow-Limited and Effort ...

The Best Peak Expiratory Flow Is Flow-Limited and Effort

The Best Peak Expiratory Flow Is Flow-Limited and Effort-Independent in Normal Subjects Claudio Tantucci, Alexandre Duguet, Pietro Giampiccolo, Thomas Similowski, Marc Zelter, and Jean-Philippe Derenne Clinica di Medicina Interna, Università di Brescia, Brescia, Italy; Laboratoire de Physio-Pathologie Respiratoire et Service de Explorations Fonctionnelles, Groupe Hospitalier Pitié-Salpêtrière, University of Paris VI, Paris, France; and Divisione di Pneumologia, Azienda Ospedaliera di Imola, Bologna, Italy Recently, it has been suggested that peak expiratory flow (PEF) may be determined by the wave speed flow-limiting mechanism. In six normal male subjects (age � 33 � 8 years) performing expiratory forced vital capacity (FVC) maneuvers, a negative expiratory pressure (NEP) of �10 cm H2O was randomly applied at the beginning of maximal expiration to assess changes in PEF as compared with baseline. During FVC maneuvers, the expiratory effort was measured by changes in esophageal pressure (Pes), as either peak expiratory Pes�Pes at end expiratory lung volume ( �Pespeak) or maximal rate of rise of Pes (dPes/dtmax). In each experimental condition, at least three FVC maneuvers with comparable expiratory effort were selected for analysis for each subject. With similar �Pespeak (107.2 � 34.9 versus 111.7 � 40.5 cm H2O) and dPes/dtmax (1181 � 518 versus 1177 � 546 cm H2O/second) PEF amounted to 10.84 � 1.08 L/second and to 10.82 � 1.03 L/second with and without NEP, respectively. These data show that PEF obtained by normal subjects to the best of their abilities (best PEF) does not increase with NEP and indicate that the best PEF is a flow-limited and effort-independent parameter, reflecting only lung and airways mechanics as the other subsequent maximal expiratory flows achieved during the FVC maneuver. Keywords: peak expiratory flow; expiratory flow limitation; negative expiratory pressure Peak expiratory flow (PEF) is the maximum flow that can be generated during a forced expiratory maneuver started from total lung capacity (TLC) and correctly performed. PEF normally occurs at high lung volumes near TLC and is a widely adopted measurement to assess and monitor airway obstruction. According to a common tenet of respiratory physiology, PEF is considered an effort-dependent parameter (1). This is because a plateau of expiratory flow cannot usually be obtained at lung volumes higher than 80% of the vital capacity by increasing pleural (and alveolar) pressure during isovolume flow–pressure curves (2, 3). Hence, PEF is believed to be limited by the force–velocity characteristics of the expiratory muscles instead of the mechanical properties of the lung and airways, as insufficient force would be available to achieve flow limitation near TLC. Thus, after standardization of the preceding inspiration, PEF might be increased by either increasing pleural pressure (i.e., by augmenting somehow the expiratory effort) or decreasing atmospheric pressure. Indeed, both procedures increment the driving pressure that, in the absence of expiratory flow limitation, is the difference in pressure between the alveoli and the mouth. ( Received in original form December 4, 2000; accepted in final form February 12, 2002) Correspondence and requests for reprints should be addressed to Claudio Tantucci, M.D., Clinica di Medicina Interna I, Università di Brescia, Spedali Civili, 25100 Brescia, Italy. E-mail: clatantu@tin.it Am J Respir Crit Care Med Vol 165. pp 1304–1308, 2002 DOI: 10.1164/rccm.2012008 Internet address: www.atsjournals.org There is evidence, on the other hand, that PEF may be determined by the wave speed flow-limiting mechanism (4), reflecting the resistance upstream to the flow-limiting segment (Rus), the cross-sectional area here (A), the compliance of the airway wall ( �A/ �Ptm, where Ptm is the transmural pressure), and the density ( �) of the breathed gas (5, 6). Under these conditions, the actual driving pressure for PEF would be only the lung elastic recoil pressure (Pel) at the corresponding volume. Consequently, a reduction of the atmospheric pressure at the beginning of expiration should not influence PEF. The aim of the present study was to assess the PEF changes occurring by applying negative expiratory pressure at the mouth (NEP) (7), as compared with baseline conditions, during forced expiratory vital capacity maneuver in normal subjects performing similar, maximal expiratory efforts. METHODS Subjects Six healthy male subjects (age � 33 � 8 years; range 26–46 years) with 2 body mass index less than 25 kg/m were investigated in a prospective open study. All subjects were non-smokers. The subjects were well motivated and familiar with respiratory maneuvers. All had normal pulmonary function tests. None of them had acute or chronic cardiopulmonary or neuromuscular diseases. No electrolytic or metabolic abnormalities were found in the routine laboratory evaluation. All subjects had a normal chest radiograph. The experimental protocol was approved by the local Ethics Committee and informed consent was obtained from all subjects. Measurements Spirometry was performed in the sitting position using a computerized system (MedGraphics 1070; Medical Graphics, St. Paul, MN). Mouth flow ( V m) was measured through a heated pneumotachograph, linear up to 13.6 L/second (model 3813; Hans-Rudolph, Kansas City, MO). Volume (V) was obtained by time integration of the flow signal and was calibrated at different flow rates with a 3-L syringe. Both flow/volume ( m/ ) and volume/time plots were simultaneously generated during forced vital capacity (FVC) maneuver. Esophageal pressure was measured with an esophageal balloon (10 cm) placed in the lower third of the esophagus following the routine technique (8) to evaluate respiratory effort. The balloon was connected via a noncompliant polyethylene catheter (100 cm, internal diameter � 1.4 mm) to a differential pressure transducer (MP-45, � 250 mm Hg; Valydine Corp., Northridge, CA). To prevent balloon collapse during forced expiration, the balloon was filled with 2 ml of air (9). Mouth pressure (Pm) was measured via a rigid polyethylene tube (internal diameter � 1.7 mm) placed near the mouthpiece and connected to a differential pressure transducer (MP-45, � 250 mm Hg; Valydine Corp.). Transducers were calibrated to 200 cm H2O before the experiments and checked at the end of the session. The system used to measure both Pes and Pm had no appreciable shift or alteration in amplitude up to 20 Hz (Figure 1). A Venturi device capable of rapidly generating a negative pressure (Aeromech Devices Ltd., Almonte, ON, Canada) was connected in series with the cone of the pneumotachograph. The dead space of the assembly was about 150 ml and its resistance amounted to 0.5 cm H2O/L/ · V · V ·

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