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University of Athens Medical School, Athens, Greece
Correspondence to: George E. Tzelepis, MD, FCCP, Department of Pathophysiology, University of Athens Medical School, 75 M. Asias St, 11527 Athens, Greece; e-mail:gtzelep{at}med.uoa.gr
To the Editor:
I read with interest the recent article in CHEST (January 2005) by Wilson et al1 on peak expiratory flow time (PEFT) in amyotrophic lateral sclerosis (ALS). They reported increasing PEFT with time, with the rate of increase in PEFT being greater than the rate of decline in either FVC or peak expiratory flow rate (PEFR).
The authors did not measure maximal expiratory pressure (PEmax). Since PEmax correlates with PEFR in patients with ALS,2 the interpretation of the PEFT data in the absence of respiratory muscle strength measurements creates more questions than it gives answers. The PEFT reflects in large part the ability of the expiratory muscles to develop rapid (explosive) force and is related to the rate of pressure rise. It is also related to the PEmax because the capacity of skeletal muscles for rapid force development declines in proportion to the ability to generate maximal force.3 If this is not the case for ALS patients, the measurement of PEmax would establish the superiority of PEFT in monitoring respiratory function in these patients. If, on the other hand, changes in PEFT relate to PEmax, then the measurement of PEmax is preferable to that of PEFT. PEFT, as for all indexes of rapid force development, is less reliable and less reproducible than PEmax.45 In addition, the measurement of PEFT will require the standardization of the forced expiratory maneuver and, specifically, the speed of inspiration prior to exhalation, which was not controlled for in the study of Wilson et al.1 A fast inspiration to total lung capacity will prestretch (eccentric contraction) the expiratory muscles, which will then develop greater pressure (and a greater rate of pressure rise) during the subsequent forceful (concentric) contraction.67 The property of skeletal muscles to produce greater force when a concentric contraction is immediately preceded by an eccentric contraction is known as the stretch-shortening cycle. Therefore, standardizing the expiratory maneuver used for measuring PEFT will help to minimize the variability of the PEFT.
Finally, the authors state that the PEFR is determined by the force-velocity characteristics of the respiratory muscles rather than by the mechanical properties of the lung. The current notion is that PEFR is determined by a flow-limiting mechanism. Some studies8 using the negative expiratory pressure technique,9 calculating PEFR on the basis of wave speed theory10 or employing specific maneuvers to augment expiratory effort through the stretch-shortening cycle, have shown that PEFR is not limited by the velocity of muscle shortening.
References
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