Abstract
Clinical assessment of expiratory flow limitation (EFL) is important for diagnosing chronic pulmonary disease (COPD). Either EFL or dynamic hyperinflation (DH) in COPD has been understood based on wave speed theory, which is widely accepted as the standard concept. However, a theoretical perspective on the relationship between EFL and DH may require another approach. This article proposed another explanation for EFL with the introduction of pulmonary entropy with thermo-statistical considerations on choke state of the pulmonary system. According to Gibbs' thermodynamic equilibrium theory, the choke state of the pulmonary system was characterized by a critical pressure (Pc) emergence in the pulmonary parenchyma, which was proportional to the elastic recoil pressure (Pel) and the slope of maximal flow-volume curve (σ). Thermodynamic balance between energies (supplied from the body as heat, stored as the entropy of lungs, and dissipated in the respiratory system) explained the work of breathing (WOB), by which it was explained that an intrinsic PEEP (PEEPi) was emerging as a difference between sufficient and insufficient WOB for energy demands of the body. It was concluded that EFL would limit the WOB into less than demanded during exercise, and that the difference between demand and performance would induce a product of PEEPi and DH in volume.
Highlights
Clinical assessment of lung function is commonly explained by a phenomenon known as expiratory flow limitation (EFL), which means that a flow approaches a maximal value as the choke state that cannot be exceeded regardless of the extra effort excreted. [1] The maximal expiratory flow ( ) decreases proportionally with lung volume ( ). [2] EFL is expressed by the volume excreted in the first one second during a forced expiration as FEV1
When FEV1 is coupled with a measure of the total gas volume expired during an entire forced expiration, known as forced vital capacity (FVC), we can differentiate between obstructive disease and restrictive disease (FVC is lower than normal but FEV1/FVC is normal).[3]
EFL has been described by wave speed theory in the conducting airway introduced based on fluid mechanics by Dawson and Elliot [4]: the pressure gradient along the airway branch is necessary to drive the air along the airway conduit
Summary
Clinical assessment of lung function is commonly explained by a phenomenon known as expiratory flow limitation (EFL), which means that a flow approaches a maximal value as the choke state that cannot be exceeded regardless of the extra effort excreted. [1] The maximal expiratory flow ( ) decreases proportionally with lung volume ( ). [2] EFL is expressed by the volume excreted in the first one second during a forced expiration as FEV1. When an aliquot of air is incompressible, it is accommodated by lateral expansion of the conduit’s upstream walls. This expansion propagates toward the other end of the conduit at a rate determined by the speed of movement of the elastic deformation of the wall, meaning that each point in the wall must radially oscillate either side of its relaxed position. The actual flow transmitted is given the product of this velocity and the amplitude of oscillations. Once these oscillations reach an amplitude equal to the conduit radius, the opposing walls bump into each other at the peak of their
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