Respiratory system mechanics in acute respiratory distress syndrome
Critical Care Division, Department of Anesthesia, University of California San Francisco at San Francisco General Hospital, 1001 Potrero Avenue, San Francisco, CA 94110 USA b Cardiovascular Research Institute, University of California San Francisco, 505 Parnassus Avenue, Box 0130, SanFrancisco, CA 94143-0130, USA c Department of Anesthesia, University of California San Francisco, 521 Parnassus Avenue, San Francisco, CA 94143-0130, USA
Research in respiratory mechanics is important to improving the ventilatory management of acute respiratory distress syndrome (ARDS). Over the years, various and sometimes contradictory ventilation strategies have been used to treat ARDS. Thesestrategies reﬂect changes in ventilator capabilities and an evolving understanding of the interplay between pathophysiology and respiratory system mechanics. This article reviews the fundamental concepts of respiratory system mechanics and describes the mechanical alterations that occur in ARDS. When appropriate, the authors speculate on the implications of respiratory system mechanics research andemphasize the limitations of this research for current clinical practice.
Fundamental concepts of respiratory system mechanics The pressure applied to the respiratory system to produce volume change reﬂects the force necessary to overcome three fundamental system properties: compliance (Crs), resistance (Rrs) and inertia (Irs). This relationship is expressed in the Newtonian equation of motionadapted to the three-dimensional respiratory system : : 1 ¨ Pappl ¼ V þ RrsV þ IrsV Crs
* Corresponding author. Department of Anesthesia, San Francisco General Hospital, NH:GA-2, 1001 Potrero Avenue, San Francisco, CA 94110. E-mail address: email@example.com (R.H. Kallet). 1078-5337/03/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1078-5337(03)00040-6298
R.H. Kallet, J.A. Katz / Respir Care Clin 9 (2003) 297–319
Where Pappl is the pressure applied to the respiratory system, V is the lung _ volume above its relaxed equilibrium value, V is gas ﬂow, Irs is the inertia of ¨ is the acceleration of both the gas molecules the respiratory system, and V and the lung–chest wall tissues. For clinical purposes, inertia is ignored: although gasmolecules undergo marked acceleration, their mass is negligible, and the lung and chest wall mass undergoing displacement experience little acceleration. Under passive ventilation conditions, the peak airway pressure (Paw) represents the total force required to overcome the resistive and elastic recoil pressures of the lungs and chest wall. When this pressure is related to the corresponding tidal volume(VT), it is referred to as the ‘‘dynamic characteristic’’  or as the ‘‘eﬀective dynamic compliance’’ of the respiratory system. Distinguishing the pressure required to overcome the resistive as opposed to the elastic properties requires the introduction of a circuit occlusion of approximately 3-seconds’ duration at end-inspiration (ie, an endinspiratory pause) [2,3]. The pressure dissipationthat occurs during this endinspiratory pause allows measurement of compliance and resistance (Fig. 1). Compliance Compliance is calculated as the VT divided by the change in Paw under zero-ﬂow conditions. Under these conditions, Crs is distinguished as dynamic or quasistatic, a determination based on the characteristics of Paw decay under zero-ﬂow conditions. The three pressure points necessary tocalculate dynamic and static Crs are the pressure at the instant of zero ﬂow (commonly referred to as P1), the equilibrated pressure or end-inspiratory plateau pressure (Pplat), and the baseline pressure. Because ARDS patients are universally managed with positive end-expiratory pressure (PEEP), the PEEP is the baseline pressure used for compliance calculations. Because dynamic hyperinﬂation is a...