![]() ![]() We first describe underappreciated lung and chest wall interactions that are clinically relevant to both normal individuals and the acutely ill who receive ventilatory support. Our purpose in this interpretive review is twofold. Detecting its presence may hold potential for therapeutic interventions. ![]() ![]() We refer to this phenomenon, whereby compression-or loading, of the chest wall in the presence of undetected end-tidal hyperinflation results in improved compliance of lungs and integrated respiratory system (and vice versa, whereby unloading of the chest wall results in worsened compliance of the lungs and integrated respiratory system), as mechanical ‘paradox’. Even though such measures invariably reduce the resting lung volume, they may improve the tidal compliance of the lung and integrated respiratory system when undetected end-tidal hyperinflation is extensive and recruitable lung tissue is relatively less. Yet, recent literature describing the mechanics of severe ARDS, primarily in patients with late-stage ARDS secondary to COVID-19, has raised awareness of the potential diagnostic (and perhaps therapeutic) value of doing the polar opposite, i.e., reducing PEEP, positioning the torso more horizontally, and loading the chest wall through application of external weights or manual pressures to the body surface. This interpretation generally holds merit for the massively obese and during the initial phase of ARDS, especially for those with overtly edematous, recruitable lungs. Quite the opposite, applying high levels of positive end-expiratory pressure (PEEP) and semi-upright positioning are generally considered beneficial when seeking to enlarge the aerated lung volume by recruiting additional lung units. While the horizontal prone position, a form of chest wall loading to the ventral body surface, is used in acute respiratory distress syndrome (ARDS) to even the distribution of transpulmonary pressures, few advocate external chest wall compression for the explicit purpose of restricting lung expansion in that setting. Scattered reports, for example, indicate that external compression of the chest wall to alleviate hyperinflation may be a temporizing and life-saving measure for status asthmaticus. Doing so for diagnostic or therapeutic purposes is seldom performed in the clinical setting, with several exceptions. Although the series-coupled lung and chest wall share a common volume, and, therefore, jointly determine not only the transpulmonary pressure ( P L) that distends the lung but also the airway pressures used to guide ventilation, the important influence of the chest wall is often discounted or ignored altogether outside such clinical extremes as morbid obesity, severe skeletal deformity, or abdominal compartment syndrome.Īctively limiting chest wall expansion through chest wall loading (applying weights or pressures to its surface) simultaneously restricts lung expansion. Indeed, the focus of lung-protective ventilation centers on the numerical values of these airway pressures, which are considered by many to determine the risk or safety of the applied tidal volume ( V T). We then apply these physiologic principles, in addition to published clinical observation, to illustrate the utility of chest wall modification for the purposes of detecting end-tidal hyperinflation in everyday practice.Ĭlinicians usually monitor mechanical ventilatory support using airway pressures-primarily the plateau and driving pressure, recorded during passive inflation. In this interpretive review, we first describe underappreciated lung and chest wall interactions that are clinically relevant to both normal individuals and to the acutely ill who receive ventilatory support. In these patients, interventions that reduce resting lung volume, such as loading the chest wall through application of external weights or manual pressure, or placing the torso in a more horizontal position, have unexpectedly improved tidal compliance of the lung and integrated respiratory system by reducing previously undetected end-tidal hyperinflation. Recent literature describing the respiratory mechanics of patients with late-stage, unresolving, ARDS, however, has raised awareness of the potential diagnostic (and perhaps therapeutic) value of this unfamiliar and somewhat counterintuitive practice. Actively limiting chest wall expansion through external compression of the rib cage or abdomen is seldom performed in the ICU. These airway pressures are influenced not only by the ventilator prescription, but also by the mechanical properties of the respiratory system, which consists of the series-coupled lung and chest wall. Clinicians monitor mechanical ventilatory support using airway pressures-primarily the plateau and driving pressure, which are considered by many to determine the safety of the applied tidal volume. ![]()
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