ABSTRACT
I. Introduction 34
II. ARDS Morphology 35
A. Terminology 35
B. Anatomical Equivalents 35
C. Structure/Morphology Relationship 36 D. Pulmonary and Extrapulmonary ARDS Morphology 36
III. Time Evolution 37
A. Late ARDS 37
B. Follow-Up 38
IV. Quantitative CT Analysis 39
A. Physical Principles of CT Scan 39
B. CT Scanning and X-ray Exposure 40
C. Computation of Gas and Tissue Volume 41
D. Hydrostatic Pressures Through the Lung: The Concept
of Superimposed Pressure 42
E. Lung Compartments 43
V. Physiological Insight 45
A. Modeling of ALI/ARDS Lung: The “Baby” Lung and the “Sponge” Lung 45
B. Recruitment and De-recruitment 46
Potential for Recruitment 46
De-recruitment and Positive End-Expiratory Pressure 48
Overinflation 50
C. Ventilation 53
Distribution of Ventilation 53
Inspiratory-Expiratory Interaction and Recruitment
Maneuvers 53
VI. CT Scan and Position 54
VII. CT Scan and Abdominal Pressure 54
VIII. Clinical Use of the CT Scan 55
References 56
I. Introduction
Rommelsheim et al. (1), to our knowledge, were the first to report the use of
computed tomography (CT) scan in acute respiratory distress syndrome (ARDS)
patients in 1983. Unfortunately, their observations did not have impact on the
scientific and clinical communities as their report appeared in a German
journal. Three years later, Maunder (2) and our group independently described
the appearances of ARDS on CT scans and its response to positive end-expiratory
pressure (PEEP) ventilation (3). This technique completely changed our vision
and interpretation of the syndrome. What was considered as a homogeneous
involvement of the lung parenchyma, due to a widespread alteration of lung
capillaries’ permeability, on CT scan appeared inhomogeneous with prevalent
involvement of the caudal and dependent lung regions. This pattern raised
relevant clinical issues: Is the ARDS lung composed of healthy and diseased
regions, as it appears at first sight? What is the nature of the observed densities?
How can these findings change our approach to the mechanical ventilation of
acute lung injury (ALI)/ARDS patients? Moreover, the widespread use of the CT scan led the clinicians to recognize clinical problems, as localized
pneumothorax, pleural effusions, bronchial and tracheal alterations, not revealed
on chest radiographs. In this chapter, we would like to integrate the knowledge
derived by the use of the CT scan in the general framework of ARDS, in its phys-
iological, pathological, and clinical aspects. We also refer to the dedicated
chapter on ARDS (Chapter 19).
