Distal airways: anatomy, histology and physiology

Distal airways are less than 2 mm in diameter, comprising a relatively large cross-sectional area that allows for slower, laminar airflow. The airways include both membranous bronchioles and gas exchange ducts, and have been referred to in the past as the ‘quiet zone’, in part because these structures were felt to contribute little to lung mechanics and in part because they were difficult to study directly. More recent data suggest that distal airway dysfunction plays a significant role in acute respiratory distress syndrome. In addition, injurious mechanical ventilation strategies may contribute to distal airway dysfunction. The presence of elevated airway resistance, intrinsic positive end-expiratory pressure or a lower inflection point on a pressure–volume curve of the respiratory system may indicate the presence of impaired distal airway function. There are no proven specific treatments for distal airway dysfunction, and protective ventilation strategies to minimize distal airway injury may be the best therapeutic approach at this time. Introduction The lung can be partitioned into the airways and parenchyma. The airways form the conduits between the outside world and the primary gas exchanging unit, the alveolus. There are three major groups of intrapulmonary airways; cartilaginous bronchi, membranous bronchioles and gas exchange ducts. Distal airways, defined in this review as airways less than 2 mm in diameter, are comprised of both membranous bronchioles and gas exchange ducts [1]. Because evaluating distal airways is challenging, relatively little has been published on their role in respiratory failure of various causes. In the present review we will highlight the experimental and clinical findings for the role of distal airways in acute respiratory distress syndrome (ARDS) as well as evaluating their function in mechanically ventilated patients. Distal airways: anatomy, histology and physiology The trachea divides into two primary bronchi that enter the lung at each hilum. After entering the lungs the primary bronchi branch downward and outward repeatedly, giving rise to smaller bronchi, which results in a dramatic increase in the number of airways and a progressive decrease in the diameter of each airway (Figure 1). Eventually, bronchi enter a pulmonary lobule and are then termed a bronchiole. Bronchioles are intralobular airways with diameters less than 5 mm that branch into five to seven terminal bronchioles. Each terminal bronchiole subdivides into two or more respiratory bronchioles that transition into alveolar ducts. Alveolar ducts open into atria that communicate with alveolar sacs, which terminate into alveoli. Saclike structures measuring about 200 μm in diameter, alveoli can evaginate from respiratory bronchioles, alveolar ducts and alveolar sacs. For the purposes of this review, distal airways will refer to airways less than 2 mm in diameter that consist of small membranous, terminal and respiratory bronchioles as well as alveolar ducts. The small membranous and terminal bronchioles carry out conductive functions, whereas respiratory bronchioles and alveolar ducts can carry out both conducting and gas exchange functions. Physiology The large increase in airway number compensates for the smaller diameter of distal airways such that the distal airway cross-sectional area is very large and resistance is relatively low [1]. In addition, distal airways are embedded in the connective tissue network of the lung, which allows transmission of tension from the parenchyma to distal Review Bench-to-bedside review: Distal airways in acute respiratory distress syndrome Manu Jain and J Iasha Sznajder Division of Pulmonary and Critical Care, Feinberg School of Medicine, Northwestern University, Room M-321, 240 E. Huron Avenue, Chicago, IL 60611, USA Corresponding author: Manu Jain, [email protected] Published: 15 February 2007 Critical Care 2007, 11:206 (doi:10.1186/cc5159) This article is online at http://ccforum.com/content/11/1/206 © 2007 BioMed Central Ltd ARDS = acute respiratory distress syndrome; ARDSNet = Acute Respiratory Distress Syndrome Network; LIP = lower inflection point; MV = mechanical ventilation; PEEP = positive end-expiratory pressure; Pinit = initial pressure; Ppeak = peak airway pressure; Pplat = plateau pressure; TNF = tumor necrosis factor; V = inspiratory flow rate.