July 2002 / Volume 47 / Number 7 / Page 778
Airway Physiology, Autogenic Drainage, and Active Cycle of Breathing
IntroductionAirway clearance techniques are used to aid in mucus clearance in a variety of disease states. Autogenic drainage and active-cycle-of-breathing technique are 2 such modalities that rely heavily on basic airway physiology to enhance clearance. In this review I discuss the equal pressure point, huffing, and asynchronous and collateral ventilation, and review the literature and theory regarding autogenic drainage and active cycle of breathing. Selection of airway clearance techniques is discussed in the light of evidence-based medicine.
Equal Pressure Point
Active Cycle of Breathing Technique
Production and clearance of airway secretions occurs throughout the respiratory tract on a daily, minute-by-minute basis. Under normal conditions the volume and viscoelastic properties of the secretions produced are easily managed by the cilia, regular respiration, and, when necessary, the occasional cough. In many disease states, however, changes in mucus characteristics or quantity, or changes in lung physiology overwhelm the normal mucociliary escalator.
Over the past 35 years a multitude of airway clearance techniques have been developed, introduced, refined, researched, and used in patient populations, from asthma to atelectasis, from cystic fibrosis (CF) to chronic obstructive pulmonary disease (COPD), to help assist normal mucus clearance mechanisms. Chest physical therapy has become almost synonymous with postural drainage and percussion (PD&P). Newer modalities include autogenic drainage (AD), active cycle of breathing technique (ACBT), positive expiratory pressure methods, high-frequency airway oscillation (flutter), high-frequency chest wall oscillation (The Vest), intrapulmonary percussive ventilation (IPV), and mechanical insufflation-exsufflation (the "in-exsufflator"). The impetus for this explosion in airway clearance options, as with any medical treatment, has been the continued search for improved efficacy and adherence to a prescribed therapy.
Mucus is moved by 3 mechanisms. First, slug flow describes the means by which a semi-solid mucus plug obstructing or partially obstructing an airway can be pushed from behind by air flow. Second, annular flow describes mucus moving along the walls of the airway, either being pulled along by expiratory air flow or transported by cilia. Third, mist flow describes aerosolized mucus that is exhaled as suspended droplets. Slug and annular flow account for the majority of airway secretion clearance.
There are only 2 over-riding physical principles to airway clearance techniques: first, there must be airflow; and, second, for the patient to have air flow, the patient must be able to get air behind the mucus.