Conference Summary Airway Clearance: Physiology, Pharmacology, Techniques, and Practice Dean R Hess PhD RRT FAARC Introduction Physiology Pharmacology Techniques Practice What to Do? Summary Clinicians and their patients are troubled by respiratory secretions, and standard practice calls for efforts to clear secretions from the lungs. On one hand, mucus production and cough are important for airway defense and protection of the lower respiratory tract against inhaled irritations. On the other hand, excessive mucus obstructs airways and excessive cough has been associated with a number of complications. The objective of this conference was to review the scientific basis and clinical evidence for the use of airway clearance therapy to guide the most appropriate approach to airway clearance. An international group of clinicians and scientists addressed the physiology of mucus production and cough, pharmacologic approaches to airway clearance, and the variety of techniques available for airway clearance. Specific issues related to airway clearance in critically ill patients, children, and the elderly were discussed. Outcome measures related to evaluating mucus clearance therapy were also presented. One of the themes repeated consistently throughout this conference was the dearth of high-level evidence related to airway clearance techniques. Appro- priately powered and methodologically sound research is desperately needed in this area. Key words: airway clearance, chest physiotherapy, cough, mucus clearance therapy. [Respir Care 2007; 52(10):1392–1396. © 2007 Daedalus Enterprises] Introduction weak cough. There is a relative lack of evidence describ- ing the effect of increased respiratory tract secretions on Many acute and chronic respiratory diseases are asso- the progression of disease, and there is even less evidence ciated with retained airway secretions due to increased that secretion clearance techniques improve the course of mucus production, impaired mucociliary transport, or a disease. Despite this lack of evidence, clinicians and pa- tients are troubled by respiratory secretions, and standard practice calls for efforts to clear secretions from the lungs. Dean R Hess PhD RRT FAARC is affiliated with the Department of Indeed, an important proportion of respiratory therapists’ Respiratory Care, Massachusetts General Hospital, and Harvard Medical time is spent in efforts to evacuate secretions from the School, Boston, Massachusetts. lower respiratory tract. In the home this can constitute a lot of time for the patient and caregiver. In recent years a Dr Hess presented a version of this paper at the 39th RESPIRATORY CARE Journal Conference, “Airway Clearance: Physiology, Pharmacology, Techniques, and Practice,” held April 21–23, 2007, in Cancu´n, Mexico. Dr Hess is a consultant for and has received research funding from Correspondence: Dean R Hess PhD RRT FAARC, Respiratory Care, Respironics, Murrysville, Pennsylvania. He reports no other conflicts of Ellison 401, Massachusetts General Hospital, 55 Fruit Street, Boston MA interest related to the content of this paper. 02114. E-mail: [email protected]. 1392 RESPIRATORY CARE • OCTOBER 2007 VOL 52 NO 10 AIRWAY CLEARANCE:PHYSIOLOGY,PHARMACOLOGY,TECHNIQUES, AND PRACTICE variety of equipment (of varying cost) has become avail- shown that a rapid series of coughing improves airway able to assist with this. clearance. The important roles of mucus production, mu- Why do we want to clear mucus from the lungs? Air- cociliary transport, and flow transport on airway clearance flow obstruction secondary to retained secretions increases was also described. the work of breathing, produces ventilation-perfusion mis- From these presentations it became clear that there is a match, and can result in gas exchange abnormalities. Re- “yin and yang” related to mucus and cough. On one hand, tained secretions can serve as a source of infection and mucus production and cough are important for airway de- inflammation. Coughing and clearing secretions is aesthet- fense and protection of the lower respiratory tract against ically unpleasant. Airway clearance may be the most com- inhaled irritations (yang). On the other hand, excessive monly prescribed therapy in patients with cystic fibrosis mucus obstructs airways, and excessive cough has been (CF), and chest physical therapy (CPT) may be the most associated with a number of complications (yin). frequently omitted treatment outside the hospital in this patient population. The objective of this RESPIRATORY CARE journal confer- Pharmacology ence was to review the scientific basis and clinical evi- dence for the use of airway clearance therapy to guide the Ruben Restrepo described the roles of sympathomimet- most appropriate approach to airway clearance. An im- ics and anticholinergics related to mucociliary clearance. pressive international group of clinicians and scientists was brought together to highlight the state of the art for For sympathomimetics, improved mucociliary clearance what is known, as well as new directions and opportunities has been shown in several species, several diseases, and for the study of mucus and sputum. with several formulations (but not in all studies). Increased mucociliary clearance may require greater doses than those Physiology used for bronchodilation. Albuterol may improve muco- ciliary clearance and cough clearance after lung transplan- The physiology of mucus secretion was addressed by tation, which may be important in this patient population. Duncan Rogers. He pointed out the physiologic impor- Overall, the clinical importance of sympathomimetics on tance of “getting airway secretions right”—not too runny, airway clearance is unclear. Moreover, sympathomimetics not too thick; not too wet, not too dry. He described human may also be counterproductive in some conditions, such as bronchial glands that consist of mucous cells (secrete mu- tracheomalacia. The effects of anticholinergics on muco- cins), serous cells (secrete antibacterials), and water secre- ciliary clearance has also been studied in several species, tion. Normally, Ͻ 10 mL/d of mucus is produced, and this several diseases, and with several formulations. Commonly serves as first-line defense and is protective of the lower used inhaled anticholinergics do not retard mucus clear- respiratory tract. With airway mucus hypersecretion, the ance in a clinically important manner. amount of mucus produced is abnormal, and this is patho- The roles of mucolytics and mucokinetics were addressed logic and no longer protective. In the case of mucus hy- by Duncan Rogers. He helped us distinguish between ex- persecretion, there is submucosal gland hypertrophy, gob- pectorants (which increase volume and/or hydration of se- let cell hyperplasia, and increased mucin synthesis. There cretions, and may induce cough), mucolytics (which re- is also plasma exudation, decreased mucociliary transport, duce viscosity of mucus), mucokinetics (which increase mucostasis, and mucus plugs. Inducers of airway mucus kinesis of mucus by cough), and mucoregulators (which secretion/hypersecretion include inflammatory mediators, reduce the process of chronic mucus hypersecretion). It cellular derived mediators, plasma-derived mediators, neu- was pointed out that there are competing effects of muco- ral mechanisms, proteinases, bacterial products, irritant active agents related to mucociliary clearance and cough. gases, reactive gas species, mechanical strain, pH, and ovalbumin sensitization. A thin mucus layer, ideal sol depth, increased elasticity, Cees van der Schans addressed the physiology of cough and decreased viscosity favors mucociliary clearance. A and airway clearance. He showed us that cough can be thick mucus layer, excess sol, lower elasticity, and higher simulated in the laboratory. With simulated cough, it can viscosity favors cough. The results of a Cochrane review be shown that airway constriction improves airway clear- suggest that mucolytics may be of benefit in COPD.1 How- ance, provided that a choke point does not occur. This is ever, it is unclear whether the mechanism was indeed air- from work conducted with a cough machine that Bruce way clearance or some other effect, such as antioxidant Rubin developed with Malcolm King about 15 years ago. effects. Although affecting mucus would seem a reason- There is an important balance between compression and able goal, the usefulness of mucolytics is essentially un- collapse of airways; with collapse of the airway, clearance proven. This continues to be an area of active scientific is obviously inhibited. With simulated cough it can also be inquiry, and new agents are being explored. RESPIRATORY CARE • OCTOBER 2007 VOL 52 NO 10 1393 AIRWAY CLEARANCE:PHYSIOLOGY,PHARMACOLOGY,TECHNIQUES, AND PRACTICE Techniques wall compression is probably better than CPT for secretion clearance; and high-frequency chest wall oscillation is per- Conventional CPT is the traditional technique used to haps a better ventilation technique than an airway clear- enhance airway clearance. This therapy was addressed by ance technique. Guidelines from the American College of Cees van der Schans, who defined CPT as any combina- Chest Physicians, although not strictly evidence-based, tion of postural drainage, percussion, chest shaking, huff- state, “In patients with CF, devices designed to oscillate ing, and directed coughing; it does not include the use of gas in the airway, either directly or by compressing the exercise, forced expiration technique,