The Science Journal of the American Association for Respiratory Care


July 2002 / Volume 47 / Number 7 / Page 757

Secretion Clearance Techniques: Absence of Proof or Proof of Absence?

Healthy lungs clear mucus from the airways via the mucociliary escalator. When the mucociliary escalator is challenged in disease, secretions are cleared with the cough reflex. Acute and chronic respiratory diseases are commonly associated with increased respiratory secretions due to increased mucus production, impaired mucociliary transport, or a weak cough. However, it is unknown whether increased respiratory secretions contribute to the deterioration of respiratory function, or whether this is merely a symptom associated with the progression of the disease. Moreover, increased mucus expectoration does not necessarily imply mucus stasis in the airways.

Clinicians and patients are troubled by respiratory secretions, and standard practice calls for efforts to clear these from the lungs. An important proportion of respiratory therapists' time is spent in efforts to remove secretions from the lower respiratory tract. For patients with diseases such as cystic fibrosis a great deal of each day can be spent using techniques designed to enhance secretion clearance. In recent years a variety of techniques for secretion clearance have become available. Many of these techniques are described in this issue of RESPIRATORY CARE. Despite clinical enthusiasm for many of these by both clinicians and patients, there is sparse high-level evidence demonstrating benefit from many of these techniques. As pointed out by others1-14 and me,15 there are a number of methodologic limitations of published reports of secretion clearance techniques. Most studies are small and use crossover designs (rather than randomized parallel designs). Sham therapy was not used in most studies -- often one technique was compared to another (for example, chest physiotherapy vs positive expiratory pressure). Many studies were limited to short-term outcomes such as sputum clearance with a single treatment session. It might be argued that short-term outcomes are irrelevant and that the focus should be on long-term outcomes such as disease progression, quality of life, and patient satisfaction.

Conducting a methodologically strong study (ie, placebo-controlled prospective randomized trial) with an adequate sample size and important outcomes (eg, disease progression, morbidity, mortality) is difficult. Such a study would be expensive and industrial support is not likely. Moreover, secretion clearance is an integral component of disease management for patients with increased sputum production, raising serious ethical concerns about placebo-controlled studies. I have always found it intriguing that a new drug to improve secretion clearance must pass through the usual Phase 1 through Phase 3 approval process, whereas a new device is not subjected to the same scrutiny.

Is absence of proof the same as proof of absence? Does the lack of evidence mean lack of benefit? Is the lack of evidence due to study methodology, or is there really no benefit from many techniques used to enhance secretion clearance? Although we should not be dogmatic about endorsing a therapy with absence of proof of its benefit, we must also not be dogmatic about abandoning a therapy because of absence of proof of its benefit -- absence of proof is not proof of absence. In fact, from a methodological and statistical standpoint, absence of proof is very difficult to prove. Given a lack of evidence, I suggest the following clinical hierarchy of questions when considering secretion clearance therapy for a patient.

  1. Is there a pathophysiologic rational for use of the therapy? Is the patient experiencing difficulty clearing secretions? Are retained secretions affecting lung function in an important way, such as gas exchange or lung mechanics? Remember that the production of large amounts of sputum does not necessarily mean that the patient is experiencing difficulty clearing sputum.
  2. What is the potential for adverse effects from the therapy? Which therapy is likely to provide the greatest benefit with the least harm?
  3. What is the cost of the equipment for this therapy? Some devices are very expensive.
  4. What are the preferences of the patient? Lacking evidence that any technique is superior to another, patient preference is an important consideration.

When a decision is made to try a secretion clearance technique, an n-of-1 trial can be conducted.16-18 For example, imagine that a decision is made to try positive expiratory pressure therapy for a patient with cystic fibrosis. The clinician and patient agree that a clinically useful outcome measure is sputum production. A 12-week trial is designed. For 1 week, the only sputum clearance technique used is huff coughing. For a second week, positive expiratory pressure (in addition to huff coughing) is used, as provided by the manufacturer. For a third week, the positive expiratory pressure device is used with pressure set at such a low level that it is probably sub-therapeutic (sham therapy). The patient is naive to the therapy and does not know whether the device should be used with or without the high-pressure setting. The order of treatments is randomized (the patient flips a coin) and the sequence is repeated 4 times. Each day, the sputum produced during the therapy session is weighed. A diary is also kept, in which events such as chest infections are logged. At the end of 12 weeks the results are analyzed (this may include statistical analysis), reviewed together by the clinician and patient, and a collaborative decision is made regarding the benefit of the therapy. In this manner an objective decision is made regarding the benefits of the therapy for this individual patient.

Despite the clinical observation that retained secretions are detrimental to respiratory function and despite anecdotal associations between secretion clearance and improvements in respiratory function, there is a dearth of high-level evidence to support any secretion clearance technique. This is problematic, given that secretion clearance is an important aspect of respiratory care practice. Although lack of evidence does not mean lack of benefit, it is desirable to have better evidence to support the practice. Appropriately powered and methodologically sound research is desperately needed. This provides an opportunity for respiratory therapists to conduct research on a very important aspect of our practice. For the effective therapy of our patients and for the efficient use of health care resources, it is incumbent upon us to improve the scientific basis for secretion clearance techniques.

Department of Respiratory Care
Massachusetts General Hospital
Harvard Medical School
Boston, Massachusetts


  1. Stiller K. Physiotherapy in intensive care: towards an evidence-based practice. Chest 2000;118(6):1801-1813.
  2. Williams MT. Chest physiotherapy and cystic fibrosis. Why is the most effective form of treatment still unclear? Chest 1994;106(6):1872-1882.
  3. Krause MF, Hoehn T. Chest physiotherapy in mechanically ventilated children: a review. Crit Care Med 2000;28(5):1648-1651.
  4. McCrory DC, Brown C, Gelfand SE, Bach PB. Management of acute exacerbations of COPD: a summary and appraisal of published evidence. Chest 2001;119(4):1190-1209.
  5. Dean E, Ross J. Discordance between cardiopulmonary physiology and physical therapy: toward a rational basis for practice. Chest 1992;101(6):1694-1698.
  6. Kirilloff LH, Owens GR, Rogers RM, Mazzocco MC. Does chest physical therapy work? Chest 1985;88(3):436-444.
  7. Wallis C, Prasad A. Who needs chest physiotherapy? Moving from anecdote to evidence. Arch Dis Child 1999;80(4):393-397.
  8. Judson MA, Sahn SA. Mobilization of secretions in ICU patients. Respir Care 1994;39(3):213-226.
  9. Eid N, Buchheit J, Neuling M, Phelps H. Chest physiotherapy in review. Respir Care 1991;36(4):270-282.
  10. Thomas J, Cook DJ, Brooks D. Chest physical therapy management of patients with cystic fibrosis: a meta analysis. Am J Respir Crit Care Med 1995;151(3 Pt 1):846-850.
  11. Jones A, Rowe BH. Bronchopulmonary hygiene physical therapy in bronchiectasis and chronic obstructive pulmonary disease: a systematic review. Heart Lung 2000;29(2):125-135.
  12. Thomas J, DeHueck A, Kleiner M, Newton J, Crowe J, Mahler S. To vibrate or not to vibrate: usefulness of the mechanical vibrator for clearing bronchial secretions. Physiother Can 1995;47(2):120-125.
  13. van der Schans C, Prasad A, Main E. Chest physiotherapy compared to no chest physiotherapy for cystic fibrosis. Cochrane Database Syst Rev 2000;(2):CD001401.
  14. Flenady VJ, Gray PH. Chest physiotherapy for preventing morbidity in babies being extubated from mechanical ventilation. Cochrane Database Syst Rev 2000;(2):CD000283.
  15. Hess DR. The evidence for secretion clearance techniques. Respir Care 2001;46(11):1276-1293.
  16. Guyatt G, Sackett D, Taylor DW, Chong J, Roberts R, Pugsley S. Determining optimal therapy: randomized trials in individual patients. N Engl J Med 1986;314(14):889-892.
  17. Guyatt G, Sackett D, Adachi J, Roberts R, Chong J, Rosenbloom D, Keller J. A clinician's guide for conducting randomized trials in individual patients. CMAJ 1988;139(6):497-503.
  18. Montori VM, Guyatt GH. What is evidence-based medicine and why should it be practiced? Respir Care 2001;46(11):1201-1211.

Correspondence: Dean R Hess PhD RRT FAARC, Department of Respiratory Care Services, Massachusetts General Hospital, 55 Fruit Street, Ellison 401, Boston MA 02114-2696. Email:

You are here: » Contents » July 2002 » Page 757