Cough and Aspiration

Abstract

To the Editor: In their recent article in CHEST (November 2008), Smith Hammond et al1Smith Hammond CA Goldstein LB Horner RD et al.Predicting aspiration in patients with ischemic stroke: comparison of clinical signs and aerodynamic measures of voluntary cough.Chest. 2008; ([Epub ahead of print])PubMed Google Scholar concluded that, for patients after a stroke, an objective assessment of voluntary cough (VC) is useful to assess the likelihood of aspiration. VC, starting with a deep inspiration, differs in muscular activities and timing from reflex cough.2Lasserson D Mills K Polkey M et al.Differences in motor activation of voluntary and reflex cough in humans.Thorax. 2006; 61: 699-706Crossref PubMed Scopus (61) Google Scholar Furthermore, the initial inspiration would cause aspiration. However, touching the glottis of animals, including humans, causes the so-called expiration reflex (ER)3Korpas J Tomori Z Cough and other respiratory reflexes. Karger, Basel, Switzerland1979Google Scholar; the lack of an initial inspiration excludes aspiration. Compared with cough, the ER has different sensors, brainstem circuits and responses to antitussive agents, anesthesia, and physiologic and pathologic inputs. Its conduction delay from larynx to expiratory muscles (mean duration in humans, 17 ms) is too short to allow initial involvement of the cerebral cortex; this makes it unlikely that the ER can be voluntarily induced, except possibly as a “huff.” There are simple methods of testing the ER (we use one).4Addington WR Stephens RE Widdicombe JG et al.Effect of stroke location on the laryngeal cough reflex and pneumonia risk.Cough. 2005; 1: 4Crossref PubMed Google Scholar Subjective measurement of VC has not been shown to give less acceptable results than the objective method. One needs to compare simultaneous objective and subjective assessments, the latter by a trained observer. In another article,5Smith Hammond C Cough and aspiration of food and liquids due to oral pharyngeal dysphagia.Lung. 2008; 186: S35-S40Crossref PubMed Scopus (34) Google Scholar, Fig 2 Smith Hammond included a figure showing that, compared with a healthy subject, a person with mild aspiration had a 90% smaller peak-to-peak oral airflow during VC, and a person with severe aspiration an 80% smaller peak-to-peak oral airflow during VC. A trained observer would easily have identified these differences subjectively (the examples may have been atypical). VC was correlated with other assessments of aspiration risk, but no respiratory outcomes were reported to indicate the significance of these findings. Furthermore, of 30 patients with acute unilateral middle cerebral artery infarcts, 11 had a weak or absent VC but a normal ER, and pneumonia developed in none.6Stephens RE Addington RA Widdicombe JG Effect of acute unilateral middle cerebral artery infarcts on voluntary cough and the laryngeal cough reflex.Am J Phys Med Rehabil. 2003; 92: 379-383Google Scholar Other studies (see Stephens et al6Stephens RE Addington RA Widdicombe JG Effect of acute unilateral middle cerebral artery infarcts on voluntary cough and the laryngeal cough reflex.Am J Phys Med Rehabil. 2003; 92: 379-383Google Scholar) have shown a correlation between a weak or absent ER and the occurrence of poststroke pneumonia. VC assessed subjectively by a trained observer may be the simplest and cheapest method of assessing cough, but more sophisticated methods should concentrate on the reflex that prevents aspiration, the ER. Comparison of methods for assessing the risk of aspiration is limited in value because they cannot be related accurately to clinical outcome. Ideally, one would take a group of patients liable to aspirate, choose a method to assess this liability, and see in which patients signs and symptoms of aspiration develop without any preventative measures (or with identical preventative measures) applied to all the patients. This should not be done, for clear ethical reasons.