Involuntary cough is a complex process, and it involves the activation and interaction of several subtypes of vagal afferent nerves, including rapidly adapting receptors, mechanoreceptors, C-fibers, and the brainstem. The rapidly adapting receptors are most prevalent in the larynx, main carina, and branching points in the tracheobronchial tree and are activated by chemical (e.g., smoke), inflammatory (e.g., histamine), and mechanical (e.g., foreign body) stimuli, causing bronchospasm and mucus secretion. Bronchospasm and mucus secretion, in turn, stimulate the mechanoreceptors and C-fibers. The cough reflex is integrated in the brainstem where motor output to the larynx, bronchial tree, and respiratory muscles initiates the cough reflex (Canning, 2006; Dicpinigaitis, 2003; Dicpinigaitis & Gayle, 2003; Leach, 2004). An esophageal-tracheobronchial reflex (also mediated by vagal innervation) is thought to cause cough associated with gastroesophageal reflux disease (GERD) from acid reflux that triggers receptors in the lower esophagus (Irwin & Madison, 2001; Poe & Kallay, 2003).

Voluntary cough is mediated via the cortex. Patients can consciously induce cough to clear the airways (Leach, 2004). The ability to initiate cough is especially helpful for patients whose disease process impairs the spontaneous cough reflex.

The cough reflex initiates a deep inspiration (inspiration phase), followed by closure of the glottis and build-up of intrathoracic pressure (compressive phase). Then, the respiratory muscles contract against the closed glottis resulting in forceful expulsion of air and other material as the glottis opens (expiratory phase) (Estfan & LeGrand, 2004; McCool, 2006; Waller & Caroline, 2000).

There are multiple causes of cough (Table 22-1). Patients with a chronic cough and a normal chest radiograph, who do not smoke, and who are not receiving treatment with ACE inhibitors should be evaluated for upper airway cough syndrome (formerly called postnasal drip syndrome), asthma, nonasthmatic eosinophilic bronchitis, and/or GERD (Irwin, Baumann, Bolser et al., 2006). Aspiration caused by pharyngeal dysfunction is another potential cause of cough.

The pressures, velocities, and energy required to clear the airways put patients at risk for multiple complications (Box 22-1). Clinicians must be aware of the profound impact cough can have on quality of life and aggressively work to eliminate, or at least minimize, cough.

Both subjective and objective measures should be used to evaluate cough. Subjective measures include patient diaries, visual analog scales, symptom distress scales (these usually measure multiple symptoms and may be disease specific), and general or cough-specific quality of life instruments (Irwin, 2006). Diaries require a motivated patient to maintain for any length of time. Visual analog scales are frequently used and, although they have not been psychometrically tested, they are recommended because they are commonly used and valid and they are likely to yield results that are different from but complementary to results of subjective instruments (Irwin, 2006).

There are three subjective instruments that have undergone extensive psychometric testing: breathlessness, cough, and sputum scale (BCSS), cough quality-of-life questionnaire (Dyspepsia Quality-of-Life Questionnaire [DQLQ]), and the Leicester Cough Questionnaire (LCQ). All three of these instruments use Likert-type scales for patient evaluation; the BCSS has 3 items, the DQLQ has 28 items, and the LCQ has 19 items (French, Irwin, Fletcher et al., 2002; Irwin, 2006). There are several disease-specific scales that include measurement of cough. Most of these have not been as extensively studied but have shown good reliability and validity in small studies. For example, the Lung Cancer Cough Questionnaire (LCCQ) and the Lung Cancer Wheezing Questionnaire (LCWQ) were used in a pilot study to measure these symptoms in patients with lung cancer (Chernecky, Sarna, & Waller, 2004). Use of one of these rating scales may assist clinicians with monitoring cough over time and with evaluating treatment.

Objective measures of cough used in palliative care settings most often involve clinician evaluation of frequency, character, and sputum production. More precise measures that may be used by a consulting pulmonologist for difficult-to-control cough include 24-hour counts of coughing either with an observer (labor intensive) or a monitoring device, use of pharmacological tussigenic challenges to assess the effect of therapy on cough, comparison of airway inflammation indexes in induced sputum samples, and comparison of exhaled nitric oxide levels (Irwin, 2006).

The etiology of a cough can be determined in 80% of patients with advanced progressive illnesses by a thorough history and physical examination (Waller & Caroline, 2000).