Cough provides an essential protective function for human airways and lungs. Without an effective cough reflex, we are at risk for retained airway secretions and aspirated material, predisposing to infection, atelectasis, and respiratory compromise. At the other extreme, excessive coughing can be exhausting; can be complicated by emesis, syncope, muscular pain, or rib fractures; and can aggravate abdominal or inguinal hernias and urinary incontinence. Cough is often a clue to the presence of respiratory disease. In many instances, cough is an expected and accepted manifestation of disease, such as during an acute respiratory tract infection. However, persistent cough in the absence of other respiratory symptoms commonly causes patients to seek medical attention, accounting for as many as 10–30% of referrals to pulmonary specialists.
Spontaneous cough is triggered by stimulation of sensory nerve endings that are thought to be primarily rapidly adapting receptors and C-fibers. Both chemical (e.g., capsaicin) and mechanical (e.g., particulates in air pollution) stimuli may initiate the cough reflex. A cationic ion channel, called the type-1 vanilloid receptor, is found on rapidly adapting receptors and C-fibers; it is the receptor for capsaicin, and its expression is increased in patients with chronic cough. Afferent nerve endings richly innervate the pharynx, larynx, and airways to the level of terminal bronchioles and into the lung parenchyma. They may also be found in the external auditory meatus (the auricular branch of the vagus nerve, called the Arnold nerve) and in the esophagus. Sensory signals travel via the vagus and superior laryngeal nerves to a region of the brainstem in the nucleus tractus solitarius, vaguely identified as the “cough center.” Mechanical stimulation of bronchial mucosa in a transplanted lung (in which the vagus nerve has been severed) does not produce cough.
The cough reflex involves a highly orchestrated series of involuntary muscular actions, with the potential for input from cortical pathways as well. The vocal cords adduct, leading to transient upper-airway occlusion. Expiratory muscles contract, generating positive intrathoracic pressures as high as 300 mm Hg. With sudden release of the laryngeal contraction, rapid expiratory flows are generated, exceeding the normal “envelope” of maximal expiratory flow seen on the flow-volume curve (Fig. 34-1). Bronchial smooth muscle contraction together with dynamic compression of airways narrows airway lumens and maximizes the velocity of exhalation (as fast as 50 miles per hour). The kinetic energy available to dislodge mucus from the inside of airway walls is directly proportional to the square of the velocity of expiratory airflow. A deep breath preceding a cough optimizes the function of the expiratory muscles; a series of repetitive coughs at successively lower lung volumes sweeps the point of maximal expiratory velocity progressively further into the lung periphery.
Flow-Volume Loop. Flow-volume curve with spikes of high expiratory flow achieved with cough.
Weak or ineffective cough compromises the ability to clear lower respiratory tract infections, predisposing to more serious infections and their sequelae. Weakness, paralysis, or pain of the expiratory (abdominal and intercostal) muscles is foremost on the list of causes of impaired cough (Table 34-1). Cough strength is generally assessed qualitatively; peak expiratory flow or maximal expiratory pressure at the mouth can be used as a surrogate marker for cough strength. A variety of assistive devices and techniques have been developed to improve cough strength, spanning the gamut from simple (splinting the abdominal muscles with a tightly-held pillow to reduce post-operative pain while coughing) to complex (a mechanical cough-assist device applied via face mask or tracheal tube that applies a cycle of positive pressure followed rapidly by negative pressure). Cough may fail to clear secretions despite a preserved ability to generate normal expiratory velocities, either due to abnormal airway secretions (e.g., bronchiectasis due to cystic fibrosis) or structural abnormalities of the airways (e.g., tracheomalacia with expiratory collapse during cough).
Table 34-1 Causes of Impaired Cough
| Save Table
Table 34-1 Causes of Impaired Cough
|Decreased expiratory-muscle strength|
|Decreased inspiratory-muscle strength|
|Impaired glottic closure or tracheostomy|
|Abnormal airway secretions|
|Central respiratory depression (e.g., anesthesia, sedation, or coma)|
The cough of chronic bronchitis in long-term cigarette smokers rarely leads the patient to seek medical advice. It lasts only seconds to a few minutes, is productive of benign-appearing mucoid sputum, and is not discomforting. Similarly, cough may occur in the context of other respiratory symptoms that, together, point to a diagnosis, such as when cough is accompanied by wheezing, shortness of breath, and chest tightness after exposure to a cat or other sources of allergens. At times, however, cough is the dominant or sole symptom of disease, and it may be of sufficient duration and severity that relief is sought. The duration of cough is a clue to its etiology. Acute cough (<3 weeks) is most commonly due to a respiratory tract infection, aspiration event, or inhalation of noxious chemicals or smoke. Subacute cough (3–8 weeks duration) is frequently the residuum from a tracheobronchitis, such as in pertussis or “post-viral tussive syndrome.” Chronic cough (>8 weeks) may be caused by a wide variety of cardiopulmonary diseases, including those of inflammatory, infectious, neoplastic, and cardiovascular etiologies. When initial assessment with chest examination and radiograph is normal, cough-variant asthma, gastroesophageal reflux, nasopharyngeal drainage, and medications (angiotensin converting enzyme [ACE] inhibitors) are the most common causes of chronic cough. Cough of less than 8 weeks' duration may be the early manifestation of a disease causing chronic cough.
Assessment of Chronic Cough
Details as to the sound, time of occurrence during the day, and pattern of coughing infrequently provide useful etiology clues. Regardless of cause, cough often worsens when one first lies down at night or with talking or in association with the hyperpnea of exercise; it frequently improves with sleep. Exceptions might ...