Cough is the most common chief complaint in the United States and Australia.1Although the differential diagnosis of cough in children is extensive, coughs can have very characteristic presentations that can aid the clinician in the diagnosis and management of this problem.

Cough is an innate defense mechanism of the lungs, used to clear the airways.2The absence of cough, or the presence of only a weak, ineffective cough, can be harmful or fatal.3

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Mucus, pus, fluid, and such inhaled substances as dust, pollen, smoke, and foreign bodies must be cleared from the airways regularly in order to prevent damage to lung tissue that may impair gas exchange.(Figure 1,Figure 2)

Cough helps to clear the 20 mL to 30 mL of mucus and secretions a person can produce per day on average. This helps explain the frequency of cough: It is estimated that a child can cough up to 11 times per day, increasing to 140 with respiratory infections. City-dwellers cough more on average than do their rural counterparts, being exposed to more air pollution and smog.2

The burden of cough

Cough can be a warning sign of pneumonia, asthma, cystic fibrosis, or other disease. As a symptom, cough can be quite detrimental, interfering with sleep, preventing participation in play or exercise, and causing missed days of school for children as well as missed days of work for their caregivers.2

Cough is a major public health issue, accounting for an estimated 29.5 million outpatients visits per year.1A cough lasting longer than eight weeks in adults is the most common reason for referral to a pulmonary specialist; in children, a chronic cough is defined as a cough lasting more than four weeks.4

The annual cost for cough treatment is unknown, as patients often choose to pursue their own remedies without consulting a health-care provider. The money spent on over-the-counter cough and cold preparations alone exceeds billions of dollars.1 

Despite high usage levels of these products by consumers, the American Academy of Pediatrics (AAP) holds that they are not effective in children under age six years, and may pose a health risk in some cases.5Studies have indicated that the use of dextromethorphan and diphenhydramine are not superior to placebo in relieving symptoms in children, and do nothing to improve quality of sleep.6

Cough physiology

A cough consists of three phases: inspiratory, compressive, and expiratory (Table 1). The initial phase, inspiratory, is the least critical of the three. It is characterized by an inhalation of air to near total lung capacity. The greater the volume of air inhaled, the greater the positive intrathoracic pressure generated to produce the kinetic energy of a cough. Cough can, however, be generated with low volumes of air.

The second, compressive phase begins with closure of the glottis at the end of inhalation. Patients with a tracheostomy or endotracheal tube, however, can still generate a cough with an open glottis through a huffing maneuver.

This closure maintains the volume of air inhaled into the lungs while simultaneously increasing the intrathoracic pressure. The high pressures that are generated can be transmitted to the nervous, cardiac, gastrointestinal, genitourinary, and musculoskeletal systems, resulting in such secondary symptoms as headache, palpitations, vomiting, incontinence, and pain.

The final, expiratory phase is the most dynamic. It is characterized by an explosive release of air and the opening of the previously closed glottis. This blast of turbulent air can reach flow rates of up to 12 liters per second in adolescents and adults, although such rates would not be achieved in younger children.

Table 1. Phases of cough
Inspiratory Compressive Expiratory
Inhalation to near total lung volume Closure of glottis Opening of glottis
Increase in intrathoracic pressure Maintenance of lung volume Dynamic release of air
Generation of kinetic energy Large increase in intrathoracic pressure Rapid decline in flow rate, lung volume, and intrathoracic pressure

The flow rate rapidly decreases as both pulmonary pressure and lung volume fall. The velocity of the flow rate is directly proportional to the degree of compressive pressures and inversely proportional to the cross-sectional area of the airways involved. 3 The greater the involvement of the more distal, smaller-diameter airways, the greater the end velocity of the cough generated.

The effectiveness of a cough depends on several factors, among them the strength of the respiratory muscles, particularly those used in exhalation, and the rheologic character of the mucus being eliminated from the airway. 

The high airway velocities within the bronchioles, mucus is torn off from the pulmonary parenchyma and suspended in the lumen. The flow rate can then produce waves of mucus. Additionally, the collapsible nature of the bronchi enhances mucus clearance through vibration. 

Cilia also play a role in airway clearance, which is enhanced by the vibratory effect of the airways. Mucus lying close to the surface of the cilia becomes less viscous and more easily cleared. Overall, increased viscosity of mucus impairs clearance by airflow alone; however, elimination can be enhanced by ciliary beat frequency.

A cough capable of eliminating debris from the lungs is produced by the complex interplay among the pressures generated by respiratory muscle strength, the volume of air inhaled, the rheology of mucus, the rigidity and collapsibility of the airway, and the efficacy of cilia. 

Certain disorders are characterized by ineffective coughs that have an impaired ability to clear mucus from the alveoli and bronchioles. In cystic fibrosis, for example, mucus becomes inspissated, or thickened, through a defect in the sodium chloride ion channels. Due to the tenacious mucus, children with cystic fibrosis lack the ability to mobilize mucus into the bronchial lumen. 

A defect in the mobility of cilia, seen in ciliary dyskinesia, will also result in an inability to mobilize mucus out of the respiratory tract. In both instances, the entrapped mucus can lead to atelectasis, pneumonia, and bronchiectasis.