Question: “What is the value of coughing in forced expiration? Is this a sign of bronchial hyperreactivity? What happens pathophysiologically?”
Answer: Coughing is a complex, centrally nervously controlled, explosive form of exhalation. However, coughing is also a protective reflex for human survival. If it fails, foreign body aspiration, asphyxiation, suffocation and permanent mucus retention and respiratory insufficiency may occur, e.g. in respiratory muscle weakness.
Up to 40% of the world’s population suffer from permanent coughing – about 30% of adults suffer from nocturnal coughing, 10% from productive coughing and 10% from unproductive coughing. 10 – 40 % of all referrals to a pneumologist include the clarification of a cough. Coughing is therefore one of the most common symptoms of pneumological diseases.
Cough is clarified according to the guidelines with the help of lung function testing, among other things, whereby cough can occur during the expiratory manoeuvres, which can be explained reflexively. Bronchial hyperreactivity is only present if obstructive ventilation disorder occurs several times in the flow volume curve during forced expiratory manoeuvres. The inhalative non-specific provocation test is proof of this!
The course of the cough reflex
The reflex arc consists of five sections: Cough receptors, afferent thigh, cough centre, efferent thigh and the musculature as effector organ. The four phases of the cough reflex consist of:
- quick inspiration with glottis and glottis open
- compression with closure of the glottis, contraction of the expiratory muscles, opening of the glottis
- acceleration with increase of transbronchial pressure, dynamic compression of the bronchial system
- Expiratory – Expulsion: rapid expiratory flow at approx. 250 m/sec
The reflex process begins with the stimulation of the mechanoreceptors (Fig. 1). The cough reflex can be initiated wherever cough receptors are present: in the entire area of the upper and lower respiratory tract, in the lung parenchyma, but also in the pleura, pericardium, diaphragm, esophagus and stomach. However, the distribution of the receptors is not uniform: they are located closest to the larynx and its surroundings (pharynx, trachea, large bronchi).
The volume of the breath (spirometry) immediately before coughing mainly determines which section of the airways (between larynx and bronchi of the 5th – 6th generation) is affected.
Cough in bronchial collapse
An effective cough reflex requires more or less intact anatomical conditions (no severe airway obstruction, no bronchial collapse, normal respiratory muscle and larynx function, normal static volumes, normal viscosity of bronchial secretion). Especially in case of bronchial wall instability (e.g. chronic bronchitis) the conditions change. If compression leads to collapse, e.g. with forced expiration, this also prevents secretion from being eliminated. Compression without total collapse is beneficial for effectiveness, because in the area of the compressed section (in healthy people: the pars membranacea) a considerable flow acceleration occurs during coughing. The flow volume curve documents an instability of the Pars membranacea in healthy patients up to tracheobronchial dyskinesia as a typical jagged curve already at the beginning of expiration without obstruction (Fig. 2).
Cough as asthma equivalent
Chronic unproductive cough without bronchial obstruction proven in lung function or clinically evident is referred to as “cough as asthma equivalent”. Breathlessness as well as whistling and humming are then missing. Therefore, this “cough as asthma equivalent” (“cough type asthma”, “cough variant asthma”) cannot be diagnosed spirometrically and whole-body plethysmographically on the basis of a reversible bronchial obstruction typical of asthma.
A non-specific inhalative provocation test is required. If bronchial hyperreactivity is detected in a CPH (chronic persistent cough) in the non-specific inhalative provocation test, the final diagnosis “cough as asthma equivalent” can only be made if it is confirmed by response to antiasthmatic therapy, since asymptomatic bronchial hyperreactivity exists in 25 – 30 % of the population. A negative inhalative provocation test, on the other hand, is very likely to rule out asthma as the cause of coughing.
Notes on therapy
Patients with cough as asthma equivalent respond very well to classical antiasthmatic therapy, usually inhalative corticosteroids, but also to beta-2 agonists or leukotriene antagonists. Some patients develop classical asthma.
Coughing with COPD
The cough in COPD is caused by a neurogenic inflammation of the bronchial mucosa. The inflammatory cytokines, especially Substance P, stimulate the cough receptors. In addition, there is the hypercrine, a physical stimulus of the cough receptor. In spirometry, the cough is typically documented at the end of exhalation, whereby bronchial hyperreactivity can worsen the degree of obstruction in multiple expiratory manoeuvres – recognizable by an increasing flattening (“sagging”) of the expiratory curve.
Conclusion for spirometry
- Cough at the beginning of the expiratory curve without obstruction: central respiratory instability, artifact
- Cough from beginning to end of expiratory curve without obstruction: instability of pars membranacea, tracheobronchial dyskinesia
- Cough at the end of the expiratory curve without obstruction: Instability of the peripheral airways (Fig. 3)
- Cough at end of expiratory curve with obstruction: COPD, asthma, bronchial hyperreactivity
Conflicts of interest: none declared
Appeared in: The General Practitioner, 2013; 35 (18) Page 54-56