I think sputum induction by nebulized hypertonic saline is the method of choice

1.For cough to be effective, the linear velocity of gas traveling through the airways should be high. Since the linear velocity of gas is related to flow and the cross-sectional area of the airways, cough is most effective when expiratory flows are great (effort independent) and dynamic compression (effort dependent) leads to a reduction of the cross-sectional area of the larger downstream airways.

file:///C:/Users/user/Desktop/New%20Research%20folder/Pathophysiology%20of%20cough.%20-%20PubMed%20-%20NCBI.html

2.The mechanisms by which it achieves this are (i) high velocity of expiratory gas flows produced in the expiratory phase (ii) the compression of the lungs and airways by high positive pleural pressure generated in the compressive phase. Cough threshold can be determined by giving acetic acid, citric acid or capsaicin inhalation challenge test. This has practical application in clinical research.file:///C:/Users/user/Desktop/New%20Research%20folder/The%20applied%20physiology%20of%20cough.%20-%20PubMed%20-%20NCBI.html

3.https://www.slideshare.net/AbinoDavid/mechanism-of-cough

5. MechanismAbout 2.5L of air isinspired Epiglottis closes, vocal cords shut tightly to entrap the air within the lung

6. Abdominal muscles contractforcefully, pushing againstthe diaphragm Internal intercosatal muscles also contract forcefully Pressure in the lungs rises to 100mmHg or more.Markedly positive intrathoracic pressure causes narrowing of the trachea.

7. Vocal cords and epiglottissuddenly open widely . The large pressure differential between the airways and the atmosphere coupled with tracheal narrowing produces rapid flow rates through the trachea. Air is expelled at velocities ranging from 75 to 100 miles/hr.

I am mentioning these references just to revise the physiology of cough which is Forces Expiratory Volume  like Lung Function Test. And from that we determine MMEFR 25-50. that represents the small airways obstruction or impending WALRI/Asthma.

Just manually in clinics eliciting cough we can determine this even when the stethoscope cannot peak up this.from normal auscultation.

thanks for nice subjects

I think it is very hard to make a child obey your commend 

so should depend on other way of examination

with regards 

 Thank you Dr Ali for your kind response. You are very right to say 'it is difficult' . In my studenthood in Medical graduation or post graduation life I have not also been taught it. But today this is a part of my routine clinical examination for every patient. It is very easy also. The only exception is when the child or newborn is in crying state. And just eliciting cough and listening its nature I can deduce the wheeze even when it is not obvious with stetho. If you ask I can demonstrate it .

Thank for your concern with this subject.

I think that cough is a gently strenuous activity. Therefore, patients with small airways obstruction will present wheezing on ausculation after coughing. In my clinical practice, I usually ask patients suspected with airway obstruction to both inhale and exhale strenuously.

Thank you Dr Lam. You are correct sir. But your young patient who cannot follow your command what you will do for them clinically to detect wheeze which is audible in stethoscope?