Pleural Effusions and Inspiratory Crackles

The two most sensitive examination findings of pleural effusion are inspiratory crackles and decreased chest expansion on the affected side. Why is pleural effusion associated with inspiratory crackles? I understand that crackles are an indication of fluid. This makes acoustic sense to me in the case of pulmonary edema when fluid is being sloshed around in the alveoli. But why crackles when fluid is in the pleural space? By analogy, I can partially fill a balloon with water but I can't imagine how this material could result in cracking noises.

As per an interpretation of the physiological effects,

"Excessive amounts of such fluid can impair breathing by limiting the expansion of the lungs during inhalation."

The normal expansion/re-bouding of the lungs is necessary for necessary natural physiological 'percussion/vibration' effect to allow natural mucous movements within the lungs. Ie. When the Type I Pneumocytes produce the mucous, the lungs require expansion for normal movement of mucous either into the lymph system, or coughed up. etc.

Thanks Canuck,

Just to clarify.

With healthy lungs, full chest expansion acts to facilitate mucous transport to the lymph system. With a pleural effusion, decreased chest expansion (from the mechanical effects of fluid in the pleura) results in mucous retention in the alveoli/airways as normal mucous transport is impeded. This mucous retention is the actual cause of crackles (not the fluid in the pleural space).

Am I correct in saying this?

Kevin

Hey Kevin - compliments on a very good question!

I think you may be putting too much faith in your stethoscope which will pick up any sounds in the chest wall from the skin to the lungs (including the pleura between)

So if you imagine fluid in the chest wall - and on inspiration the airways "attempting" to open i would think the crackles are from some dislocation of this fluid (minute amounts)........am i wrong in thinking this or am i just confusing the matter???