The buffering effect of middle ear negative pressure by retraction of the pars tensa

Objective: To measure the middle ear (ME) volume displaced when the pars tense (PT) retracts during conditions of atelectasis (AT). Study Design: Measurements were carried out on AT tympanic membrane (TM). Setting: Measurements were carried out on ambulatory patients in a private clinic. Patients: Thirty-nine patients treated for otologic disorders who came for a routine follow-up. Interventions: Quantitative assessment of ME volume displaced by retraction of the PT was achieved by measuring the amount of fluid needed to fill up the retracted part of a TM. The fluid-3% aqueous boric acid solution was colored with gentian violet. Main Outcome Measures: Retraction of the PT may prevent formation of high ME negative pressure by reducing the ME volume. Retraction of the PT acts as a pressure buffer according to Boyls law, in which volume x pressure = constant. Results: Direct correlation was found between the degree of AT and the ME volume displaced. by the AT. Measurements of 12 AT ears grade I showed them to have a volume of 5 mu L to 31 mu L (average, 19.5 mu L) and ten AT ears grade II had a volume of 15 mu L to 87 mu L (average, 39.5 mu L). Eight AT ears grade III had a volume of 26 mu L to 67 mu L (average, 54.3 mu L), and nine AT ears grade IV had 80 mu L to 200 mu L (average, 130.6 mu L). Conclusions: Retraction of the PT may counteract an important potential negative ME pressure (up to several hundred millimeters of water) depending on the degree of retraction and the extent of mastoid pneumatization.