Gas Supply, Demand and Middle Ear Gas Balance: Specific Aim 3a Part ii
Condition: Healthy
Study type: Observational

Summary: 
      The middle ear is an airspace located behind the eardrum that consists of two connecting
      compartments. The compartment directly behind the eardrum is called the tympanum and contains
      the three small bones of the middle ear, the hammer, anvil and stapes, that function to
      transfer eardrum movements to the inner ear so that you can hear. Behind the tympanum is the
      mastoid cavity which is a larger airspace subdivided into small air cells of unknown
      function. For normal hearing, it is important that the air pressure in the middle ear is
      similar to that of the environment so that the eardrum can move freely in response to sounds.
      The air pressure of the environment is not constant and is affected by changes in weather
      conditions (high and low pressure systems that move through the area) and by changes in
      elevation above sea level (the fullness in your ears that can be noticed when you ride in an
      elevator or in an airplane). The air pressure in the middle ear also changes because middle
      ear gas is constantly leaking from that airspace to the blood that flows through the walls of
      the middle ear. These effects (changing environmental air pressures and changing middle ear
      air pressure) are independent and cause the middle ear and environmental pressures to be
      different from each other. Periodically and during swallowing or yawning, any existing
      difference between middle ear and environmental air pressure is reset to zero by the opening
      of a biological tube (the Eustachian tube) that connects the middle ear to the back of the
      nose. This allows gas flow between the middle ear and the environment which increases or
      decreases middle ear pressure to the level in the environment at that time. Most people
      cannot open their Eustachian tubes at will and the number of automatic openings varies from
      infrequent to often in a population. Whether or not a person's usual frequency of Eustachian
      openings is good enough to keep the middle ear pressure the same as environmental levels
      depends on how fast gas is lost from the middle ear by gas leakage (diffusion) to blood. For
      example, in ears with very slow rates of gas loss, the Eustachian tube does not need to open
      very frequently to keep the middle ear at environmental pressure. Some researchers believe
      that the mastoid compartment functions to control the rate of gas loss to blood, with larger
      mastoid volumes associated with lesser rates of middle ear gas loss. In this experiment, the
      investigators plan to test this by measuring mastoid and tympanum volumes using Computer
      tomography (CT) and the rate of blood to middle ear gas transfer using a technique that
      involves breathing air that contains laughing gas (Nitrous Oxide=N2O) and measuring middle
      ear pressure change using tympanometry (a technique that involves putting an ear plug into
      the ear canal and measuring the pressure). From past studies in patients undergoing short
      surgical or dental procedures, the investigators know that breathing gas mixtures that
      contain N2O will increase the blood levels of that gas, cause gas to go from blood to the
      middle ear and increase middle ear pressure. The investigators predict that the rate of
      change in middle ear pressure while breathing a gas mixture containing 25% N2O and the normal
      oxygen level (20%) of air will be less for those ears with larger mastoid volumes. If the
      investigators prediction is correct, they will be able to explain why ears with larger
      mastoid volumes are better able to keep their pressure like that of the environment even if
      the Eustachian tube does not open often.
    

Detailed description: 
      The middle ear (ME) pressure balance is an important contributor to the maintenance of ME
      health. ME pressure is a measure of the number of contained gas moles and processes that add
      or remove gas moles from the relatively fixed volume ME cavity change its pressure. Three
      routes for ME gas exchange have been identified; 1) passive, gradient-dependent species
      exchange with the local blood via diffusion across the ME mucosa; 2) passive,
      gradient-dependent species exchange with the ambient environment via diffusion across the
      tympanic membrane, and 3) bolus exchange of mixed gases between the nasopharynx and ME during
      transient openings of the Eustachian tube. Total ME pressure and the trajectory of ME
      pressure under specified conditions depend on the relative rates of species gas exchanges
      across these pathways. The ME airspace can be subdivided into two compartments, the anterior
      tympanum and the posterior mastoid air cell system (MACS). Theory suggests that the
      transmucosal gas exchange rate for the ME (transMEM) may be different for the tympanum and
      the MACS and this possibility has important implications with respect to the role of the MACS
      in ME pressure regulation and protection from ME disease. This study measures the rate of
      inert gas exchange across the mucosa of the two ME compartments in adults. These data will be
      fitted to a model of gas exchange parameterized based on the relative surface areas and
      volumes of the two compartments, and the hypothesis that volume gas exchange across the MACS
      mucosa is less than that for the tympanum will be tested. The results will be used to reject
      or confirm one mechanism by which large MACS volume protects from otitis media under certain
      conditions.
    

Observational model: Cohort
Time perspective: Prospective
Sampling method: Non-Probability Sample
Study population: 
        Healthy adult volunteers
      
Keywords: ear disease; otitis; Eustachian tube; mastoid air cells; Normal healthy volunteers