The purpose of this investigation was to find an effetive method for topical lidocaine anaesthesia of the tympanic membrane for minor therapeutical procedures. Twelve subjects with healthy ears were tested in a randomized double-blind crossover trial. Xylocaine® (lidocaine) aerosol 10%, lidocaine 4%, and lidocaine hydrochloride 5% were tested against placebo. Xylocaine aerosol was sprayed into a small container, shaken, and tempered. This made the application painless. The eardrum was stimulated by a point of a needle effective 25 min after application (p = 0.008 and p < 0.05, respectively). Lidocaine hydrochloride showed no effect. No side effects were observed. The effectiveness seems to be dependent on the ionization of the drug, since the two effective drugs lidocaine and Xylocaine are un-ionized, whereas the ineffective lidocaine hydrochloride is ionized.
In studying mechanisms regulating the middle ear pressure it is essential to know the gas compositions of the middle ear. We have constructed a device which made it possible to sample middle ear gas without creating a negative pressure, thus eliminating the risk of admixture of atmospheric air. The samples were analysed by gas chromatography. In 26 normal subjects we found relatively stable values of carbon dioxide (median 52 mmHg, range 31-69 mmHg) and nitrogen (median 605 mmHg, range 563-627 mmHg). The values of oxygen were more fluctuating (median 54 mmHg, range 23-111 mmHg). In 10 of the subjects, arterial gases were determined simultaneously. No correlation could be shown between middle ear and arterial oxygen and carbon dioxide tensions. This investigation provides evidence contradicting the classical theory of a high negative middle ear pressure in Eustachian tube closure. We found strong indications that the Eustachian tube plays an active role in regulating the pressure in the normal middle ear, but variations in blood flow through the middle ear capillaries may also be an important regulating factor.
Methods for storage and analysis of gas samples were studied prior to measuring the composition of gas in the human middle ear cavity. Storage and analysis of gas samples are beset with several pitfalls. The dead space of the sampling syringe must be considered and the gas sample can equilibrate with the surrounding atmosphere, especially if the analysis cannot be done immediately. In this study the equilibration rate was found to fit an exponential function with a time constant of 0.004 min-1. The time constant was reduced by more than a factor of ten when butyl rubber stoppers were used to seal the syringes. Silicone rubber stoppers were less efficient. Disposable plastic syringes were found just as efficient as 'gas-tight' syringes.
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