Mouthfill equalization, often shortened to mouthfill, is a technique used for middle ear equalization during deep freediving. Gas is transferred from the lungs to the mouth and upper pharynx during descent and isolated from the lungs, forming a supraglottic gas reservoir that can be used for repeated equalization as the diver continues to descend.^[1]

During a breath-hold descent, increasing ambient pressure progressively reduces lung volume. As the lungs are compressed towards residual volume, transfer of additional pulmonary gas to the upper airway for equalization becomes increasingly limited. The depth at which this occurs varies between individuals.^[1]

In mouthfill equalization, gas is transferred to the oral and pharyngeal spaces before pulmonary gas transfer becomes ineffective and is then isolated from the lungs by closure of the glottis.^[1][2] The retained gas is then used for repeated Frenzel-type equalizations. In the Frenzel manoeuvre, contraction of the muscles of the floor of the mouth and the superior pharyngeal constrictors with the glottis closed compresses gas in the nasopharynx. The resulting increase in nasopharyngeal pressure opens the Eustachian tube, allowing gas to enter the middle ear.^[3] Gas from the mouthfill reservoir can also be used to equalize the paranasal sinus air spaces during continued descent.^[1]

Mouthfill is principally used during deeper breath-hold dives, where progressive lung compression limits the availability of pulmonary gas for equalization. It is one of the specialized techniques described in the medical literature as allowing freedivers to reach depths beyond conventional predictions based on residual lung volume.^[4]

The technique is used in contemporary recreational and competitive freediving. In a 2024 survey of 140 dives associated with episodes of freediving-induced pulmonary syndrome, mouthfill was among the equalization techniques reported by the divers studied.^[5]

Mouthfill equalization was documented in freediving literature by the beginning of the 21st century. Eric Fattah's Frenzel-Fattah Equalizing Workshop, first copyrighted in 2001 and later revised in 2006, described a mouthfill technique combined with continued use of the Frenzel manoeuvre during descent.^[2] By 2014, mouthfill had been described in the academic literature on breath-hold diving physiology as a method of isolating gas in the oral cavity and using it repeatedly to equalize the middle ears and sinuses during deep descent.^[1] It has since been subsequently recognized in clinical diving-medicine literature as a specialized technique used in deep freediving.^[6]

• Middle ear equalization
• Frenzel maneuver
• Freediving
• Glossopharyngeal insufflation