Considerations in Contact Lens Use Under Adverse Conditions Proceedings of a Symposium (1991) / Chapter Skim
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Effect of Aircraft Cabin Altitude and Humidity on Oxygen Tension Under Soft and Hard Gas-Permeable Contact Lenses
Effect of Aircraft Cabin Altitude and Humidity on Oxygen Tension Under Soft and Hard Gas-Permeable Contact Lenses
Pages 106-118
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From page 106... ...
This cabin environment is shown to result in calculated oxygen levels under contact lenses that may be substantially reduced from normal and thus needs consideration. MILITARY AIRCRAFT CABIN PRESSURIZATION At sea level the ambient air pressure is about 760 mmHg (14.7 psi)
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From page 107... ...
At higher altitudes the oxygen under a contact lens must therefore be lower. In addition, lower humidity is known to result in partial dehydration of soft lenses (Andrasko and Schoessler, 1980~.
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From page 108... ...
This tear exchange increases the level of oxygen under HGP lenses by about 7-15 mmHg PO2 above the amount from diffusion (Efron and Carney, 1983; Fatt and Liu, 1984~. A number of studies in altitude chambers and aircraft have assessed the subjective, visual, and corneal responses to contact lenses at aviation altitudes and low humidity levels (Eng et al., 1978, 1982; Brennan and Girvin, 1985; Flynn et al., 1986; Dennis et al., 1988~.
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From page 109... ...
However, these calculated oxygen levels allow relative comparisons between lenses under military cabin environments and can also be compared to the proposed "acceptable" oxygen level. The calculations suggest that corneal hypoxic conditions could occur, particularly with soft lenses, that may approach the hypoxia that occurs during night wear.
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From page 110... ...
and for hard lenses worn during eye closure (O'Neal et al., 1984; Benjamin and Rasmussen, 19851. Using this equation, the relationship between lens oxygen transmissibility and calculated oxygen tension under a contact lens is shown in Figure 3 for soft lens open-eye wear at sea level and at cabin altitudes of 8,000 and 16,000 feet.
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From page 111... ...
Using data in their Table 4, the oxygen transmissibility (Dk/L) of the soft lens in the vial, the Dk/L during normal wear, and the Dk/L at a low 18 percent relative humidity were calculated and are listed in Table 1 for two frequently used water content soft lenses, 55 percent and 71 percent H2O soft lenses having 0.09- and 0.21-millimeter average lens thicknesses, respectively.
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From page 112... ...
Although data on lens changes are not available for these very low humidities, the low 18 percent humidity data noted above shows a dramatic effect on lens oxygen transmissibility. Using the calculated Dk/L in Table 1, the calculated oxygen tension under 55 percent and 71 percent H2O soft lenses in the vial, during normal wear, and at 18 percent relative humidity is shown in Table 2 for sea level and for cabin altitudes of 8,000 and 16,000 feet.
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From page 113... ...
For the low 18 percent humidity condition, when the soft lens partially dehydrates and the hard lens does not, the calculated oxygen tension under the hard lens is two to three times that TABLE 3 Effect of Altitude on Oxygen Tension Under HGP Lens Calculated Oxygen Tension (mmHg) at: L ens Cabin at Cabin at DK/La Sea Level 8,000 ft 16,000 ft 20 70 49 3 1 25 80 59 37 30 89 67 43 35 97 74 48 40 105 80 52 NOTE: PO2 added for tear exchange = 0.075 x ambient PO2.
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From page 114... ...
The aircraft cabin frequently has a very low 5-10 percent humidity that would result in even lower oxygen levels and greater corneal hypoxia. The calculated oxygen levels under soft lenses further suggest that normal everyday extended wear of soft contact lenses may not be a viable choice for military aircrew.
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From page 115... ...
HGP lenses would generally have much more oxygen under the lens than the proposed minimum oxygen level at both cabin altitudes. This higher oxygen level occurs because HGP lenses do not dehydrate in low humidity, as soft lenses do, and they get additional oxygen under the lens from the tear exchange that occurs with blinking, which does not occur with soft lenses.
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From page 116... ...
Investigative Ophthalmology and Visual Science 18 :93-95. Effect of blinking on the level of oxygen beneath hard and soft gas permeable contact lenses.
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From page 117... ...
Mertz 1984 Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Investigative Ophthalmology and Visual Science 25:1161-1167.
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From page 118... ...
Bonanno 1987 Gas-permeable hard contact lens extended wear: ocular and visual responses to a 6-month period of wear. Contact Lens Association of Ophthalmologists Journal 13:31-3B.
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