How much oxygen does the cornea really need?

Eric Papas, PhD MCOptom, DCLP Eric Papas is Executive Director of Research & Development at the Vision Co-operative Research Centre, based at the University of New South Wales, Sydney, Australia. He originates from England where he obtained degrees in physics and optometry, before spending several years in specialist contact lens practice. Most of his subsequent career has been devoted to research and he has managed clinical research groups for corporations such as Hydron and Allergan. More recently he was Director of Clinical Research in the Cornea and Contact Lens Research Unit (CCLRU) at UNSW. His latest role involves supervision of a range of vision correction projects that are either government funded or run in direct collaboration with industrial partners. Currently, his major research interests are the tear film, ocular surface sensation and vascular behaviour.

Review of the paper entitled: Estimation of Human Corneal Oxygen Consumption by Noninvasive Measurement of Tear Oxygen Tension While Wearing Hydrogel Lenses

Joseph A. Bonanno, Thomas Stickel, Tracy Nguyen, Trina Biehl, Donna Carter, William J. Benjamin, and P. Sarita Soni

IOVS 2002, 43, 371-376

The question of how much oxygen the cornea consumes has, historically, not been easy to answer. While there have been several attempts to measure this quantity, the methods used have not been ideal from the point of view of arriving at a answer appropriate to the normal human situation. One group, for example, used excised rabbit corneas while another had subjects wearing tightly clamped fluid filled goggles. The method described in this article by Bonanno et al provides an elegant alternative approach that permits information to be extracted from beneath regular contact lenses, in human subjects, with a minimum of disruption.

Their technique involves the use of an oxygen sensitive dye that can be attached to the lens surface. When excited by a brief light flash, this dye exhibits a phosphorescent glow that fades with time. The fact that the speed of this decay is related to the concentration of oxygen in the immediate vicinity provides a means of probing oxygen concentration in the post lens region. Once the concentration stabilizes to a steady value, it is possible to make an estimate of the corneal oxygen consumption rate (Qc) beneath that lens.

Estimates of Qc were made for three different lenses with oxygen transmissibilities of 4.2, 14 and 99 x 10-9 cm.mLO2/mL.sec.torr, in both open and closed eye conditions.

The results show that the higher the transmissibility of the lens, the more oxygen is consumed. This is the case irrespective of whether the eye is open or closed, though for a given lens type, closed eye consumption is always lower than when the eye is open.

Wearing low transmissibility lenses in the closed eye depressed oxygen consumption to 3.7 x 10-6 mLO2cm³.sec. Conversely, the highest consumption was 2.2 x 10-4 mLO2cm³.sec, corresponding to high transmissibility lenses in open eyes. While this 60 fold difference illustrates the fantastic resilience of the cornea to hypoxic stress; a similar drop in pulmonary oxygen intake would be rapidly fatal; it is also clear that, given the choice, corneal tissue is very oxygen thirsty. Even with a contact lens in place, 2.2 x 10-4 mLO2cm³ . sec is some two to three times higher than previous measurements in rabbits.

How closely does the high transmissibility lens value represent true, i.e. non-lens, open eye; corneal oxygen consumption? Unfortunately, because the phosphorescent dye must be attached to a surface in order to be useful, this method does not lend itself to a direct estimate of non-contact lens wearing consumption rates. Nevertheless, as lenses with successively higher transmissibility obviously allowed correspondingly higher consumption, it seems probable that not wearing a lens would result in an even higher intake value than that recorded with the highest transmissibility lens. This would simply be because any residual lens-related resistance to oxygen flow would have been removed.

While we can’t be certain how much higher the true no-lens level would be, inspection of the distribution of Qc data derived from the measured lenses implies that the increase may not be very great. In saying that, one does need to be aware of the dangers of extrapolating curves beyond their data points, particularly when these are relatively few in number.

So, while we still do not have a definitive value for true, no-lens corneal oxygen consumption, we do know which ball park we are in. Furthermore, this work gives a very direct demonstration of the value to corneal metabolism of utilizing high transmissibility materials for contact lens wear in both the closed and open eye situations.