Vapor pressure osmometry: minimum sample microvolumes

PURPOSE

Vapor pressure osmometers are currently designed to handle sample volumes as small as 2.0 microliters (microl) but smaller sample volumes are desirable in tear fluid studies. We determined the minimum sample size required for adequate validity and repeatability of osmolality measurements.

METHODS

A standard saline solution (290 mmol/kg) was sampled by a variable, calibrated pipette. Forty samples were processed with a Wescor 5520 vapor pressure osmometer at each of the following volumes: 2.0, 1.6, 1.2, 1.0, 0.8, 0.7, 0.6 and 0.5 microl. Prior to each test series, the instrument was calibrated with the identical volume of 290 mmol/kg saline. Relevant descriptive statistics were computed and an analysis of variance (ANOVA) was performed on the resulting data.

RESULTS

The mean osmolalities of the eight 40-sample groups ranged from 288.42 to 290.68 mmol/kg and were not significantly different from 290 mmol/ kg or each other (p>0.05). The standard deviations were inversely correlated with the sample volumes, gradually increased to approximately 1% as sample volume was reduced to 0.8 microl, then more rapidly increased as the sample volume was lowered still further.

CONCLUSION

Sample microvolumes as small as 0.8 microl can be collected for accurate and repeatable results with the Wescor 5520 vapor pressure osmometer when a standard deviation of approximately 1% is acceptable. Microvolumes from 0.7 to 0.5 microl may also be used if the expanded spread of data can be offset by multiple repeated readings. Using a 2.0 microl sample volume, the ultimate accuracy and repeatability of the Wescor vapor pressure osmometer was +/-2% at 290 mmol/kg: 99%, of all readings (+/-3 standard deviations) should fall within +/-6 mmol/kg of the true value.

Features of the partially expanded human inferior conjunctival sac

Space in the inferior conjunctival sac has not been studied in sufficient detail to allow optimal physical design of conjunctival inserts. We analyzed shape and volume of partially expanded, anesthetized, left inferior conjunctival sacs in 10 young adult subjects by injection of a liquid polysiloxane molding compound. Mean volume, greatest thickness, central thickness, horizontal width, and vertical height of 60 molds were 125.7 microliters (SD = 55), 1.56 mm (SD = 0.69), 1.46 mm (SD = 0.62), 20.7 mm (SD = 2.2), and 8.9 mm (SD = 0.8), respectively. Volumetric and linear dimensions varied between subjects, but certain features were common to all molds: 1) a crescent shape horizontally; 2) a thick inferior horizontal ridge; and 3) a wedge-like shape sagittally. We postulate several advantages of conjunctival inserts with features representative of our molds of conjunctival sacs, e.g. such inserts may be more comfortable, less expelled, larger in volume, and contact more tissue area.

Human corneal oxygen demands at superior, central, and inferior sites

BACKGROUND

Oxygen depletion of the superior and central cornea in response to standard conditions is of clinical interest because of pathological conditions, usually resulting from the hypoxia of contact lens wear, that affect these areas. A study was designed to determine if there are differences in oxygen depletion at these two sites.

METHODS

Oxygen depletion times from a standard reservoir following corneal exposure to three different oxygenated environments (20.9%, 5%, and 0% oxygen) were assessed micropolarographically at central, superior, and inferior corneal surface positions on both corneas of six healthy young adult human subjects.

RESULTS

Based on 48 measurements for each corneal position at each oxygen percentage (432 total measurements), the superior cornea 1.0 mm from the limbus was found to have longer oxygen depletion times (slower oxygen uptake rates) than the central or inferior corneal locations. The differences reduced to nearly zero during the course of the initial 1.5-day testing procedure, but were re-established months later when testing was resumed.

CONCLUSIONS

There is a small reversible increase in superior corneal oxygen depletion time (a decrease of the oxygen uptake rate) due to the chronic hypoxia that results from the presence of an overlying superior eyelid or other physiological factors.

Oxygen performance of a prototype nonconventional hydrogel

INTRODUCTION. Oxygen permeability (Dk) of available hydrogel materials used for soft contact lenses is currently restricted by the oxygen permeability of water at 80 Dk units. A predicted upper limit for conventional hydrogels amenable for use as a contact lens, has been 37 Dk units obtained with a water content of 80%. Methods for increasing oxygen permeability beyond that determined by water content must be explored, in order to further advance the physiological compatibility of hydrogel contact lenses.

METHODS

Oxygen transport characteristics of a prototype nonconventional hydrogel contact lens material were derived in terms of human equivalent oxygen percentage (EOP) and thickness (L). The oxygen permeability of the material and oxygen transmissibilities (Dk/L) of various thicknesses were computed on the bases of a recently published nonlinear EOP vs. Dk/L relationship.

RESULT

The material's oxygen permeability (Dk) was determined to be approximately 35 Dk units compared to 12.7 Dk units calculated for a conventional gel material having the same water content (52% H2O). Oxygen permeability equivalent to that obtained with high-water conventional gels was achieved using this mid-water nonconventional hydrogel. The prototype material was apparently also capable of low critical thicknesses typical of low-water conventional gels.

DISCUSSION

Should new gel materials be durable with critical thicknesses in the "ultra-thin" range, nonconventional gel lenses could be capable of super and perhaps even hyper transmissibility with central minus lens EOP between 15-18.5% and Dk/L between 50-90 Dk/L units. The conventional relationships between water content and oxygen transmissibility of hydrogel contact lenses appear to have been surpassed. However, significant additional hurdles have yet to be overcome before nonconventional hydrogel materials can be ready for the ophthalmic marketplace.

Human basic tear fluid osmolality. II. Importance of processing strategy

Osmolalities of 100 human inferior tear prism fluid samples collected from two subjects and 100 standard solution samples (290 mOsm/Kg) were determined from their melting-point temperatures with the Clifton Nanoliter Osmometer. Accuracy and reliability comparisons were made for endpoints obtained using a single-sample simultaneous-recalibration (SS/SR) strategy, vs a lowest-of-six periodic-recalibration (LS/PR) strategy. Tear fluid osmolality values based on the LS/PR strategy were significantly lower (hypotonic) than for the SS/SR strategy (overall mean = 302.4 and 307.8 mOsm/Kg, respectively; p < 0.0012). The mean difference (5.4 mOsm/Kg) resulted from the combined effects of machine drift and reduced reliability of endpoint determination for tear fluid in comparison to that of a homogenous standard solution. LS/PR osmolality was linearly correlated to SS/SR osmolality with a slope of 0.433 and the two endpoint strategies resulted in equivalent values at 298.4 mOsm/Kg. Eighty-four percent (84%) of individual osmolality readings were greater than 298.4 mOsm/kg, and an 'averaging-type' effect caused LS/PR osmolalities to be increasingly less than SS/SR values above this value. These outcomes from two subjects were approximated by a parallel statistical model. We suggest that physiological representation of basic human tear fluid is more accurate using the single-sample simultaneous-recalibration strategy. Basic human tear prism fluid is more hypertonic and has a greater within-subject range in normal (non-dry eye) humans than previously realized.

Human basic tear fluid osmolality. I. Importance of sample collection strategy

Osmolalities of 200 human tear prism fluid samples collected from two subjects were determined from their melting-point temperatures with the Clifton Nanoliter Osmometer by calibration with 200 standard solution samples (290 mOsm/kg). Comparisons were made between tear fluid osmolalities obtained using a single-sample simultaneous-recalibration method: 1) for tear samples collected using finely-drawn microcapillaries without biomicroscopy, vs secondly with biomicroscopic observation by illumination of only the sampling area on the inferior tear prism, the two collections separated by an interval of 10 min; and 2) for samples collected without biomicroscopy before, vs after a 10-min interval. Tear fluid collection using a biomicroscope resulted in values that were significantly lower than those collected without (overall mean = 299.5 and 306.6 mOsm/kg, respectively; p < 0.0001). The difference (7.1 mOsm/kg) may have resulted from mechanical, photic, and/or psychogenic reflex stimulation due to biomicroscopy, as the 10-min interval had no osmotic effect on samples collected without biomicroscopy. Based on these results from two subjects, we suggest that relatively rapid, repetitive collections of human tear prism fluid can be made without significantly disturbing the osmotic outcome. However, physiological representation of basic human tear fluid is more accurate by avoidance of reflex-inducing collection methods that were formerly considered unobtrusive. Basic human tear prism fluid is more hypertonic, by at least 7 mOsm/kg, than generally realized.

EOP and Dk/L: the quest for hyper transmissibility

The relationship between human equivalent oxygen percentage (human EOP) and oxygen transmissibility (Dk/L) was examined for 28 parallel- and nearly parallel-surfaced contact lenses having oxygen transmissibilities ranging from 14.2 to 134.4 Dk/L units as determined with a gas-to-gas method. EOP and Dk/L were positively related such that increases in Dk/L led to progressively smaller increases in human EOP, until Dk/L asymptoted at an EOP of approximately 20 percent. Human EOPs were semi-logarithmically correlated to Dk/L in a linear fashion (R2 = 0.95), though the semi-logarithmic linear equation didn't as closely predict the expected EOP vs. Dk/L curve at its extreme ends. Classifications of oxygen transmissibility and permeability were also standardized into ranges of equivalent oxygen percentage: contact lenses of low, medium, high, super, and hyper transmissibility and materials of low, medium, high, super, and hyper permeability were defined. While most available hydrogel lenses and materials are currently limited by their water contents to the low and medium Dk/L and Dk groups, some available rigid lenses and materials have attained super-Dk/L and hyper-Dk status. They should be better able to be fitted to the individual eye and to incorporate more complicated optical corrections, having partially alleviated dependency of corneal oxygen supply on lens design. The quest for hyper-permeable materials stable in thinner designs may produce lenses of hyper transmissibility with corneal oxygen supply nearly insensitive to lens design, as variation of thickness over lens apertures will then result in alterations of corneal oxygenation (EOP) having minimal clinical significance.(ABSTRACT TRUNCATED AT 250 WORDS)

Cleaning and storage of rigid contact lenses prior to dispensing

Most care regimens used with rigid contact lenses are composed of three solutions: 1) a "cleaning" solution; 2) a "combination" solution intended for storage, disinfection, and wetting prior to placement of rigid lenses on the eye; and 3) an in-eye "rewetting" or "lubricating" solution. While solutions (1) and (3) have only single functions, solution (2) must fulfill three fairly disparate functions. A "2-solution" care regimen is offered by Sherman Pharmaceuticals and consists of (a) a solution for cleaning, storage, and disinfection; and (b) a "wetting" solution that doubles as an in-eye lubricant. Through a more compatible and efficient matching of multiple functions within solutions, Sherman Pharmaceuticals claims to more adequately clean and prepare rigid gas-permeable (RGP) lens surfaces for wear. We investigated this strategy in terms of in-eye wettability of initially "ill-prepared" RGP lens surfaces, as measured with the in vivo contact angle technique. For this study, lenses were intentionally not cleaned of residues or solvents by the manufacturer at the end of production so as to create the well-known "first-day non-wetting syndrome." The ability of the 2-solution care regimen to even then provide wettable surfaces was compared to that of a saline control "regimen" and the most popular competing "3-solution" care system. We concluded that use of both care regimens dramatically enhanced in-eye wettability and, therefore, reduced the incidence of the "first-day non-wetting syndrome". However, the 2-solution care regimen provided significantly better wettability overall, lower incidence of functionally non-wetting surfaces, and more consistently wettable RGP lenses.

Physiology of aging and its influence on the contact lens prescription

The normal progressive physiological and visual effects of aging and its influence on the ability of contact lenses to provide ametropic correction is examined in this article. Of primary significance is the aging eye's perceptual ability especially in the mesopic range of vision. In addition, a recap of current available techniques is included for correction of presbyopia with contact lenses from the perspective of prescribing for an aging patient--not merely a patient who has lost only the ability to accommodate. One of the hallmarks of the profession of optometry has been its traditional support of the premise that patients should be provided with the best possible optical correction. While there are presbyopic patients who can achieve satisfactory vision via the contact lens modalities now in use, the physiological influences discussed here have been generally ignored by the profession. A redefinition of the criteria indicating patient "success" has evolved from achieving satisfaction through excellent vision derived from the optimum optical correction, to satisfaction obtained through patient selection and indoctrination to wear an optically inferior device. Unless the physiological, visual, and psychological effects of aging are adequately compensated by future contact lens designs, most elderly patients will remain likely to return to spectacles as the deficiencies of contact lens correction become progressively more apparent to them.

Effect of humidity on the deswelling function of the human cornea

The present study determined the effect of the full range of humidities on the deswelling function of the human cornea. The closed-eye deswelling function and the open-eye deswelling responses for five different levels of humidity (0%, 25%, 60%, 85%, and 100%) were assessed for 8 normal, young-adult subjects. Open-eye corneal deswelling for the 8 subjects was unaffected by relative humidities from 0 to 100%. Therefore, osmotically-driven corneal thinning effect of tear evaporation does not significantly contribute to the deswelling function of the human cornea. We conclude, contrary to recent reports, that the endothelial pump is the primary mechanism that maintains normal corneal thickness and provides recovery from stromal edema.

Tonicity of human tear fluid sampled from the cul-de-sac

A 'freezing point' depression technique was used to determine the osmolality of 384 samples of tear fluid originating from the middle of the lower tear prism and the lower cul-de-sac of one eye of each of 12 young adults. Tear fluid from the cul-de-sac (mean 341.0 mosm/kg) was found overall to be significantly hypertonic (p less than 0.0001) relatively to fluid from the tear prism (mean 315.5 mosm/kg). However, the difference between the two sampling sites was highly variable between individuals, ranging from a maximum mean site difference of 64.5. mosm/kg for one of the six cul-de-sacs found to be significantly hypertonic, to a mean site difference of 25.0 mosm/kg for one of the two cul-de-sacs found to be significantly hypotonic. These results suggest that a unique localised tear environment exists inside the lower cul-de-sac, which has several clinical consequences--for example, for pharmaceutical absorption, ocular microbiology, and hydrophilic contact lens performance.

Clinical findings correlated with contact angles on rigid gas permeable contact lens surfaces in vivo

Functional wettability of daily wear rigid gas permeable (RGP) contact lenses worn by 16 human subjects was monitored over a 4-month period during which wettability was also assessed with an equilibrium sessile-drop contact angle method in vivo. In all, 2,128 lens surface break-up time (LBUT) and in vivo contact angle data points were accumulated. Six hundred thirty-one associated graded evaluations of lens surface deposition and patient discomfort were also recorded. The four factors (LBUT, contact angle, deposition, and discomfort) were found to be correlated to each other, their paired values having statistically significant correlation coefficients. Contact angles were of predictive value for extremes of functional wettability, based on criterion of 20 degrees equivalent to an LBUT of 5 seconds. We confirm that surface deposition and subject discomfort are related to RGP lens wettability on the eye. In addition, the in vivo contact angle is perhaps the first contact angle measurement to be correlated with functional RGP wettability.

Oxygen consumption of the superior cornea following eyelid closure

Oxygen depletion curves were recorded from the superior and central cornea with a micropolarographic system immediately following eyelid closure in 10 adult human subjects. Superior corneal oxygen uptake rates for the closed-eye state ranged from 13% below to 58% above, with a mean 15% greater than equivalent central corneal oxygen uptake rates. Chronic superior corneal hypoxia induced by upper eyelid coverage when the eyes are open is therefore accompanied by a deeper hypoxia relative to the central cornea of some individuals when their eyes are closed. This hypoxia adds to the physiological compromise endured by the superior cornea and is especially emphasized when contact lenses are worn.

Human cornea: superior and central oxygen demands

Corneal oxygen demands, expressed as ratios of uptake rate relative to baseline rates derived from unstressed corneas, were determined with a micropolarographic system for central (closed eye) and superior (open eye) locations of one cornea of each of seven human subjects. The closed-eye central corneal measurements were repeated during two additional experimental sessions. Intrasubject variability of the three mean closed-eye central corneal rate ratios ranged as high as 23%, possibly representing effects of homeostatic mechanisms on the palpebral conjunctival capillaries of some subjects. For the open-eye superior cornea, which was covered by the upper eyelid of every subject prior to measurement, oxygen demand was found to have a greater intersubject range, but was diminished in magnitude relative to the demand associated with the closed-eye central cornea. Superior corneal oxygen demand was not found to be predictable from closed-eye central corneal oxygen demand or extent of eyelid overlap onto the cornea and thus indicated localized open-eye superior corneal environments that were significantly different from those of the corresponding closed-eye central corneas. Such localized environments may be critically important when gauging the susceptibility of particular eyes to superior corneal pathology during contact lens wear.

Human cornea: individual responses to hypoxic environments

Using a micropolarographic system, the responses of seven human corneas to oxygen atmospheres ranging from 20.9% to 0% were measured. Under normal atmospheric conditions, ratios of oxygen demand were observed between corneas as great as 1.23:1. Following 0% oxygen conditions, however, these ratios rose to a maximum of 1.93:1, i.e., representing demand differences between particular corneas of nearly 2:1. Such differences may prove increasingly important in the gauging of patient suitability and safety in relation to hypoxic stress, e.g., the extended wear of contact lenses.

Closed-lid factors influencing human corneal oxygen demand

A micropolarographic system was used to measure the flux of oxygen crossing the tear-epithelial interface of the human cornea just following stabilization of four lid closure conditions. In all, 215 oxygen flux measurements were made on the corneas of 6 young healthy subjects, including 2 reference conditions: the normal open eye and anterior surface oxygen deprivation. When compared with the temperatures associated with certain of those conditions, oxygen demands greater than those predicted by a previous Q10 measurement were found. Proportional contributions of the non-temperature factor are presented, and the nature of its components suggested.

Tear osmotic differences across the ocular surface

We used a freezing point depression method to determine the osmolalities of 502 non-stimulated 200-nanoliter tear samples. The samples were collected hourly at 9 times on each of 6 days. Four reference (tear prism) sites on the ocular surface were used: mid-superior, mid-inferior, and the tear prisms at the medial and lateral canthi. While 256 of the samples were from a young (25 year old) male volunteer having no complaints of dry eyes, the remaining 246 samples were taken from a second male volunteer of the same age range, but who had occasional complaints of dry eyes. The mean osmolalities for all sites and all times for both subjects were found to be 315 and 331 mOsm/kg, respectively, and were significantly different (P = 0.0004). Two of the six time-averaged intersite comparisons investigated here were found to be significantly different for each of the subjects, with the inferior tear prism and the medial canthus being the only shared pairs present. The magnitudes and patterns of these osmotic site differences were found to shift for both subjects over time, although this was more prominent and with a greater hypertonic bias for the subject with dry eye complaints. While these data do demonstrate statistically different osmolalities across the ocular surfaces of the two subjects examined in this report, these findings must be considered a preliminary view of broader population patterns yet to be studied.

Human corneal oxygen demand: the closed-eye interval

A micropolarographic system was used to measure the flux of oxygen crossing the tear epithelial interface of the human cornea just following lid-closure periods of 0-15 min, and just following a series of brief lid openings (0.2-0.3 s) of frequencies from 0 to 30 times/min. In all, 462 measurements spanning those two formats were made on the corneas of seven young healthy subjects. In the extended-closure format the average rate increased over the first 5 min, then stabilized at 2.51 times the oxygen-uptake rate under normal open-eye conditions. In the brief lid-opening format, frequencies of 30 openings/min reduced the oxygen uptake rate to 70% of that of the stabilized closed eye. Individual differences noted among subjects may influence their relative success in extended contact lens wear, and natural lid and eye movements during sleep may have at least transient effects on corneal oxygen supply and demand during those periods.

Human cornea: oxygen uptake immediately following graded deprivation

A micropolarographic system was used to measure the flux of oxygen crossing the tear epithelial interface of the human cornea just following stabilized exposure to eight oxygen atmospheres ranging from 0% to 20.9%. In all, 336 measurements were made on the corneas of seven young healthy subjects. The rate coefficients of their immediate postexposure oxygen uptakes were found to increase nearly linearly over the atmospheric range of 20.9% (air) down to about 1.5%. Below that level, however, a marked increase in the immediate recovery rate coefficient occurred, suggesting that at least two oxygen demand phases can be distinguished for the aerobically dependent pathways of the cornea.

Human tears: osmotic characteristics

Using a freezing point depression method, requiring volumes of only 200 nanoliters, the osmolalities of 324 tear samples collected at consecutive 10-minute intervals throughout 8.5-hour periods from each of six healthy young adults were measured. The population mean was found to be 318 mOsm/kg, with a population median of 315 mOsm/kg. Individual subject means ranged from 310 to 334 mOsm/kg. Short-term oscillations were observed for all subjects, with a net positive rate toward hypertonicity for the population of 1.43 mOsm/kg X hr-1 as the day progressed. Two subjects did demonstrate mild net rates of decrease, however. All eyes in the study were asymptomatic, suggesting a broader range of tear osmotic pressure among normals than previously suspected.

Nanoliter tracking of the tears

Microliter sampling opened the way in the 1970's for at least limited quantitative studies of the tear fluid. Now, nanoliter sampling offers a much less imposing technique for viewing even more subtle changes within the eye's most immediate environment. Examples, based on nanoliter sampling, are given of certain transient effects that even a soft lens can induce during the earliest periods of wear.