April 2013
Volume 54, Issue 4
Free
Physiology and Pharmacology  |   April 2013
Spectrophotometric Retinal Oximetry in Pigs
Author Affiliations & Notes
  • Sindri Traustason
    Department of Ophthalmology, Glostrup University Hospital, Glostrup, Denmark
    Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark
  • Jens F. Kiilgaard
    Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark
    Department of Ophthalmology, Rigshospitalet, Copenhagen, Denmark
  • Robert A. Karlsson
    Oxymap ehf, Reykjavik, Iceland
  • Sveinn H. Hardarson
    Department of Ophthalmology, University of Iceland, Reykjavik, Iceland
  • Einar Stefansson
    Department of Ophthalmology, University of Iceland, Reykjavik, Iceland
  • Morten la Cour
    Department of Ophthalmology, Glostrup University Hospital, Glostrup, Denmark
    Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark
  • Correspondence: Sindri Traustason, Department of Ophthalmology, Glostrup University Hospital, Nordre ringvej 57, 2600 Glostrup, Denmark; sindri@halur.net
Investigative Ophthalmology & Visual Science April 2013, Vol.54, 2746-2751. doi:10.1167/iovs.12-11284
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      Sindri Traustason, Jens F. Kiilgaard, Robert A. Karlsson, Sveinn H. Hardarson, Einar Stefansson, Morten la Cour; Spectrophotometric Retinal Oximetry in Pigs. Invest. Ophthalmol. Vis. Sci. 2013;54(4):2746-2751. doi: 10.1167/iovs.12-11284.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose.: To assess the validity of spectrophotometric retinal oximetry by comparison to blood gas analysis and intravitreal measurements of partial pressure of oxygen (pO2).

Methods.: Female domestic pigs were used for all experiments (n = 8). Oxygen fraction in inspired air was changed using a mixture of room air, pure oxygen, and pure nitrogen, ranging from 5% to 100% oxygen. Femoral arterial blood gas analysis and retinal oximetry were performed at each level of inspiratory oxygen fraction. Retinal oximetry was performed using a commercial instrument, the Oxymap Retinal Oximeter T1. The device simultaneously acquires images at two wavelengths (570 nm and 600 nm), and specialized software automatically detects retinal blood vessels. In three pigs, invasive pO2 measurements were performed after the initial noninvasive measurements.

Results.: Comparison of femoral arterial oxygen saturation and the optical density ratio over retinal arteries revealed an approximately linear relationship (R 2 = 0.74, P = 3.4 × 10−9). In order to test the validity of applying the arterial calibration to veins, we compared noninvasive oximetry measurements to invasive pO2 measurements in three pigs. This relationship was approximately linear (R 2 = 0.45, P = 0.04).

Conclusions.: Noninvasive spectrophotometric oximetry is sensitive to changes in oxygen saturation in pigs and correlated with intravitreal pO2 measurements and with femoral artery pO2. Pigs present a higher intraindividual variability in retinal oxygen saturation and a lower overall saturation than do humans. The difference between porcine and human eyes makes direct comparisons of measurements difficult.

Introduction
Traditionally, the pathology of retinal disease has primarily been defined by structural and functional changes in the retina. These changes are, however, often secondary to metabolic alterations, such as changes in retinal oxygenation. 
Measurement of systemic oxygen saturation in peripheral blood became available with the invention of pulse oximetry in late 1970s, and is now performed routinely in the clinic. 1  
Attempts at noninvasive spectrophotometric oxygen saturation measurements in the retina were first made by Hickam et al. in 1959 using broadband light filters and photographic films. 2 Various later groups developed photoelectric and spectroscopic methods of retinal oximetry. 3 More recently, digital cameras have made it possible to acquire wide-field snapshot images for automatic retinal oximetry, as first described in 1999 by Beach et al. 4 Charge-coupled device (CCD) image sensors offer a clear advantage over earlier methods based on film in that the solid-state electronics circuits are highly linear, sensitive, and reproducible in their detection of reflected light. 
The Oxymap Retinal Oximeter is based on the method described by Beach et al. 4 and Hardarson et al. 5  
The purpose of the present study was to assess the validity of spectrophotometric retinal oximetry by comparison to blood gas analysis and intravitreal pO2 measurements. 
Methods
Animals
Female domestic pigs of Danish Landrace (Duroc/Hampshire/Yorkshire) breed were used for all experiments. Eight pigs (age 3–4 months, weighing 22–34 kg) were used in total. 
Anesthesia was induced by intramuscular injections of midazolam, tiletamine, zolazepam, ketamine, xylazine, and methadone as previously described in detail. 6 Anesthesia was maintained by continuous infusion of 15 mg/kg/h propofol (B. Braun AG, Melsungen, Germany). 
The animals were tracheally intubated and artificially ventilated with room air. Catheters were placed in the left femoral artery, and the animals were covered with a blanket to maintain normothermia. Heart rate and peripheral oxygen saturation were monitored continuously using a pulse oximeter that was placed on the tail. 
All experiments were performed in compliance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research, and the Danish Animal Experiments Inspectorate granted permission for the use of the animals (permission No. 2007/561-1386). Handling and preparation of the animals was done by experienced animal technicians. 
Noninvasive Oximetry
Retinal oximetry was performed using a commercial instrument (Oxymap Retinal Oximeter T1; Oxymap ehf, Reykjavik, Iceland). The instrument consists of a fundus camera (Topcon TRC-50DX; Topcon, Tokyo, Japan) coupled with a custom-made beam splitter and two digital cameras (Spot Insight IN1800; Diagnostics Instruments, Sterling Heights, MI). Images are acquired simultaneously at two wavelengths (570 nm and 600 nm). Specialized software (Oxymap Analyzer; Oxymap ehf) automatically detects retinal blood vessels and selects measurement points for the estimation of retinal vessel oxygen saturation. 
Spectrophotometric oximetry is based on quantifying the color changes in blood that are related to changes in blood oxygen saturation. The optical density (light absorbance) of blood is measured at two wavelengths. Optical density (OD) is defined as the logarithmic ratio between incident and transmitted light intensity and can be described by the equation  where I 0 is the light intensity before passing through the test solution and I is the intensity of transmitted light.  
Blood oxygen saturation can be defined as the fraction of available hemoglobin molecules that are bound to oxygen. Hemoglobin and oxyhemoglobin have different light absorption spectra, which intersect at the so-called isosbestic wavelengths, where hemoglobin and oxyhemoglobin have the same optical density. 7 By comparing the optical density at an isosbestic wavelength to that of a nonisosbestic wavelength, it is possible to estimate oxygen saturation. 
The Oxymap Retinal Oximeter (Oxymap ehf) performs measurements using 570 nm as the isosbestic wavelength and 600 nm as the nonisosbestic. The optical density ratio (ODR) is therefore defined as  where OD600 is the optical density at 600 nm and OD570 is the optical density at 570 nm. The ODR has an approximately linear relationship to the oxygen saturation of hemoglobin as described by the linear equation  where a and k are constants, acquired by calibration. These constants can be determined by solving a system of linear equations or by linear regression.  
Ideally, spectrophotometric oximetry is performed by shining light through the blood vessels. Since this is not feasible in the eye, measurements are made on light that is reflected by the fundus, as an approximation. Thus, the optical density of the blood column can be assessed by comparing the intensity of reflected light at the vessel to that of light in the immediate surrounding background. 
All noninvasive retinal oximetry analysis was performed using a specialized edition of Oxymap Analyzer version 2.3.2 (Oxymap ehf) that was adapted for use on porcine fundus images. Adapting the software was necessary as retinal vessels on porcine fundus images appear considerably wider than those on human fundus images. The software algorithms for blood vessel tracking and selection of measurement points for incident and reflected light intensities were adjusted to porcine fundi. No hardware changes were made to the Oxymap Retinal Oximeter (Oxymap ehf). 
Measurements were made on the most prominent artery and vein in the superior retina. Segments of approximately one optic disc diameter in length were selected, immediately below the first major vessel branching (Fig. 1). 
Figure 1
 
Porcine fundus image, with graphic overlay showing measurement areas for arteries (a) and veins (v). The brackets indicate approximate segments used for spectrophotometric oximetry, and the circle indicates the approximate area over which the pO2 probe was placed for intravitreous pO2 measurements. The fundus image was taken at 570 nm, at which there is no difference between the optical density of hemoglobin and oxyhemoglobin.
Figure 1
 
Porcine fundus image, with graphic overlay showing measurement areas for arteries (a) and veins (v). The brackets indicate approximate segments used for spectrophotometric oximetry, and the circle indicates the approximate area over which the pO2 probe was placed for intravitreous pO2 measurements. The fundus image was taken at 570 nm, at which there is no difference between the optical density of hemoglobin and oxyhemoglobin.
Invasive Partial Pressure of Oxygen (pO2) Measurements
Invasive oxygen tension measurements were made with a fluorescence quenching optical probe system (Oxylab pO2; Oxford Optronix, Oxford, UK). The probe was inserted into the vitreous through a 17-gauge catheter and advanced with a micromanipulator. The probe positioning was directed by indirect ophthalmoscopy, and the probe was placed 0.5 mm above the most prominent major vein in the superior retina, approximately one-half disc diameter below the first major branching (Fig. 1). 
Measurements were made continuously and recorded with an analog–digital converter to a computer. 
Experimental Procedure
Oxygen fraction in inspired air was controlled by manually adjusting the inflow of room air, pure oxygen, and pure nitrogen into the respirator, resulting in oxygen fraction ranging from 5% to 100%. All animals were subjected to 100% oxygen, 21% oxygen (room air), and a mixture of room air and nitrogen aimed at 10% oxygen fraction (actual range, 11%–12%). One pig was found to have unusually low femoral oxygen saturation at room air and was supplied with 0.1 L/min 100% O2, bringing the inspiratory oxygen fraction to 23% and the femoral oxygen saturation to a normal level. 
For three pigs, the order of the inhalation mixtures was 21%–100%–10% oxygen fraction. For five pigs, the order was 21%–10%–100%. Additionally, two pigs were subjected to 5% oxygen breathing and a stage between room air and 100% oxygen (43% and 61%). 
At each level of inspiratory oxygen fraction, femoral arterial blood gas analysis and retinal oximetry were performed when a stable level had been reached on the peripheral pulse oximeter. 
In three pigs, intravitreal pO2 measurements were performed above a retinal vein after the initial noninvasive measurements. 
All statistical analyses were performed using the statistics software package R, version 2.15.1 (R Foundation for Statistical Computing, Vienna, Austria). 8  
Results
We first varied the oxygen percentage in the inspired air and studied the relationship between the femoral arterial oxygen saturation and the optical density ratio over retinal arteries, revealing an approximately linear relationship (Fig. 2A, ODR = −0.35 × SatO2 + 0.48, SE = 0.068, adjusted R 2 = 0.74, P = 3.5 × 10−9). 
Figure 2
 
( A) Retinal artery ODR values and femoral arterial saturation at 5% to 100% volumetric inspiratory O2, with regression line (n = 8). Each symbol denotes values from an individual pig. (B) Calibrated oxygen saturation values in retinal arteries (red) and veins (blue), drawn with regression lines and 95% confidence intervals (n = 8). Retinal oxygen saturation values were calibrated using the reversal of the relationship presented in (A). Different symbols represent different levels of inspiratory oxygen fraction.
Figure 2
 
( A) Retinal artery ODR values and femoral arterial saturation at 5% to 100% volumetric inspiratory O2, with regression line (n = 8). Each symbol denotes values from an individual pig. (B) Calibrated oxygen saturation values in retinal arteries (red) and veins (blue), drawn with regression lines and 95% confidence intervals (n = 8). Retinal oxygen saturation values were calibrated using the reversal of the relationship presented in (A). Different symbols represent different levels of inspiratory oxygen fraction.
By linear regression analysis of the reverse relationship, the linear calibration equation (Equation 3) was determined to be SatO2 = 1.21 − 2.11 × ODR. Using these calibration constants we used the ODR values obtained from retinal veins to calculate the oxygen saturation in these veins. These results are drawn in Figure 2B, along with regression lines and 95% confidence intervals. 
Results for retinal vessel oximetry and femoral blood gas analysis are presented in the Table
Table. 
 
Mean Oxygen Saturation and Vessel Diameter Values at 10%, 21%, and 100% Inspiratory Oxygen (%, Mean ± SD, n = 8)
Table. 
 
Mean Oxygen Saturation and Vessel Diameter Values at 10%, 21%, and 100% Inspiratory Oxygen (%, Mean ± SD, n = 8)
Inspiratory Oxygen Fraction
10% 21% 100%
Femoral artery saturation 37.0% ± 12.5% 91.4% ± 3.0% 99.7% ± 0.3%
Retinal artery saturation 45.1% ± 22.2% 85.2% ± 7.8% 94.8% ± 18.2%
Retinal vein saturation 10.0% ± 14.5% 25.2% ± 18.7% 64.6% ± 15.0%
Retinal arterio–venous difference 35.0% ± 11.4% 60.0% ± 24.3% 30.2% ± 20.1%
Retinal artery diameter, pixels 29.6 ± 2.7 26.6 ± 3.4 22.9 ± 4.1
Retinal vein diameter, pixels 45.2 ± 4.9 39.3 ± 5.5 32.5 ± 6.4
In order to test the validity of applying the arterial calibration to veins, we compared noninvasive oximetry measurements to invasive pO2 measurements in three pigs. In this way we could compare the ODR in a major vein to the pre-venous vitreal pO2 in situations in which the arterial pO2 was manipulated by changing the oxygen percentage in the inspired air. 
The results of this comparison are presented in Figure 3, where the retinal intravenous oxygen saturation values have been converted to pO2 values, using the porcine oxyhemoglobin dissociation curve. 9 The results are drawn along with regression line and confidence intervals. This relationship was approximately linear (y = 0.56 × x + 10, SE = 12.81, adjusted R 2 = 0.45, P = 0.04). 
Figure 3
 
Retinal intravenous pO2 values compared to prevenous vitreal pO2 at corresponding inspiratory oxygen fraction levels in the same pig (n = 3), drawn with regression line and 95% confidence intervals. Intravenous pO2 values were calculated from noninvasive oxygen saturation values using the porcine oxyhemoglobin dissociation curve. 9 Each symbol denotes values from an individual pig.
Figure 3
 
Retinal intravenous pO2 values compared to prevenous vitreal pO2 at corresponding inspiratory oxygen fraction levels in the same pig (n = 3), drawn with regression line and 95% confidence intervals. Intravenous pO2 values were calculated from noninvasive oxygen saturation values using the porcine oxyhemoglobin dissociation curve. 9 Each symbol denotes values from an individual pig.
Discussion
Noninvasive spectrophotometric oximetry was sensitive to changes in oxygen saturation in pigs and correlated with intravitreal pO2 measurements and with femoral artery pO2. This is in agreement with previous studies, which have shown a significant correlation between systemic and retinal oxygen saturation in systemic hypoxemia due to Eisenmenger syndrome 10 and chronic obstructive pulmonary disease, 11 as well as during induced systemic oxygen saturation changes on retinal saturation in healthy subjects. 4 Furthermore, studies using multispectral scanning laser ophthalmoscopy have found a strong correlation between femoral artery saturation and retinal artery saturation, suggesting that comparison of these values is a reasonable method for calibrating retinal oximetry. 12  
Using porcine experiments offers the opportunity to test noninvasive oxygen saturation measurements at a much wider range of systemic oxygen saturation than is possible in humans. 
The levels of inspiratory oxygen percentage used in the present study were similar to those used on human subjects by Beach et al. 5 in 1999 (5%–100% and 8%–100%, respectively), but resulted in considerably lower femoral artery saturation values (Table). This may partly be explained by the difficulties in ventilation of the anesthetized pigs, but pigs have also been shown to have lower oxygen levels than humans in conscious states and present lower oxygen saturation values at comparable levels of pO2, as determined by the difference in human and porcine oxyhemoglobin dissociation curves. 9,13,14 In comparison to the human curve, the porcine dissociation curve is shifted to the right, resulting in a saturation difference between humans and pigs at the same pO2 that is more pronounced at lower levels of pO2 (Fig. 4). The calculated mean difference is 13.9 percentage points for blood gas measurements made at 10% inspiratory oxygen, 2.8 percentage points for measurements made at room air, and virtually nonexistent at 100% inspiratory oxygen. 
Figure 4
 
Dissociation curves for human and porcine oxyhemoglobin.
 
The curves were made using models from Severinghaus 14 and Serianni et al. 9 The marks represent femoral blood gas analysis values from the present study. Eight pO2 values above 200 mm Hg have been omitted (329–544 mm Hg, 99.3%–100.1% saturation).
Figure 4
 
Dissociation curves for human and porcine oxyhemoglobin.
 
The curves were made using models from Severinghaus 14 and Serianni et al. 9 The marks represent femoral blood gas analysis values from the present study. Eight pO2 values above 200 mm Hg have been omitted (329–544 mm Hg, 99.3%–100.1% saturation).
Pigs present a higher intraindividual variability in retinal vessel oxygen saturation compared to retinal saturation values in human subjects and a lower overall retinal saturation (Table, Fig. 2B). 4,5,15 The higher variability may be due to the different optics of the porcine eye, which is shorter on the anteroposterior axis and has a substantial level of corneal astigmatism, making it difficult to get uniform focus over the porcine retina. 16,17  
Vessel diameter on retinal images has previously been shown to have an artifactual effect on spectrophotometric retinal oximetry and is corrected for in the human version of Oxymap Analyzer. 4,15 The mean vessel diameter on our porcine fundus images taken at normal air (Table) is approximately two times greater than values reported for first-degree human retinal vessels. 18  
Reports of normal retinal vessel diameter values in porcine eye are scarce. A study by Jeppesen et al. 19 suggests that first-degree retinal arteries are approximately 140 μm in diameter in enucleated porcine eyes. Human studies have reported an arterial diameter of approximately 120 μm in healthy subjects. 20,21 This could suggest that porcine retinal arteries are not significantly larger than human retinal arteries in vivo and that the observed difference is mainly due to a different level of magnification. 
Porcine retinal vessels lie superficially in the inner retina, whereas they lie more deeply embedded in the nerve fiber layer in humans. 22 The superficial position of the porcine retinal vessels may cause an inconsistency in the paths of incident and transmitted light, especially since fundus photography of pigs usually requires that the fundus camera be tilted and that images be taken at a superior angle. 
The generally low retinal venous saturation values may be due to a combination of the low femoral saturation values and the calibration method used for retinal saturation. Highly variable raw ODR values from retinal arteries were compared to femoral arterial values. Given the relatively small sample size, the linear regression is sensitive to random effects. 
This is perhaps most prominent at 21% inspiratory oxygen fraction, where the reported arterio–venous difference is considerably higher than at 10% or 100% oxygen. Theoretically, one could expect the arterio–venous difference to be higher at room air than either at 10% oxygen, where the total amount of oxygen is limited, or at 100%, where the concentration of oxygen dissolved in plasma and the contribution of the choroid keep the hemoglobin oxygen saturation high even on the venous side. This physiological effect is most likely intensified by random effects of measurement noise. 
One possible source of measurement error that should be taken into consideration is the anesthesia. Previous studies suggest that propofol causes a reduction in systemic blood pressure, heart rate, and cardiac output, mainly through peripheral vasodilatation. 2325 Although altered, these vital signs are stable during propofol anesthesia and therefore unlikely to have an effect on our results. 
In conclusion, spectrophotometric snapshot oximetry is sensitive to systemic oxygen saturation changes over a wide range in pigs, but the variability between subjects is high. Pigs have a lower systemic oxygen saturation than do humans at comparable levels of inspiratory oxygen fraction, which results in low retinal oxygen saturation values. The difference in optical properties between porcine and human eyes makes direct comparisons of measurements difficult and reduces the value of porcine eyes as a model for spectrophotometric oximetry in humans. 
Acknowledgments
Supported by Th. Elmquist's Fund for Eye Research, Carl and Nicoline Larsen's Fund, Fight for Sight Denmark, and Synoptik Foundation. 
Disclosure: S. Traustason, Oxymap (R); J.F. Kiilgaard, None; R.A. Karlsson, Oxymap (I, E), P; S.H. Hardarson, Oxymap (I), P; E. Stefansson, Oxymap (I), P; M. la Cour, None 
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Harris A Dinn RB Kagemann L Rechtman E. A review of methods for human retinal oximetry. Ophthalmic Surg Lasers Imaging . 2003; 34: 152–164. [PubMed]
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Hardarson SH Harris A Karlsson RA Automatic retinal oximetry. Invest Ophthalmol Vis Sci . 2006; 47: 5011–5016. [CrossRef] [PubMed]
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Blondal R Sturludottir MK Hardarson SH Halldorsson GH Stefansson E. Reliability of vessel diameter measurements with a retinal oximeter. Graefes Arch Clin Exp Ophthalmol . 2011; 249: 1311–1317. [CrossRef] [PubMed]
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Figure 1
 
Porcine fundus image, with graphic overlay showing measurement areas for arteries (a) and veins (v). The brackets indicate approximate segments used for spectrophotometric oximetry, and the circle indicates the approximate area over which the pO2 probe was placed for intravitreous pO2 measurements. The fundus image was taken at 570 nm, at which there is no difference between the optical density of hemoglobin and oxyhemoglobin.
Figure 1
 
Porcine fundus image, with graphic overlay showing measurement areas for arteries (a) and veins (v). The brackets indicate approximate segments used for spectrophotometric oximetry, and the circle indicates the approximate area over which the pO2 probe was placed for intravitreous pO2 measurements. The fundus image was taken at 570 nm, at which there is no difference between the optical density of hemoglobin and oxyhemoglobin.
Figure 2
 
( A) Retinal artery ODR values and femoral arterial saturation at 5% to 100% volumetric inspiratory O2, with regression line (n = 8). Each symbol denotes values from an individual pig. (B) Calibrated oxygen saturation values in retinal arteries (red) and veins (blue), drawn with regression lines and 95% confidence intervals (n = 8). Retinal oxygen saturation values were calibrated using the reversal of the relationship presented in (A). Different symbols represent different levels of inspiratory oxygen fraction.
Figure 2
 
( A) Retinal artery ODR values and femoral arterial saturation at 5% to 100% volumetric inspiratory O2, with regression line (n = 8). Each symbol denotes values from an individual pig. (B) Calibrated oxygen saturation values in retinal arteries (red) and veins (blue), drawn with regression lines and 95% confidence intervals (n = 8). Retinal oxygen saturation values were calibrated using the reversal of the relationship presented in (A). Different symbols represent different levels of inspiratory oxygen fraction.
Figure 3
 
Retinal intravenous pO2 values compared to prevenous vitreal pO2 at corresponding inspiratory oxygen fraction levels in the same pig (n = 3), drawn with regression line and 95% confidence intervals. Intravenous pO2 values were calculated from noninvasive oxygen saturation values using the porcine oxyhemoglobin dissociation curve. 9 Each symbol denotes values from an individual pig.
Figure 3
 
Retinal intravenous pO2 values compared to prevenous vitreal pO2 at corresponding inspiratory oxygen fraction levels in the same pig (n = 3), drawn with regression line and 95% confidence intervals. Intravenous pO2 values were calculated from noninvasive oxygen saturation values using the porcine oxyhemoglobin dissociation curve. 9 Each symbol denotes values from an individual pig.
Figure 4
 
Dissociation curves for human and porcine oxyhemoglobin.
 
The curves were made using models from Severinghaus 14 and Serianni et al. 9 The marks represent femoral blood gas analysis values from the present study. Eight pO2 values above 200 mm Hg have been omitted (329–544 mm Hg, 99.3%–100.1% saturation).
Figure 4
 
Dissociation curves for human and porcine oxyhemoglobin.
 
The curves were made using models from Severinghaus 14 and Serianni et al. 9 The marks represent femoral blood gas analysis values from the present study. Eight pO2 values above 200 mm Hg have been omitted (329–544 mm Hg, 99.3%–100.1% saturation).
Table. 
 
Mean Oxygen Saturation and Vessel Diameter Values at 10%, 21%, and 100% Inspiratory Oxygen (%, Mean ± SD, n = 8)
Table. 
 
Mean Oxygen Saturation and Vessel Diameter Values at 10%, 21%, and 100% Inspiratory Oxygen (%, Mean ± SD, n = 8)
Inspiratory Oxygen Fraction
10% 21% 100%
Femoral artery saturation 37.0% ± 12.5% 91.4% ± 3.0% 99.7% ± 0.3%
Retinal artery saturation 45.1% ± 22.2% 85.2% ± 7.8% 94.8% ± 18.2%
Retinal vein saturation 10.0% ± 14.5% 25.2% ± 18.7% 64.6% ± 15.0%
Retinal arterio–venous difference 35.0% ± 11.4% 60.0% ± 24.3% 30.2% ± 20.1%
Retinal artery diameter, pixels 29.6 ± 2.7 26.6 ± 3.4 22.9 ± 4.1
Retinal vein diameter, pixels 45.2 ± 4.9 39.3 ± 5.5 32.5 ± 6.4
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