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J Appl Physiol 86: 1497-1504, 1999;
8750-7587/99 $5.00
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Vol. 86, Issue 5, 1497-1504, May 1999

Perfluorocarbon emulsion improves oxygenation of the cat primary visual cortex

Lissa B. Padnick1, Robert A. Linsenmeier1,3,4, and Thomas K. Goldstick1,2,3,4

Departments of 1 Biomedical Engineering, 2 Chemical Engineering, and 3 Neurobiology and Physiology, and 4 Institute for Neuroscience, Northwestern University, Evanston, Illinois 60208-3107


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX A
APPENDIX B

Tissue PO2 was measured in the primary visual cortex of anesthetized, artificially ventilated, normovolemic cats to evaluate the effect of small doses [1 g perfluorocarbon (PFC)/kg] of a PFC emulsion (1 g PFC/1.1 ml emulsion; Alliance Pharmaceutical, San Diego, CA) on brain oxygenation. The change in tissue PO2 (Delta PO2), resulting from briefly changing the respiratory gas from room air to 100% oxygen, was measured before and after intravenous infusion of the emulsion. Before emulsion, Delta PO2 was 51.1 ± 45.6 Torr (n = 8 cats). Increases in Delta PO2 of 34.0 ± 26.1 (SD) % (n = 8) and 16.3 ± 8.4% (n = 6) were observed after the first and second emulsion infusions, respectively. The further increase in Delta PO2 after the third dose (7.9 ± 10.5%; n = 7) was not statistically significant. The observed increases in tissue oxygenation as a result of the PFC infusions appear to be the result of enhanced oxygen transport to the tissue.

tissue oxygen tension; brain


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX A
APPENDIX B

PERFLUOROCARBON (PFC)-based artificial oxygen carriers have been proposed as a replacement for donor blood as well as a therapy for minimizing ischemic tissue damage (29, 38). The administration of a small dose (1 g PFC/kg body wt) of a PFC-based oxygen carrier (1 g perflubron/ml emulsion; Alliance Pharmaceutical, San Diego, CA) has been shown to enhance substantially oxygen transport in the retina of normovolemic cats while the animal was breathing 100% oxygen (5). This effect was surprising considering that the addition of such a small amount of PFC to the blood must have increased arterial oxygen-carrying capacity by 1% or less (APPENDIX A). To be efficacious in certain clinical applications, PFC emulsions would have to improve oxygenation of organs other than the retina, including the brain. Although the retina is often considered representative of the brain, the unique organization of the retinal vasculature (e.g., Ref. 3) may cause tissue oxygenation to differ in the two neural tissues.

The intravenous infusion of PFC emulsions has been shown to be beneficial in the treatment of stroke in cat (17, 26) and dog (13) models. Peerless et al. (26) concluded that the first-generation PFC emulsion, Fluosol (Green Cross, Osaka, Japan), had a protective effect on ischemic brain tissue in cats. Macroscopic and histological tissue evaluations were consistent with the animals' neurological status after recovery from anesthesia. Fluosol-treated animals (5.25 g PFC/kg) were in better neurological condition, both histologically and clinically, than animals given an equal volume of an isotonic saline solution; however, differences between animals hemodiluted with Fluosol and animals hemodiluted with a mannitol solution were marginal. Kline et al. (17) also studied the effect of Fluosol on middle cerebral artery ligation in cats. After a 2-h ischemic period and a 2-h reperfusion period, animals were hemodiluted with Fluosol (5.25 g PFC/kg) or a dextran solution. The delay in treatment was meant to mimic a human clinical situation. Eight hours after reperfusion, cytochrome aa3 in the ischemic penumbra was in a more oxidized state in the animals treated with Fluosol than in those left untreated or those treated with dextran. This suggested that the Fluosol-treated cats were in better condition, metabolically, possibly because of an improved cerebral oxygen supply. In a canine cerebral ischemia model (13), the magnitude of the auditory-evoked potential was used as the measure of brain stem function. Five hours after reperfusion, the animals pretreated with a PFC emulsion (1.5 g PFC/kg) exhibited an 88% recovery in evoked potential magnitude, whereas those animals given an equal volume of saline showed only a 23% recovery (13).

Infusion of PFC emulsions has also been shown to increase oxygen availability during hyperoxia in the brain of rabbits (9, 33) and cats (31). These studies measured oxygen availability rather than PO2 because the investigators used relatively large cathodes. Large polarographic electrodes consume enough oxygen so that an artificial PO2 gradient may be formed within the tissue, thus altering tissue PO2. Actual brain PO2 during hyperoxia has been reported to be increased in dogs hemodiluted with perflubron emulsion (1). Batra et al. (1) measured the average PO2 in a fairly large region of the parietal cortex with a commercial oxygen electrode system (Eppendorf, Madison, WI).

The purpose of the present research was to directly examine the efficacy of a PFC emulsion (AF0104, 1 g perflubron/1.1 ml emulsion; Alliance Pharmaceutical) in enhancing brain PO2 by recording locally from the visual cortex of the cat. Cortical tissue PO2 was recorded during 2-min episodes of ventilation with 100% oxygen (hyperoxia), both before and after small doses of the PFC emulsion (1 g PFC/kg body wt) were delivered intravenously. By using small (5-10 µm) polarographic microelectrodes, we were able to obtain more local measurements than did most previous investigators. A unique aspect of this study was that visual-evoked potentials, recorded from the same site as oxygen measurements, were used to examine cortical activity and to verify electrode penetration of the tissue (24).


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX A
APPENDIX B

Animal preparation. The surgical procedures and experimental setup have been described previously (25). Three of the cats used in the previous study (25) were also used in this one, along with six additional cats for a total of nine animals. All measurements were made with double-barreled oxygen-voltage electrodes. Polarographic oxygen electrodes are not influenced by the presence of PFC emulsions (5, 6).

Experimental protocol. Tissue PO2 transients were measured during 2-min episodes in which the animal breathed 100% oxygen (hyperoxia). Two-minute periods were used to avoid any physiological changes that may be induced by long-term hyperoxia. These periods of hyperoxia were at least 10 min apart to allow the animal to completely return to its basal state. Each PFC dose consisted of 1.1 ml emulsion/kg. This dose resulted in the addition of 1 g PFC/kg. Each of the three emulsion doses contributed an incremental, theoretical fluorocrit (volume %PFC in blood) of 0.7% (assuming 70 ml blood/kg). All doses of the PFC emulsion were administered over ~3 min during ventilation with room air.

Before the PFC was administered, three control episodes of hyperoxia were used to check for reproducibility of the tissue PO2 transients and stability of the baseline. When the control transient responses were too variable, negative, or when only an immeasurably small PO2 change (Delta PO2) could be detected during hyperoxia, the electrode was repositioned in the brain. In some cortical locations, a positive-going oxygen transient in response to hyperoxia was not observed. Of all the intracortical locations sampled for a response to hyperoxia, approximately one-third of them had a negative-going oxygen transient or showed no measurable Delta PO2. These locations were not used to test the effect of the PFC emulsion on brain oxygenation. When no satisfactory intracortical location could be found, the electrode was positioned 50-150 µm above the brain surface, and the experiment was performed with the electrode in the chamber fluid (n = 4).

Data analysis. The effect of the first dose of the perflubron emulsion was determined by calculating the percent increase in the difference between tissue PO2 during ventilation with 100% oxygen and room air (Delta PO2). The transient immediately before and the transient immediately after emulsion infusion were used to calculate the percent Delta PO2. To determine whether inherent brain variability was influencing the results, the two values of Delta PO2 immediately before emulsion infusion were compared with each other in the same manner. All changes between consecutive transients were compared to zero with a two-tailed, paired t-test. Data were excluded from analysis in one cat because of baseline instabilities, and in another cat (cat 169) an outlying result for the second emulsion infusion (2.35 SD away from mean), in which the recording became unstable during or just after the recording of the postinfusion transient, was excluded. In addition, a computer error resulted in loss of data for the third PFC infusion in one cat (cat 165).

The effectiveness of the second and third doses (each 1 g PFC/kg) was evaluated in the same manner as described above for the first dose. Because the same recording site could not always be used for all recordings, the cumulative effect of the second and third doses could not be obtained from direct comparison with the control PO2 transient recorded before the first emulsion infusion. Consequently, the hypothetical cumulative dose effects had to be calculated from the individual effects as described in APPENDIX B.

Statistics. Statistical significance was determined by a two-tailed, paired Student's t-test except in the case in which the results from the present study and a separate retinal study were compared. Here, a two-tailed, unpaired Student's t-test was performed. A P value of <0.05 was used as the criterion for statistical significance.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX A
APPENDIX B

Response in tissue PO2 to ventilation with 100% oxygen (hyperoxia). Local responses in cortical PO2 to brief (2-min) episodes of hyperoxia were examined. As in previous studies (18, 19, 21, 23, 36), a variable response was observed. Different locations showed an increase, decrease, or no measurable Delta PO2 after the respiratory gas was changed from room air to 100% oxygen. In three of the cats, all three types of responses were observed in the same animal with the same electrode.

Figure 1A shows two responses from different locations in the same cat recorded at a similar depth 2 h apart. As is demonstrated, intracortical areas that had a higher baseline PO2 and a positive-going transient in response to hyperoxia tended to have a larger transient amplitude (Delta PO2) than did areas with smaller baseline PO2 values. A significant correlation between baseline PO2 and Delta PO2 was found for intracortical (r2 = 0.665, P <0.0001, n = 43 transients) and extracortical (r2 = 0.187, P = 0.009, n = 36 transients) data (Fig. 1B).


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  		Fig. 1.   Hyperoxic transient amplitudes [change in PO2 (Delta PO2)] generally increased with baseline PO2. A: 2 transients recorded in same cat in different locations at similar depths 2 h apart. B: Delta PO2 as function of baseline PO2 for positive-going responses. There was a significant correlation between baseline PO2 and magnitude of positive-going oxygen transient for intracortical (r2= 0.665, n = 43 transients) and extracortical (r2 = 0.187, n = 36 transients) recordings. Only 7 of these transients, all collected before any perfluorocarbon (PFC) was infused, were used as control transients for PFC measurements. Control transient in 1 cat had a baseline of 0.1 Torr and Delta PO2 of 136.2 Torr, which was an outlier for this relationship, and so it has been omitted from this figure.

Effect of the PFC emulsion during ventilation with room air (normoxia). After the first 1 g PFC/kg infusion, while the animal was still breathing room air, the average increase in tissue PO2 was 8.2 ± 18.5 Torr (n = 7; PO2 during infusion not recorded in 1 cat). This increase was not statistically significantly different from zero (P = 0.29), probably due to the variability of the response (3 increases, 2 decreases, 3 no change in PO2). The top trace in Fig. 2 shows a slight increase in PO2 during emulsion infusion; the bottom trace shows a slight decrease in PO2 during emulsion infusion. The average changes in normoxic PO2 during the second and third infusions were 0.9 ± 2.7 and 1.9 ± 3.3 Torr, respectively. Neither of these changes was significantly different from zero.


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  		Fig. 2.   Consecutive oxygen transients separated by first PFC infusion in 2 cats. A: data from cat 169 were recorded intracortically at an electrode depth of 500 µm. B: data from cat 174 were recorded 150 µm above surface of brain.

Infusion of the first emulsion dose seemed to affect the systemic blood pressure slightly. On infusion of the first dose, the blood pressure sometimes transiently decreased 5-10 mmHg (while the animal was still breathing room air). This decrease lasted for only a few minutes. All hyperoxic episodes were imposed after the blood pressure had recovered. The second and third emulsion infusions had no significant effect on the blood pressure.

Effects of the PFC emulsion during ventilation with 100% oxygen (hyperoxia). Figure 2 shows examples of oxygen transients recorded immediately before and after infusion of 1 g PFC/kg body wt. Figure 2A was recorded intracortically, and Fig. 2B was recorded slightly above the brain in the artificial cerebrospinal fluid. Often a trace with a stable baseline showed increased fluctuations during hyperoxia, as shown in Fig. 2A.

Table 1 gives the effect of the first emulsion dose for each cat. Recording depths ranged from 150 µm above the cortex (negative) to 500 µm within the cortex (positive). The first PFC emulsion infusion always resulted in an increase in Delta PO2 (n = 8), but it was of variable amplitude. The mean increase in Delta PO2 was 34.0 ± 26.1 (SD) % (P = 0.008). On the other hand, the average Delta PO2 between the two transients taken immediately before emulsion infusion was not significantly different from zero. For some unexplained reason, there was, however, a nearly significant (P = 0.054) decrease in Delta PO2 of 17.8 ± 21.6% (n = 8) between the first and second transients after the first emulsion infusion.

                              
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  		Table 1.   Effect of the first 1 g PFC/kg dose of PFC emulsion

Table 2 provides the baseline PO2 and average transient amplitudes, both before and after PFC infusion, for the second and third doses. The second PFC infusion (1 g PFC/kg) resulted in an increase in Delta PO2 of 13.7 ± 10.2% (n = 7, P = 0.004). The third emulsion infusion (1 g PFC/kg) resulted in an increased Delta PO2 of 7.8 ± 10.5% (n = 7), but this effect was not statistically significant (P = 0.074). Figure 3 shows the effects of all three individual doses.

                              
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  		Table 2.   Effect of the second and third 1 g PFC/kg doses of PFC emulsion



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  		Fig. 3.   Individual effects of each PFC dose. Individual cat results (solid symbols) as well as mean effect (open symbols) are shown together. Error bars represent 1 SD from mean. PFC effects for 1st, 2nd, and 3rd individual doses were 34.0 ± 26.1, 16.3 ± 8.4, and 7.9 ± 10.5%, respectively.

It is important to note that the mean baseline PO2, reported in Tables 1 and 2, is not representative of average cortical tissue PO2. Because it was necessary to collect data at sites selected for sizable positive-going hyperoxic PO2 transients, our baseline PO2 was skewed toward a higher value (Fig. 1B). Average cortical PO2 has been reported to be 12.8 Torr (25). In addition, approximately one-half of the baseline values averaged for Tables 1 and 2 were obtained extracortically.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX A
APPENDIX B

Measurement of brain oxygen enhancement. The present study demonstrates the ability of a PFC emulsion (AF0104, Alliance Pharmaceutical) to enhance cortical oxygenation during transient hyperoxic conditions. Previous studies examining the effect of PFC infusions have used large electrodes to obtain a relative measurement of cortical oxygenation (19, 31, 33). The present study is one of the first to use small (5- to 8-µm tip) recessed-cathode microelectrodes inside of a sealed skull chamber. The smaller electrodes have several advantages. Polarographic electrodes consume oxygen in the process of measuring it, and the larger the electrode, the greater the oxygen consumption. Because of this phenomenon, larger electrodes create greater artificial oxygen sinks, and therefore gradients, making the PO2 in the presence and absence of the electrode different. For this reason, investigators using larger electrodes are forced to report oxygen availability rather than PO2. With recessed cathode microelectrodes, such a small amount of oxygen is consumed that tissue PO2 is not affected (30). In addition, a smaller electrode should compress the tissue and blood vessels less than a larger one. Also, smaller electrodes give very local measurements of PO2, whereas larger electrodes give spatial averages of tissue oxygenation. We attempted to make all measurements within the brain to exploit these advantages of our microelectrodes, but this was not always possible. Fortunately, measurements in the chamber fluid near the brain gave very similar results to intracortical ones. It is possible that the PFC effect may not be uniform at all cortical locations, and it would be worth investigating this with a technique in which PO2 could be measured in multiple cortical locations simultaneously.

Braun et al. (5) observed a substantial increase in retinal oxygenation after the intravenous administration of 1 g PFC/kg. The present study was done to determine whether this enhancement was similar in other neural tissue. To compare the PFC effect on the brain and the retina, we had to utilize similar analyses. Considering that in the preretinal vitreous only positive values of Delta PO2 were observed, we could only examine cortical locations with a positive Delta PO2 so that a fair comparison between the brain and the retina could be made.

Mechanism of the PFC-induced tissue oxygenation enhancement. After a single dose of 1 g PFC/kg, the brain, like the retina, exhibited an increase in tissue oxygenation that was much greater than expected from the minute increase in the oxygen-carrying capacity of the blood. Also, arterial blood PO2 does not change after emulsion infusions. In a previous experimental series, in which the effect of the emulsion on the retina was examined under virtually identical systemic conditions (5), no changes in normoxic or hyperoxic arterial PO2 were observed after emulsion administration. There was also no measurable effect of the infusion on hematocrit with the use of the standard evaluation methods (5). Apparently, the blood volume is regulated to be constant, by either renal mechanisms or blood and/or extracellular fluid shifts, so that the increase in oxygen-carrying capacity of the blood after the addition of 1 g PFC/kg to the blood is only 1.2% under hyperoxic conditions (APPENDIX A). If this regulation of blood volume did not occur, blood volume would increase by at most 1.66%, and the oxygen content in 100 ml of hyperoxic arterial blood would decrease by ~0.4% after the first PFC infusion. The observed enhancement of tissue PO2 must have resulted from something more than simply a change in the oxygen-carrying capacity of the blood.

Either a PFC-mediated increase in cerebral blood flow or a decrease in brain oxygen consumption could explain the enhancement. Experimental data suggest that cerebral blood flow and metabolism are not affected by the specific PFC emulsion used in this study. No experimental evidence has been published that shows vasoactivity of small doses of the PFC emulsion used here. Systemic hemodynamics do not change after intravenous infusion of the emulsion (e.g., Refs. 4, 20). Furthermore, it should be noted that increased oxygen transport has been observed in vitro in oxygenators, where flow was experimentally maintained at a constant rate (34). In addition, while the animal was breathing room air, neither brain PO2 nor electrophysiological activity (flash visual-evoked potential) was altered by the administration of the PFC, suggesting that metabolism was not affected. A previous study that used the same emulsion formulation as examined in the present work did not show any decrements in brain stem activity after emulsion infusion (13). Because systemic hemodynamics, cerebral blood flow, and cortical electrical activity appear not to be changed by the presence of small amounts of PFC, alternative explanations have been postulated.

Other possible mechanisms involve an increase in the overall mass transfer coefficient of oxygen from blood to tissue. Whereas it is not possible to quantify the contributions of these mechanisms, the basic ideas can be discussed. It is important to note that PFC particles essentially do not enter brain tissue. The PFC leaves the bloodstream unchanged through excretion by the lungs (29, 38). Only the transport within the blood will be discussed, as it seems unlikely that transport characteristics of the tissue would be changed by the intravascular infusion of the emulsion.

The dominant resistance to oxygen transport from blood to tissue is thought to be in the plasma. There is comparatively little resistance to oxygen transport present in the red blood cell (RBC) (22). For vessels that are 100-200 µm in diameter, the identical PFC droplets as used in the present study (~0.3 µm in diameter) have been found to be concentrated in an erythrocyte-free annulus near the wall (32). This phenomenon effectively decreases the overall resistance of the plasma phase to oxygen transport (8) by creating an annulus of PFC droplets that may act as "stepping stones" for oxygen (11). This radial separation of large and small particles with blood flow in a 200-µm tube was first shown with small latex beads that were 2.5 µm in diameter (10) and is known as the "near-wall excess" phenomenon.

The near-wall excess phenomenon, as it has been studied to date, applies only to arteriolar-sized vessels, but the brain is mostly oxygenated by a three-dimensional capillary mesh (27). It is, therefore, important to consider also mechanisms in capillaries that may be improving tissue oxygenation.

Capillary oxygen transport can be separated into two types. Some capillaries may be perfused only with plasma, preventing them from significantly contributing to tissue oxygenation. Assuming that a 1 g PFC/kg dose is evenly distributed in the plasma, the plasma would undergo a 28.0% increase in oxygen solubility (hematocrit = 43.7, the average in 8 cats). Most of the oxygen would still be carried to the tissue by the RBCs, however, with the amount of oxygen dissolved in the plasma phase (plasma and PFC oxygen) only comprising 4.8% of the total amount of oxygen in the blood. In addition, only 3.6% of capillaries within the first 250 µm of the rat cortex were found to be cell free in an in vivo confocal laser microscopy study (35), so this mechanism is not likely to be important.

Most capillaries contain RBCs that flow in single file with spaces between single cells or groups of stacked cells (rouleaux). In such a case, the cell-free gaps probably contribute little to oxygen transport. By adding PFC droplets to the circulation, however, oxygen transport can take place over the entire capillary surface area, increasing overall oxygen delivery to the tissues. This can be viewed as a "gap excess" of PFC droplets. In these capillaries, oxygen may first diffuse from the RBC to the PFC droplets within the plasma before diffusing to the tissue. Although some capillaries are smaller than RBCs, a cell-free annulus next to the vessel wall still exists where the tiny PFC droplets (0.3 µm in diameter) may create a near-wall excess, even at the microcirculatory level. Hochmuth et al. (14) have shown that flowing RBCs in 4- to 10-µm-diameter glass tubes display a cell-free annulus 0.6-1.9 µm thick.

Faithfull and Cain (11) believe that the PFC droplets also facilitate oxygen transport through the capillary endothelium, because PFC particles have been found residing within endothelial cells of both ischemic and healthy tissue. The trapped droplets may enhance oxygen transport by giving it a pathway of decreased resistance across the endothelium. Other mechanisms, unknown to us at this time, may also be operating.

Data from successive PFC doses further support a mechanism that lowers plasma oxygen transport resistance. Under control conditions, when there is no PFC in the bloodstream, the plasma is the dominant resistance to oxygen transport (22). Each dose of PFC lowers this resistance. At the point when the plasma oxygen transport resistance becomes approximately the same as other resistances to oxygen transport, further doses of PFC would show little effect. A calculation of the cumulative effect of multiple doses (APPENDIX B) is shown in Fig. 4. Saturation of the enhancement of transport was observed to occur between 2 and 3 g PFC/kg in the present study.


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  		Fig. 4.   Calculated cumulative effects of each dose of PFC. Individual cat results (solid symbols) as well as mean effect (open symbols) are shown together. Error bars represent 1 SD from mean. Calculated cumulative effects for 2nd and 3rd doses were 47.0 ± 21.3% (n = 7) and 51.7 ± 17.2% (n = 6), respectively.

All of the above-mentioned mechanisms would also affect cortical PO2 during normoxia (ventilation with room air); however, the magnitude of the effect would be substantially lower than that observed under hyperoxic conditions. In the present study, a statistically insignificant increase of 8.2 ± 18.5 Torr (n = 7 cats) was observed during the first 1 g PFC/kg infusion only. We believe that this increase reflects a slight PFC effect and that mechanisms that lower the plasma resistance to oxygen transport are operating. The variability among cats resulted in the statistical insignificance of the increase in normoxic cortical PO2. As with the hyperoxic results, the source of this variability is unknown. Again, a technique that measures brain PO2 at multiple sites simultaneously would be valuable in determining the effect of PFC during normoxia.

Magnitude of oxygen transport enhancement. Although the enhanced tissue oxygenation attributable to the PFC tended to be smaller in the cortex than in the retina (Fig. 5), the two results were not statistically significantly different. Retinal data in this figure were obtained from a previous study by Braun et al. (5). The tendency for a somewhat smaller effect in the cortex may be due to differences in the vascular anatomy of the two organs. The retina is oxygenated by both arterioles and capillaries, as is apparent from the capillary-free zones surrounding retinal arterioles (e.g., Ref. 28). Also, the retina frequently has metarteriolar branches, which feed into the capillary network, that leave arterioles at a 90° angle. Plasma skimming may be facilitated by this branching pattern, and, as a result, it has been theorized that retinal capillaries have a lower hematocrit than do other capillaries (2), an idea supported by experimental observations (15). This lower capillary hematocrit, and, therefore, lower oxygen-carrying capacity, in the retinal capillaries may be the underlying reason for the somewhat larger enhancement of oxygenation by small doses of intravenous PFC in the retina compared with the brain. Another possibility may be that preretinal PO2 was influenced by oxygen diffusing through the vitreous humor from distant arteries and arterioles (7), in which the PFC effect may have been larger than in capillaries.


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  		Fig. 5.   Comparison of effect of first 1 g PFC/kg infusion in a previous retinal study (5) and the present cortical study. Both individual cat results (solid symbols) and mean effect (open symbols) are shown. Error bars represent 1 SD from mean. Difference in mean %increase of Delta PO2 was not statistically significantly different in the two studies (P = 0.083).

Whereas the mean effect on the brain was not statistically significantly different from the mean effect on the retina, the responses to the PFC emulsion in the brain were more variable based on the coefficient of variation (0.77 in brain vs. 0.45 in retina). This may simply be a result of inherent brain variability. On the other hand, it may be that there is variability in the effect of PFC in different locations in each brain and that, if multiple locations were sampled simultaneously, the average effect would be more similar among cats. We could not detect a basis for variability, because the magnitude of the PFC effect was not correlated with recording depth, control transient variation, or hematocrit. The age of the emulsion was also considered because the small PFC droplets can enlarge over time. This also did not appear to influence the effect of PFC on tissue oxygenation.

The observed effect of the PFC on cat brain Delta PO2 was less than the comparable effect on rabbit brain oxygen availability (33). In that study, van Rossem et al. (33), using the same PFC emulsion as used here, measured the difference between oxygen availability during ventilation with 100% oxygen and room air in a manner similar to the present study. The change in oxygen availability nearly doubled with a single, larger 5.4 g PFC/kg dose. It is doubtful that the higher dose completely explains the larger effect observed, because the PFC effect appears to be nonlinear with dose (Fig. 4). In the present study, the calculated cumulative effect nearly saturated at 52% with a dose of 3 g PFC/kg, and it is unlikely that additional doses of PFC would have increased the effect. Two previous studies that used an earlier PFC emulsion (Fluosol), one in rabbits (9) and the other in cats (31), showed a very similar difference in emulsion effect (100 vs. 33%). Both of those studies (9, 31) measured oxygen availability and used similar PFC doses. Another study that used the same PFC emulsion as the present work, with a hemodiluted dog model, showed an increase in hyperoxic brain PO2 of 33% after emulsion infusion (1). It is, therefore, reasonable to conclude that the discrepancy between rabbits and higher mammals might simply be a species effect.

Treatment with a PFC before or after experimentally induced stroke has proven beneficial in dogs (12) and cats (9, 26). The increased oxygenation of the brain with the addition of small amounts of a PFC could explain the protective effects that PFC emulsions have been observed to have during stroke.


    APPENDIX A
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX A
APPENDIX B

Oxygen-Carrying Capacity of the Blood and the Perflubron Emulsion

Natural blood oxygen-carrying capacity. The overall blood oxygen-carrying capacity (O2 blood) is the sum of the oxygen that is held by hemoglobin (Hb; O2 Hb), the oxygen that is dissolved in the RBC (O2 RBC), and the oxygen that is dissolved in the plasma (O2 plasma) (36)
O<SUB>2 blood</SUB> = O<SUB>2 Hb</SUB> + O<SUB>2 RBC</SUB> + O<SUB>2 plasma</SUB>
Hemoglobin fractional saturation, Y, is modeled by the Hill equation (12)
<IT>Y</IT> = <FR><NU>(P/ P<SUB>50</SUB>)<SUP><IT>n</IT></SUP></NU><DE>1 + (P/ P<SUB>50</SUB>)<SUP><IT>n</IT></SUP></DE></FR>
where P50 is partial pressure of oxygen (in Torr) at which the Hb is 50% saturated (38.8 Torr for cat Hb) (16), P is oxygen partial pressure of the blood (in Torr), and n is the value of the exponent that most closely fits experimental data (2.95 for cat Hb) (16).

The amount of oxygen dissolved in the RBC and the plasma is determined by the oxygen solubility (4.7 × 10-3 ml O2 · 100 ml RBC-1 · mmHg-1 and 2.9 × 10-3 ml O2 · 100 ml plasma-1 · mmHg-1, respectively) and the PO2 of the blood (12, 36).

The total amount of oxygen in 100 ml of blood is therefore
O<SUB>2 blood</SUB> = <IT>Y</IT> ⋅ O<SUB>2 max</SUB> + 4.7 × 10<SUP>−3</SUP> ⋅ V<SUB>RBC</SUB> ⋅ P
 + 2.9 × 10<SUP>−3</SUP> ⋅ V<SUB>plasma</SUB> ⋅ P
where O2 max is maximum oxygen carrying capacity of Hb (100% saturation) (in ml O2/100 ml blood) and equals 0.45 × %hematocrit (5), VRBC is the fractional volume of the RBC, and Vplasma is the fractional volume of the plasma.

Under hyperoxic conditions (PO2 = 500 Torr), the amount of oxygen carried by a 4-kg animal (70 ml/kg blood) with a hematocrit of 43.7 is 60 ml O2.

PFC emulsion oxygen-carrying capacity. The oxygen-carrying capacity of a PFC emulsion is determined by its solubility for oxygen. The emulsion used in the present work is able to hold 32 × 10-5 ml O2 · ml emulsion-1 · mmHg-1.

When a 1 g PFC/kg (1.1 ml emulsion/kg) dose is added to the blood of a 4-kg cat, the extra amount of oxygen carried by the blood at arterial PO2 = 500 Torr is 0.7 ml O2. Assuming total blood volume remains constant, the increase in the amount of oxygen carried by 100 ml of blood is only 1.2%. The increase in the amount of oxygen carried by the plasma is increased to 4.8% from 3.7% of the total oxygen in the blood (increase of 30%).

If total blood volume does not remain constant, the volume of the emulsion would simply add to the initial blood volume of 280 ml, so the oxygen-carrying capacity of the blood would change to 60.7 ml O2 in 284.4 ml blood, a 0.4% decrease in arterial oxygen-carrying capacity during hyperoxia.


    APPENDIX B
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX A
APPENDIX B

Calculations to Arrive at Cumulative Effects for the Second and Third PFC Doses

The following equations were used to obtain the hypothetical cumulative effects after the second and third doses from the data available. Let
&Dgr;1 = <FR><NU>post(1) − pre(1)</NU><DE>pre(1)</DE></FR>
&Dgr;2 = <FR><NU>post(2) − pre(2)</NU><DE>pre(2)</DE></FR>
&Dgr;3 = <FR><NU>post(3) − pre(3)</NU><DE>pre(3)</DE></FR>
where pre(n) is Delta PO2 before the nth dose and post(n) is Delta PO2 after the nth dose of 1 g PFC/kg body wt.

Assuming that post(1) = pre(2), the hypothetical cumulative dose effects can be calculated from
cumulative(2) = <FR><NU>&Dgr;2 ⋅ post(1)</NU><DE>pre(1)</DE></FR> + &Dgr;1 = <FR><NU>post(2) − pre(1)</NU><DE>pre(1)</DE></FR>
cumulative(3) = <FR><NU>&Dgr;3 ⋅ post(2)</NU><DE>pre(1)</DE></FR> + <FR><NU>&Dgr;2 ⋅ post(1)</NU><DE>pre(1)</DE></FR> 
+ &Dgr;1 = <FR><NU>post(3) − pre(1)</NU><DE>pre(1)</DE></FR>
where cumulative(2) and cumulative(3) are the hypothetical cumulative effects after the second and third doses, respectively. Because the electrode was sometimes moved between doses, the additional data manipulation is necessary to arrive at the above equations.


    ACKNOWLEDGEMENTS

We thank Dr. Jameel Ahmed, Dr. Monique McRipley, and Jennifer Kang for assistance during experiments and Dr. David Ferster for advice regarding animal preparation and useful discussion. We also thank Dr. Peter Keipert for encouragement, help in obtaining PFC emulsion, and assistance in editing this manuscript.


    FOOTNOTES

This work was supported by Alliance Pharmaceutical (San Diego, CA) and by National Eye Institute Grant EY-05034.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests and other correspondence: R. A. Linsenmeier, Northwestern Univ., Dept. of Biomedical Engineering, 2145 Sheridan Rd., Evanston, IL 60208-3107 (E-mail: r-linsenmeier{at}nwu.edu).

Received 27 January 1998; accepted in final form 6 January 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX A
APPENDIX B

1.   Batra, S., B. P. Peters, J. D. Symons, D. Y. Hazard, U. del Balzo, J. D. Bradley, P. E. Keipert, and S. F. Flaim. Perflubron emulsion increases tissue oxygenation in a canine model of hemodilution (Abstract). Artif. Cells Blood Substit. Immobil. Biotechnol. 24: 307, 1996.

2.   Bek, T., and T. Ledet. Vascular occlusion in diabetic retinopathy. Acta Ophthalmol. Scand. 74: 36-40, 1996[Medline].

3.   Bill, A. Ocular circulation. In: Physiology of the Eye: Clinical Application (6th ed.), edited by R. A. Moses. St. Louis, MO: Mosby, 1975, p. 210-231.

4.   Biro, G. P. Central hemodynamics and blood flow distribution during infusion of perflubron emulsion or its vehicle: effects in anesthetized dogs. Artif. Cells Blood Substit. Immobil. Biotechnol. 22: 1343-1353, 1994[Medline].

5.   Braun, R. D., R. A. Linsenmeier, and T. K. Goldstick. New perfluorocarbon emulsion improves tissue oxygenation in the cat retina. J. Appl. Physiol. 72: 1960-1968, 1992[Abstract/Free Full Text].

6.   Brizel, D. M., R. Dodge, and M. W. Dewhirst. The effect of the perflubron emulsion OxygentTM on the calibration characteristics of polarographic oxygen electrodes. Radiother. Oncol. 33: 262-265, 1994[Medline].

7.   Buerk, D. G., R. D. Shonat, C. E. Riva, and S. D. Cranstoun. O2 gradients and countercurrent exchange in the cat vitreous humor near retinal arterioles and venules. Microvasc. Res. 45: 134-148, 1993[Medline].

8.   Chakrabarti, S. Mathematical Analysis of the Enhancement of Oxygen Transport by a Perfluorocarbon Emulsion for Blood Flowing in Oxygenator Tubes (MS thesis). Evanston, IL: Northwestern University, 1995.

9.   Clark, L. C., Jr., R. B. Spokane, R. E. Hoffmann, S. Ranjan, H. M. Homan, A. C. Maloney, S. J. Jacobs, T. L. Stroup, and P. E. Winston. Polarographic cerebral oxygen availability, fluorocarbon blood levels and efficacy of oxygen transport by emulsions. Artif. Cells Blood Substit. Immobil. Biotechnol. 16: 375-393, 1988.

10.   Eckstein, E. C., J. F. Koleski, and C. M. Waters. Concentration profiles of 1 and 2.5 µm beads during blood flow: hematocrit effects. ASAIO Trans. 35: 188-190, 1989[Medline].

11.   Faithfull, N. S., and S. M. Cain. Critical levels of O2 extraction following hemodilution with dextran or Fluosol-DA. J. Crit. Care 3: 14-18, 1988.

12.   Goldstick, T. K. Oxygen transport. In: Engineering Principles in Physiology, edited by J. H. U. Brown, and D. S. Gann. New York: Academic, 1973, vol. 2, p. 257-282.

13.   Guo, J., J. A. White, and H. H. Batjer. Intravenous perflubron emulsion administration improves the recovery of auditory evoked potentials after temporary brain stem ischemia in dogs. Neurosurg. 36: 350-357, 1995[Medline].

14.   Hochmuth, R. M., R. N. Marple, and S. P. Sutera. Capillary blood flow. I. Erythrocyte deformation in glass capillaries. Microvasc. Res. 2: 409-419, 1970[Medline].

15.   Jensen, P. S., and M. R. Glucksberg. Regional variation in capillary hemodynamics in the cat retina. Invest. Ophthalmol. Vis. Sci. 39: 407-415, 1998[Abstract/Free Full Text].

16.   Kiwull-Schöne, H., B. Gärtner, and P. Kiwull. The effects of CO2 and fixed acid on the O2-Hb affinity of rabbit and cat blood. Pflügers Arch. 408: 451-457, 1987[Medline].

17.   Kline, R. A., W. Negendank, L. McCoy, and R. Berguer. Beneficial effects of isovolemic hemodilution using a perfluorocarbon emulsion in a stroke model. Am. J. Surg. 162: 103-106, 1991[Medline].

18.   Lübbers, D. W. Oxygen delivery and microcirculation in the brain. In: Microcirculation in Circulatory Disorders, edited by H. Manube, B. W. Zweifach, and K. Mossemer. New York: Springer-Verlag, 1988, p. 33-50.

19.   Maas, A. I. R., W. Fleckenstein, D. A. de Jong, and H. van Santbrink. Monitoring cerebral oxygenation: experimental studies and preliminary clinical results of continuous monitoring of cerebrospinal fluid and brain tissue oxygen tensions. Acta Neurochir. Suppl. (Wien) 59: 50-57, 1993[Medline].

20.   Mattrey, R. F., P. L. Hilpert, C. D. Long, D. M. Long, R. M. Mitten, and T. Peterson. Hemodynamic effects of intravenous lecithin-based perfluorocarbon emulsions in dogs. Crit. Care Med. 17: 652-656, 1989[Medline].

21.   Metzger, H., W. Erdmann, and G. Thews. Effect of short periods of hypoxia, hyperoxia, and hypercapnia on brain O2 supply. J. Appl. Physiol. 31: 751-759, 1971[Free Full Text].

22.   Nair, P. K., J. D. Hellums, and J. S. Olson. Prediction of oxygen transport rates in blood flowing in large capillaries. Microvasc. Res. 38: 269-285, 1989[Medline].

23.   Nair, P. K., W. J. Whalen, and D. Buerk. PO2 of cat cerebral cortex: response to breathing N2 and 100 percent O2. Microvasc. Res. 9: 158-165, 1975[Medline].

24.  Padnick, L. B., and R. A. Linsenmeier. Properties of the flash visual evoked potential recorded in the cat primary visual cortex. Vision Res. In press.

25.   Padnick, L. B., R. A. Linsenmeier, and T. K. Goldstick. Oxygenation of the cat primary visual cortex. J. Appl. Physiol. 86: 1490-1496, 1999[Abstract/Free Full Text].

26.   Peerless, S. J., R. Ishikawa, I. G. Hunter, and M. J. Peerless. Protective effect of Fluosol-DA in acute cerebral ischemia. Stroke 12: 528-563, 1981[Abstract/Free Full Text].

27.   Purves, M. J. The Physiology of the Cerebral Circulation. London: Cambridge University Press, 1972.

28.   Records, R. E. Physiology of the Human Eye and Visual System. Hagerstown, MD: Harper & Row, 1979.

29.   Riess, J. Fluorocarbon-based in vivo oxygen transport and delivery systems. Vox Sang. 61: 225-239, 1991[Medline].

30.   Schneiderman, G., and T. K. Goldstick. Oxygen electrode design criteria and performance characteristics: recessed cathode. J. Appl. Physiol. 45: 145-154, 1978[Abstract/Free Full Text].

31.   Sutherland, G. R., J. K. Farrar, and S. J. Peerless. The effect of Fluosol-DA on oxygen availability in focal cerebral ischemia. Stroke 15: 829-835, 1984[Abstract/Free Full Text].

32.   Szeto, S. Radial Segregation of Perfluorocarbon Droplets in Ghost Cell Suspensions Flowing Through a Capillary Tube (MS thesis). Evanston, IL: Northwestern University, 1997.

33.   Van Rossem, K., H. Vermarien, N. S. Faithfull, L. Wouters, and K. Decuyper. Effects of perflubron emulsion and 100% oxygen breathing on local tissue PO2 in brain cortex of unanaesthetized rabbits. Adv. Exp. Med. Biol. 411: 403-409, 1996.

34.   Vslef, S. N., and T. K. Goldstick. Enhanced oxygen delivery induced by perfluorocarbon emulsions in capillary tube oxygenators. ASAIO J. 40: M643-M648, 1994[Medline].

35.   Villringer, A., A. Them, U. Lindauer, K. Einhaupl, and U. Dirnagl. Capillary perfusion of the rat brain cortex. An in vivo confocal microscopy study. Circ. Res. 75: 55-62, 1994[Abstract/Free Full Text].

36.   Whalen, W. J., R. Ganfield, and P. Nair. Effects of breathing O2 and O2-CO2 and the injection of neurohumors on the PO2 of cat cerebral cortex. Stroke 1: 194-200, 1970[Abstract/Free Full Text].

37.   Zander, R. Oxygen solubility in normal human blood. In: Advances in Physiological Sciences: Oxygen Transport to Tissue, edited by A. G. B. Kovach, E. Dóra, M. Kessler, and I. A. Silver. Budapest, Hungary: Akademiai Kiado, 1981, vol. 25, p. 331-332.

38.   Zuck, T. F., and J. G. Riess. Current status of injectable oxygen carriers. Crit. Rev. Clin. Lab. Sci. 31: 295-324, 1994[Medline].

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