Perfluorocarbon solutions (PFCs) are biochemically inert organic compounds with a high affinity for oxygen and CO₂ that can improve tissue oxygenation when used with high inspiratory concentrations of oxygen [1]. PFCs have long been proposed as potential blood substitutes to reduce the need for allogeneic blood transfusions in a variety of clinical situations. Although PFCs were introduced in the 1970s, their applications and safety profiles have yet to be defined [1], [2].

The impact of PFCs on cerebral function is still under investigation, including its effects on cerebral blood flow (CBF). It is well known that hemodilution, characterized by decreased hematocrit (Hct), lowered circulating oxygen content (CaO₂), and decreased blood viscosity can increase CBF [3]–[7]. Results of animal studies suggest that PFC administration decreases cerebrovascular resistance and blood viscosity, thereby increasing CBF [8], [9]. In most cases, increased CBF reflects increased metabolic demand or decreased arterial oxygen content (CaO2), and may indicate a need to increase cerebral oxygen delivery.

Because of the tiny particle size, PFC emulsions readily perfuse in the microcirculation, resulting in augmented oxygen delivery to the tissues more effectively than oxygen-carrying blood cells in the large vessels [9]. Because of this simple physical transport system, PFC emulsions load and unload oxygen twice as fast as does hemoglobin [10]. Although the exact capacity of oxygen delivery by PFC compared to Hb is not known, several reports indicate that oxygen delivery potential of a 2.7 g/kg dose of PFC is approximately equivalent to 4 g/dL [Hb] [11], [12]. Although the clinical importance of PFC-induced increases in CBF is unclear, animal studies suggest that PFCs may be neuroprotective in patients with traumatic and ischemic brain injury [13]–[16].

Oxycyte® is a perfluorocarbon emulsion that has the ability to dissolve large amounts of oxygen, and has been shown to be a safe blood alternative in Phase I clinical trials. Oxycyte® particles can be efficiently carried to tissues perfused at critically low flow rates, while eliciting no vasoconstriction or associated pharmacological disadvantage that prevents tissues from utilizing this oxygen. In a model of permanent middle cerebral artery occlusion in rats Oxycyte® has been shown to decrease ischemic brain damage [17]. A recent study showed that early administration of Oxycyte® immediately after the onset of vascular occlusion exerted a neuroprotective effect in these rats [18].

Several studies showed that PFC emulsions have higher oxygen carrying capacity compared to Hb, and are able to deliver twice the oxygen to the tissues when combined with hyperoxic ventilation [12], [9]–[11]. Furthermore, Adminsitration of PFC emulsions has been well tolerated by infused animals and has no negative effects on microvascular perfusion [12], [19]. To determine whether oxygen utilization by cerebral tissue is modified when Oxycyte® is used for hemodilution, the present study was performed to evaluate the effects of stepwise isovolemic hemodilution with Oxycyte® on regional CBF (rCBF) in experimental rats.

In this study we show that acute isovolemic hemodilution with Oxycyte® maintains significantly higher circulating oxygen content than does isovolemic dilution with albumin alone. Furthermore, administration of both Oxycyte® and albumin increased rCBF after acute isovolemic hemodilution, albeit in different patterns. With albumin, rCBF increased progressively in inverse proportion to the decreased Hct and CaO₂. Under the most extreme isovolemic hemodilution conditions, rCBF was approximately 170% higher in the albumin-treated group than in the Oxycyte®-treated group when the two groups were compared at the same concentrations of Hct and circulating oxygen content (i.e., Hct <10% and CaO₂ <7.5 ml/dl), indicating that Oxycyte® has maintained baseline CBF. Table 1. shows that pH and PCO₂ were kept constant in both groups.

Laser Doppler flowmeter (LDF) was used in the present study to show the increase in rCBF after acute hemodilution with Oxycyte® or albumin. As Tonnesen et al [21] have pointed out, LDF is a valuable tool for continuously measuring relative changes in rCBF; for example, under conditions of hypovolemic hemodilution and hemorrhagic shock caused by controlled hemorrhage, the LDF method may yield significantly higher relative rCBF values than the ¹³³xenon injection technique. The advantage of using LDF in cerebral perfusion studies is that this technique can measure rCBF in vivo, continuously, for up to several hours. Harada et al have reported the reproducibility of the LDF monitoring in minimizing the risk and reducing intraanimal variability and unexpected death for rat MCAO model [22].

The increase in CBF caused by hemodilution with PFC emulsion has long been recognized; the mechanisms by which PFC emulsions increase CBF are probably multifactorial. Lower blood viscosity resulting from isovolemic hemodilution, the small particle size of the PFC emulsion and decreased circulating oxygen content are all contributing factors. Hiraga et al [23] used [14C] iodoantipyrine and quantitative autoradiographic techniques to measure local CBF in rats breathing 100% oxygen after blood-PFC exchange. The CBF increased approximately two-fold after complete blood-PFC exchange and 1.5-fold after partial exchange. The authors attributed the increase in CBF to an autoregulatory response of the central nervous system vasculature to maintain an adequate supply of oxygen to the central the nervous system tissue. Lee et al [24], [25] used a radiolabeled-microsphere technique to evaluate cerebrocirculatory responses to total perfluorocarbon (FC-43) for-blood replacement (Hct less than 1%) in anesthetized, ventilated rats breathing 100% O₂. Exchange transfusion with FC-43 doubled the total and regional CBF, causing preferential flow increases to the cortex and cerebellum. Estimated cerebrovascular resistance fell to 50% of the pre-exchange value. Both hemodilution and hypoxia experiments produced CBF responses similar to those obtained in FC-43 animals. Their results suggested that decreased oxygen content and, perhaps, lower viscosity of the circulating fluorocarbon were responsible for increases in CBF required to maintain sufficient delivery of oxygen to the brain.

In the present study, partial Oxycyte®-for-blood exchange increased rCBF approximately 150%. However, rCBF did not further increase in response to decreased Hct and circulating oxygen content, but instead returned toward almost the baseline level, while Hct decreased to less than 10% and CaO₂ decreased to less than 7.5 ml/dl. Increased rCBF in response to decreased circulating oxygen content is a normal physiological response of the brain. The reason for different of rCBF changes under extreme conditions between our study and others [23]–[25] is unclear; however, the different cerebral regions of measured blood flow (striatum in our study) and the different structures of PFC emulsions (Oxycyte® in our study) may be partially responsible for this discrepancy. The impact of perfluorocarbon emulsion on neural cells' structure and function warrants further investigations. The present study's findings suggest that when Oxycyte® is used for hemodilution, care should be taken to keep circulating oxygen content from becoming too low (<7.5 ml/dl) to maintain adequate oxygen tissue perfusion.

Under normal physiological conditions, increased CBF is associated with improved cerebral oxygenation. Findings from animal studies have suggested that the increased CBF and improved oxygenation may be beneficial for patients with ischemic and traumatic brain injury [13]–[16].

In the present study, the rats were anesthetized, and their physiological parameters were kept within normal ranges (e.g., normal range of PaCO₂/pH). The increased rCBF after isovolemic hemodilution could be attributed to a cerebral autoregulatory response that services to maintain a sufficient supply of oxygen and normal cerebral functions. The reason for the smaller increase in rCBF after hemodilution with Oxycyte® than with albumin, even when the circulating oxygen content was the same, is unknown. Although calculated circulating oxygen content is the same, the oxygen availability of red blood cells (RBCs) and PFC emulsions may be different. The average particle diameter of PFC emulsions is about 0.2 micron, compared to 5–7 microns for RBCs. An emulsion of PFC, therefore, can further enhance tissue oxygenation by filling the plasma gaps between RBCs and blood vessel walls and by perfusing constricted microcapillaries too small for RBCs to pass, thereby improving local tissue oxygenation [26]. The smaller increase in rCBF after hemodilution with Oxycyte® may therefore account for the greater amount of available circulating oxygen in the Oxycyte® recipients.

As a recent study examined the rheologic effects of PFC emulsion in the presence of clinically used volume expanders. The authors observed that hemodilution with a new perfluorocarbon emulsion significantly increased plasma and whole-blood viscosity. Viscosity values were maintained at physiologic range when human albumin-PFC mixture was used. Red blood cells deformability was also observed unchanged [27]. It has also observed that hemodynamic stability was better maintained when colloid solutions, including albumin were used in hemodilution [19]. In the present study, albumin was added to Oxycyte to minimize possible increased blood viscosity caused by Oxycyte.

PFCs have been designed to substitute for blood transfusion used in lower hematocrit status (hemodilution, hemorrhage), therefore so far there is no study to look into the effect of PFCs on CBF under normal hematocrit status. In our study, stepwise hemodilution was performed, there was no significant effect of Oxycyte on rCBF when Hct was only minimally (Hct 30–40) reduced, suggesting Oxycyte per se (in small doses) may have no significant effect on CBF when given to normal Hct status. Few studies in which second generation PFCs were added to the pump prime have demonstrated improved tissue oxygenation and increased cerebral blood flow [28]–[31].

One of the limitations in the present study was that moderate phenylephrine infusion was necessary to maintain the hemodynamic stability during acute hemodilution. Phenylephrine, a pure alpha-adrenergic agonist, produces both venous and arterial constriction. Excessive vasoconstriction and increased systemic vascular resistance may decrease cardiac output, but the direct or indirect effects of phenylephrine infusion on rCBF are unknown. In the present study, rCBF was measured in the striatum only; therefore, our data do not reflect blood flow changes that might have occurred in other regions of the brain.

We therefore conclude that although Oxycyte® and albumin administration both were accompanied by an increase of rCBF following acute isovolemic hemodilution, each does so with different variability. The different patterns of increased rCBF could be due to the ability of Oxycyte®, as a true oxygen therapeutic, to maintain higher levels of circulating oxygen content under similar physiological conditions.