Abstract
To determine the effect of perfluorocarbon (PFC) hemodilution on myocardial vessel capacity to autoregulate, circumflex coronary artery pressure-flow relations were studied in anesthetized dogs under three conditions: maximal vasodilatation before and after PFC; autoregulation before and after PFC with 100% oxygen supplemented with room air ventilation, and autoregulation with PFC hemodilution during either room air or 100% oxygen supplemented with room air ventilation. During coronary vasodilatation, PFC did not modify coronary conductance or zero-flow pressure. During autoregulation after PFC, the lower pressure limit of the autoregulatory pressure-flow relation was shifted leftward. This leftward shift occurred because endocardial blood flow was maintained at a lower coronary perfusion pressure with PFC while epicardial blood flow was unchanged. Endocardial blood flow was also preserved at 50% of control blood flow levels as evidenced by the higher endocardialepicardial blood flow ratio with PFC. After PFC with 100% oxygen supplemented with room air ventilation, oxygen transport increased significantly when coronary perfusion pressure was below the lower pressure limit; the effect was most prominent in the endocardial tissue layer. Thus, PFC shifts the lower pressure limit to the left because of the increased ability of the endocardial vessel to autoregulate. Consequently, PFC can be considered a useful intervention for improving endocardial oxygen transport at low coronary perfusion pressures. (Circulation 1988;78:746-753) C oronary blood flow is maintained over a wide range of coronary perfusion pressures for a given myocardial metabolic demand because of the capacity of the myocardial vessel to autoregulate. 1 At a certain point of the autoregulatory process, coronary blood flow decreases with further reductions in coronary perfusion pressure. This point has been defined as the lower pressure limit of the autoregulatory pressure-flow curve (LPL) by Klocke et al.2 These investigators have also shown that the steady-state pressure-flow relation is likely to be shifted upward and to the right with increases in myocardial oxygen demand. Pressureflow relations vary in the different myocardial layers. The LPL of the endocardial tissue layer is higher than that of the epicardium.3,4 Interventions that modify blood viscosity5 or myocardial oxygen From the Quebec Heart Institute,
Effects of Perfluorochemical Hemodilution on Coronary Blood Flow Distribution in Dogs
John G. Kingma Jr, PhD, Jacques R. Rouleau C oronary blood flow is maintained over a wide range of coronary perfusion pressures for a given myocardial metabolic demand because of the capacity of the myocardial vessel to autoregulate. 1 At a certain point of the autoregulatory process, coronary blood flow decreases with further reductions in coronary perfusion pressure. This point has been defined as the lower pressure limit of the autoregulatory pressure-flow curve (LPL) by Klocke et al.2 These investigators have also shown that the steady-state pressure-flow relation is likely to be shifted upward and to the right with increases in myocardial oxygen demand. Pressureflow relations vary in the different myocardial layers. The LPL of the endocardial tissue layer is higher than that of the epicardium. 3, 4 Interventions that modify blood viscosity5 or myocardial oxygen demand26 may be expected to produce changes mostly in the LPL of the endocardial tissue layer.
Effects of Perfluorochemical Hemodilution on Coronary Blood Flow Distribution in Dogs
Perfluorochemical emulsions are of potential medical interest because of their ability to transport oxygen in solution without hemoglobin, which acts as the oxygen carrier. Fluosol-DA (20%) (PFC) has been found useful as red blood cell substitutes for maintenance of intravascular volume in animals and patients.7-14 However, the effects of PFC hemodilution on myocardial blood flow, oxygen transport, and regional blood flow distribution in the heart during maintained autoregulation and on coronary "back pressure" and vascular conductance during vasodilatation have not been examined. Such studies may be relevant to determine whether myocardial oxygen transport can be improved and whether coronary back pressure and vascular conductance are affected during PFC hemodilution.
Effects of Perfluorochemical Hemodilution on Coronary Blood Flow Distribution in Dogs
If the ability of the endocardial vessel to autoregulate increases after PFC hemodilution, we would expect a leftward shift of the LPL, which would result in increased oxygen transport in the endocardial tissue layer. This would have clinical implications during reduced coronary perfusion pressure. The present study was undertaken to test this hypothesis.
Materials and Methods Animal Prepraration
Adult, male mongrel dogs (body weight, [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] During maintained autoregulation (group B), circumflex artery pressure-flow relations before and after PFC hemodilution were initially measured with the electromagnetic flow probe. Thereafter, each autoregulatory pressure-flow relation was repeated, and radiolabeled microspheres were injected before and after PFC hemodilution at four points on the coronary pressure-flow relation: at control coronary blood flow, at the LPL, at the coronary perfusion pressure that produced a coronary blood flow level at 75%, and at 50% of control circumflex artery blood flow. Dogs in groups A and B were ventilated with 100% oxygen supplemented with room air throughout the experiment to increase the oxygen carrying capacity of the PFC emulsion.
Materials and Methods Animal Prepraration
Circumflex artery pressure-flow relations in group C dogs were obtained after PFC hemodilution as described in group B dogs; however, these dogs were ventilated with either room air or with 100% oxygen supplemented with room air to evaluate the emulsifier and the oxygen-carrying capacity effects of PFC.
Materials and Methods Animal Prepraration
Regional myocardial blood flow was determined with radiolabeled microspheres (15 ,um) as previously described. 18, 19 The purpose of the microsphere injections was to determine PFC-induced alterations in myocardial blood flow distribution of the left circumflex artery perfusion bed.
Materials and Methods Animal Prepraration
Arterial and coronary sinus blood samples were withdrawn immediately after microsphere injection for determination of P02, Pco2, pH, hematocrit level, fluorocrit level (i.e., relative volume of packed PFC particles/l00 ml blood), hemoglobin levels, and percent oxygen saturation. The 02 content of arterial and coronary sinus blood samples was determined with a Lex-02-Con K apparatus (Lexington Instruments, Lexington, Massachusetts). Myocardial oxygen consumption (mIImin/100 g) was calculated as the product of total coronary blood flow measured with microspheres (ml/min/100 g) and the arterocoronary sinus oxygen content (ml/ml) difference. Oxygen transport (ml/min/100 g) was calculated as the product of arterial oxygen content (ml/ml) and endocardial or epicardial blood flow (ml/minIl00 g). Blood samples were also obtained for arterial and coronary sinus lactate measurements in dogs with maintained autoregulation. Lactate was determined as previously described,20 and percent lactate extraction was calculated as the quotient of the arterocoronary sinus differences and arterial lactate measurements.
Materials and Methods Animal Prepraration
At the end of each experiment, India ink was injected into the left circumflex artery to define the posterior perfusion bed, and simultaneously the heart was arrested by injecting a saturated potassium chloride solution into the left atrium. The hearts were excised, washed, and subsequently fixed by immersion in 10% (w/v) neutral formalin. The atria, ventricles, and septum were sectioned as previously reported by Rouleau et al. 4 Regional myocardial flow to the left ventricle was calculated with a least-squares fit method.19
Data Analysis
Hemodynamic and metabolic data for the different experimental groups were compared by an unpaired Student's t test21 and ANOVA. Two-way analysis of variance with replication22 was used to evaluate the effect of PFC treatment on coronary blood flow and myocardial oxygen transport at different coronary perfusion pressure levels. Regional myocardial blood flow and oxygen consumption changes were assessed by single factor ANOVA at four different coronary perfusion pressures, and differences in blood flow within myocardial tissue layers were assessed by Newman-Keuls multiple range tests.
Data Analysis
All data are expressed as the mean ± SD, and a probability of less than or equal to 0.05 was considered to be statistically significant.
Results
Experiments were performed on 24 dogs for this study. Eight dogs were excluded because acute congestive heart failure (n=4) or severe hypotension (n = 4) occurred after PFC administration. Data from the remaining 16 dogs provide the basis for this study.
Cardiac Hemodynamic Data
Hemodynamic and blood gas measurements for group A dogs (n = 6) were made before vasodilatation and before PFC hemodilution as shown in Table 1 . After vasodilatation, heart rate increased (p<0.05) and left ventricular (LV) systolic pressure decreased (p<0.05); however, LV end-diastolic and mean coronary sinus pressures were unchanged. Administration of PFC decreased the hematocrit level from 40 to 31 vol%, whereas other measurements remained unchanged.
Cardiac Hemodynamic Data
In group B dogs (n = 5) with PFC hemodilution, heart rate increased (p<0.05), whereas LV systolic pressure decreased (p<0.O5). However, the ratepressure product, and LV end-diastolic and mean coronary sinus pressures were unchanged. In group C dogs (n=5) with PFC hemodilution during room air or 100% oxygen supplemented with room air ventilation, heart rate, LV systolic pressure, rate-pressure product, LV end-diastolic and mean coronary sinus pressures, hematocrit level, fluorocrit level, and arterial carbon dioxide tension were unchanged, thereby normalizing for all variables except arterial oxygen tension. Lactate was not produced during maintained autoregulation either before or after PFC hemodilution.
Cardiac Hemodynamic Data
In group B and C dogs, myocardial oxygen consumption (14.5 + 3.6 ml/min/100 g) was unchanged both between treatment groups and within groups at control coronary blood flow levels, at the LPL, and at 75% and 50% of control coronary blood flow levels throughout these experiments. Lactate was not produced throughout these experiments. Mean percent lactate extraction levels varied from 24% to 48% before and after PFC hemodilution. The only significant difference observed was for an increased percent lactate extraction (26.4 + 11.9 vs. 35.3 ± 23. 1, p<0.05) at the LPL in group C dogs with 100% oxygen supplemented with room air ventilation compared with room air ventilation alone.
Pressure-Flow Relations
Circumflex artery pressure-flow relations during maximal vasodilatation (group A) before and after PFC hemodilution for the total posterior LV myocardium (radiolabeled microspheres) are shown in Figure 1 . Total blood flow in the LV posterior wall was correlated with mean circumflex coronary pressure. The pressure-flow relations for total blood flow and blood flow in the epicardial and endocardial layers were linear throughout the range of coronary perfusion pressures studied both before and after PFC hemodilution. The slopes of the calculated regression lines and the zero-flow intercepts on the pressure axis were unchanged for the total myocardium and for each of the myocardial layers.
Pressure-Flow Relations
Circumflex artery pressure-flow relations during maintained autoregulation (group B) before and after PFC hemodilution are illustrated in Figure 2 . All of the pressure-flow relations were qualitatively similar. The effect of PFC hemodilution on mean circumflex artery blood flow and on total endocardial and epicardial blood flow at control (i.e., 100% mean blood flow, at the LPL, at 75%, and at 50% of control blood flow is shown in Table 2 . Circumflex artery blood flow, measured with either the electromagnetic flow probe (ml/min) or with radiolabeled microspheres (ml/min/100 g) decreased from the control coronary perfusion pressure to the LPL before and after PFC hemodilution; after PFC hemodilution, circumflex artery blood flow increased, but these changes did not achieve a level of statistical significance. Because coronary blood flow did not change significantly during maintained autoregulation, we have set the higher coronary perfusion pressure at 80 mm Hg in all dogs for the purpose of comparison. Before PFC hemodilution, the mean LPL was S559 mm Hg, whereas after PFC hemodilution, it was 39± 10 mm Hg (p<0.001). This leftward shift of the LPL with PFC hemodilution was observed in each dog (Table 2 ) with the electromagnetic flow probe (coronary blood flow) pressure-flow relation and was further confirmed with microsphere blood flow data (myocardial blood flow) and was a significant treatment effect (p<0.022). During PFC hemodilution with 100% oxygen supplemented with room air ventilation, mean coronary blood flow, total blood flow, and endocardial and epicardial blood flow were unchanged at the lower LPL.
Pressure-Flow Relations
When circumflex artery blood flow was lowered from 75% to 50% of control levels, coronary diastolic perfusion pressure decreased to 26+4 and 21±9 mm Hg before and after PFC hemodilution, respectively. At these lower levels of coronary With maintained autoregulation, total blood flow remained constant over a wide range of coronary perfusion pressures; however, with perfluorocarbon hemodilution, the lower pressure limit was shifted to a lower coronary perfusion pressure (p<0.00J vs. no perfluorocarbon hemodilution). This shift can be explained by the differences in pressure-flow relations in the endocardial (i.e., Figure 2B ) and epicardial (i.e., Figure 2C) range, and no effect was observed when 100% oxygen supplemented with room air was compared with room air ventilation after PFC hemodilution. Below the LPL, both endocardial and epicardial oxygen transport increased (p<0.03) during 100% oxygen supplemented with room air ventilation; however, the beneficial effect of PFC hemodilution on oxygen transport was greater (p<0.009) in the endocardial tissue layer. Discussion The major finding of these studies was that PFC hemodilution preserved coronary blood flow distribution distal to a coronary stenosis by shifting the lower pressure limit of the autoregulatory pressureflow curve to the left. Coronary blood flow and oxygen transport in the endocardial tissue layer of PFC-treated dogs was maintained despite significant reductions in coronary perfusion pressure.
Effects of Perfluorocarbon Hemodilution
Fluosol-DA 20% is a low viscosity, stable emulsion comprising two fluorocarbon compounds (perfluorodecalin and perfluorotripropylamine) with high oxygen solubility, a nonionic surface active agent (Pluronic F-68), balanced salts, and other emulsion stabilizers. Pluronic F-68 may be responsible for some of the beneficial effects that have been demonstrated during myocardial ischemia. 10 Figure 2A) , the increased heart rate, lower LV blood pressures, and the nonsignificant reduction of LV end-diastolic pressure. These results rule out significant changes in viscosity or edema after PFC hemodilution and are similar to the observations reported by O'Benar and Bellamy.35 Biro36 also reported a significant reduction in the hematocrit level after PFC hemodilution in dogs, but only a marginal change in vascular conductance was observed.
Effects of Perfluorocarbon Hemodilution
With maintained autoregulation, the LPL can be shifted upward and to the right during tachycardia,6 increased myocardial oxygen demand,2 and downward and to the right with increased blood viscosity.5 In the present study, the LPL of the coronary pressure-flow relation before PFC hemodilution was achieved at 55 ± 9 mm Hg. After PFC hemodilution, myocardial oxygen consumption was unchanged, but the LPL shifted leftward at 39 ± 10 mm Hg. In each dog, the LPL was determined before injection of microspheres and again at the time of microsphere injection; the LPL indicated a similar leftward shift. Furthermore, at the LPL, a good correlation for blood flow measurements was obtained with two independent blood flow measurement techniques (Table 2) . Total blood flow to the circumflex artery perfusion bed was maintained despite the lower coronary perfusion pressure. This was primarily due to the ability of the endocardial vessels to maintain autoregulation at the lower coronary perfusion pressure of 39 ± 10 mm Hg because myocardial oxygen consumption was unchanged; there- after, blood flow was pressure dependent (p<0.002). The epicardial vessel capacity to autoregulate was substantially greater than the endocardial vessels and was not lost for the range of coronary perfusion pressures studied either before or after PFC hemodilution. Based on data from group A, a significant viscosity effect was ruled out because circumflex artery pressure-flow relations were not changed.
Effects of Perfluorocarbon Hemodilution
Among the factors that control steady-state coronary pressure-flow relations, changes in back pressure rather than PFC hemodilution may have been responsible for the leftward shift of the LPL. However, in the present study, zero-flow pressure was unchanged during PFC hemodilution and LV enddiastolic, and coronary sinus pressures were always lower than zero-flow pressure throughout these experiments. Myocardial ischemia, which could also have modified the coronary pressure-flow relation at low coronary perfusion pressure, was ruled out because after PFC hemodilution none of the dogs produced lactate even at 50% of control blood flow levels. Prevention of ischemia with oxygenated Fluosol-DA has been shown by Cleman et a17 during coronary angioplasty and also by Tokioka et a131 who showed preservation of myocardial function and metabolism during brief coronary occlusions.
Effects of Perfluorocarbon Hemodilution
An intrinsic component of the coronary vessel, which is responsive to arterial oxygen concentration, may also be responsible for the adjustment of vascular tone and consequently the alteration in flow3 during PFC hemodilution. The beneficial effect of PFC hemodilution may occur as a consequence of increased diffusion of oxygen from fluorocarbons into the tissue, which is perhaps due to the small particle size of the fluorocarbon emulsion, or as a consequence of reduced affinity of the blood and PFC mixture for oxygen even in the absence of increased oxygen content or blood flow. PFC hemodilution without 100% oxygen supplemented with coronary pressure group by two-way ANOVA with room air ventilation also produced a leftward shift of the LPL in the present study. However, with 100% oxygen supplemented with room air ventilation, endocardial oxygen transport was significantly increased compared with PFC hemodilution with room air ventilation alone. This suggests that it is not only the oxygen-carrying capacity of the fluorocarbon emulsion that is responsible for its beneficial effects. The results of this study support the hypothesis that the reduction of ischemic injury by PFC may be partly due to a mechanism other than direct enhancement of oxygen delivery25 because fluorocarbon emulsions, even during room air ventilation, may increase washout of protons and metabolic by-products of ischemia or may recruit capillary beds at the lower coronary perfusion pressures. An important question that has not been resolved in this study is whether PFC hemodilution shifts the LV pressure-volume relation. Such a shift may have contributed to the treatment effect by reducing myocardial oxygen demand; however, myocardial oxygen consumption remained constant throughout these studies.
Conclusions
Our data are the first to demonstrate that an intervention such as PFC hemodilution can preserve myocardial blood flow distal to a coronary stenosis. This is achieved both by a leftward shift of the LPL and by an increased oxygen-carrying capacity. The ability of PFC emulsions to transport oxygen is of potential clinical importance in patients with coronary artery disease because it may improve the efficacy of the coronary angioplasty technique by permitting increased durations of balloon inflation. Thus, the development of PFC emulsions that are better tolerated than Fluosol-DA 20%, but which have similar coronary dynamic properties, may be important for improving transmural blood flow distribution and oxygen transport economy.

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