Brain microvascular function during cardiopulmonary bypass

Emboli in the brain microvasculature may inhibit brain activity during cardiopulmonary bypass. Such hypothetical blockade, if confirmed, may be responsible for the reduction of cerebral metabolic rate for glucose observed in animals subjected to cardiopulmonary bypass. In previous studies of cerebral blood flow during bypass, brain microcirculation was not evaluated. In the present study in animals (pigs), reduction of the number of perfused capillaries was estimated by measurements of the capillary diffusion capacity for hydrophilic tracers of low permeability. Capillary diffusion capacity, cerebral blood flow, and cerebral metabolic rate for glucose were measured simultaneously by the integral method, different tracers being used with different circulation times. In eight animals subjected to normothermic cardiopulmonary bypass, and seven subjected to hypothermic bypass, cerebral blood flow, cerebral metabolic rate for glucose, and capillary diffusion capacity decreased significantly: cerebral blood flow from 63 to 43 ml/100 gm/min in normothermia and to 34 ml/100 gm/min in hypothermia and cerebral metabolic rate for glucose from 43.0 to 23.0 mumol/100 gm/min in normothermia and to 14.1 mumol/100 gm/min in hypothermia. The capillary diffusion capacity declined markedly from 0.15 to 0.03 ml/100 gm/min in normothermia but only to 0.08 ml/100 gm/min in hypothermia. We conclude that the decrease of cerebral metabolic rate for glucose during normothermic cardiopulmonary bypass is caused by interruption of blood flow through a part of the capillary bed, possibly by microemboli, and that cerebral blood flow is an inadequate indicator of capillary blood flow. Further studies must clarify why normal microvascular function appears to be preserved during hypothermic cardiopulmonary bypass.     

Hemodilution elevates cerebral blood flow and oxygen metabolism during cardiopulmonary bypass in piglets

BACKGROUND: Hemodilution continues to be widely used during cardiopulmonary bypass (CPB) for both adults and children. Previous studies with nonbypass models have suggested that an increase in cerebral blood flow (CBF) compensates for the reduced oxygen-carrying capacity; however, this increased CBF is achieved by an increase in cardiac output. We hypothesized that even with the fixed-flow perfusion of CPB, CBF would be increased during hemodilution. METHODS: Two experiments were conducted and analyzed separately. In each experiment, 10 piglets were randomized to two different groups, one with a total blood prime yielding a high hematocrit (25% or 30%), and the other with a crystalloid prime resulting in a low hematocrit (10% or 15%). Animals were cooled with pH-stat strategy at full flow (100 or 150 mL.kg(-1).min(-1)) to a nasopharyngeal temperature of 15 degrees C, a period of low flow (50 mL.kg(-1).min(-1)) preceding deep hypothermic circulatory arrest (45 or 60 minutes), and a period of rewarming at full flow. Cerebral blood flow was measured at the beginning of CPB, at the end of cooling, at the end of low flow, 5 minutes after the start of rewarming, and at the end of rewarming by injection of radioactive microspheres. RESULTS: Mean arterial pressure was significantly greater with higher hematocrit at each time point (p< 0.05). Cerebral blood flow and the cerebral metabolic rate of oxygen decreased during cooling and further during low flow bypass but were significantly greater with lower hematocrit during mild hypothermia and at the end of rewarming (p< 0.05). CONCLUSIONS: Hemodilution is associated with decreased perfusion pressure, increased CBF and increased the cerebral metabolic rate of oxygen during hypothermic CPB.     

Somatosensory evoked potentials and cerebral metabolism during cardiopulmonary bypass with special reference to hypotension induced by prostacyclin infusion

Somatosensory evoked potentials and cerebral metabolism were studied during cardiopulmonary bypass in 41 patients undergoing coronary bypass. Twenty-two patients received prostacyclin 50 ng/kg/min during cardiopulmonary bypass for platelet protection and 19 patients served as controls. Mean arterial blood pressure in the prostacyclin group was below 30 mm Hg during the first 30 minutes of bypass, but it remained above 50 mm Hg in the control group. Central conduction time, a measure of the electrical conduction time in the central nervous system, was prolonged in both groups during bypass up to 30 minutes of rewarming. The prolongation was greater in the control group early during bypass. At 20 minutes of cardiopulmonary bypass, central conduction time was increased by 81% (standard deviation 38) of the prebypass value in the control group and by 44% (standard deviation 17) in the prostacyclin group (p less than 0.001). Arteriovenous oxygen difference across the brain was greater in the prostacyclin group early during bypass. It was 36 ml/L (standard deviation 9) in the control group and 60 ml/L (standard deviation 18) in the prostacyclin group (p less than 0.001) at 10 minutes of bypass. There was no difference between the groups in regard to glucose and lactate. We conclude that cardiopulmonary bypass with hypothermia prolongs central conduction time. The hypotension induced by prostacyclin (50 ng/kg/min) did not further impair conduction in the central nervous system.     

异丙酚对体外循环中脑氧代谢的影响

目的：探讨异丙酚对体外循环（ＣＰＢ）各阶段脑氧及乳酸代谢的影响。方法：选择心内直视手术病人３１例，随机分为异现酚组（Ａ组）１６例，对照组（Ｂ组）１５例。分别于ＣＰＢ前、降温及３３℃和３０℃，低温期，复温至３０℃和３３℃以及ＣＰＢ后１５分钟七个时点动脉，颈内静脉血气及乳酸值（ＬＡ）并计算脑摄氧率（Ｏ２Ｅｘｔ）及动脉－颈内静脉乳酸差值。     

Rapid Rewarming Causes an Increase in the Cerebral Metabolic Rate for Oxygen that Is Temporarily Unmatched by Cerebral Blood Flow: A Study during Cardiopulmonary Bypass in Rabbits

Background: Jugular venous hemoglobin desaturation during the rewarming phase of cardiopulmonary bypass is associated with adverse neuropsychologic outcome and may indicate a pathologic mismatch between cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO2). In some studies, rapid rewarming from hypothermic cardiopulmonary bypass results in greater jugular venous hemoglobin desaturation. The authors wished to determine if rewarming rate influences the temperature dependence of CBF and CMRO2.

Methods: Anesthetized New Zealand white rabbits, cooled to 25 degrees Celsius on cardiopulmonary bypass, were randomized to one of two rewarming groups. In the fast group (n = 9), aortic blood temperature was made normothermic over 25 min. Cerebral blood flow (microspheres) and CMRO2 (Fick) were determined at baseline (25 degrees C), and at brain temperatures of 28 degrees, 31 degrees, 34 degrees, and 37 degrees Celsius during rewarming.

Results: Systemic physiologic variables appeared similar between groups. At a brain temperature of 28 degrees C, CMRO2 was 47% greater in the fast rewarming group than in the slow group (2.2 +/-0.5 vs. 1.5+/-0.2 ml O2 *symbol* 100 g sup -1 *symbol* min sup -1, respectively; P = 0.01), whereas CBF did not differ (48+/-18 vs. 49+/-8 ml *symbol* 100 g sup -1 *symbol* min sup -1, respectively; P = 0.47). Throughout rewarming, CBF increased as a function of brain temperature but was indistinguishable between groups. Cerebral metabolic rate for oxygen differences between groups decreased as brain temperatures increased.     

Brain hyperperfusion during cardiac operations. Cerebral blood flow measured in man by intra-arterial injection of xenon 133: evidence suggestive of intraoperative microembolism

Cerebral blood flow (CBF) was measured by intra-arterial injection of xenon 133 in 29 patients during cardiac operations. Marked changes occurred in all patients. A normal and significant correlation with temperature and plasma PCO2 (p less than 0.01) support the reliability of the method. Mean CBF measured between sternotomy and the onset of extracorporeal circulation (ECC) was 38 ml/100 gm . min. The first minute of ECC was associated with a decrease in CBF in nine of 12 patients (p less than 0.02). During steady-state hypothermic ECC (temperature 29 degrees C), CBF increased unexpectedly to 64 ml/100 gm . min (p less than 0.01). Following rewarming steady-state normothermic ECC, mean CBF decreased to 42 ml/100 gm . min with signs of impairment of cerebral autoregulation. Ten and 20 minutes after termination of ECC, mean CBF was 40 and 41 ml/100 gm . min, respectively. Arterial PCO2 was found to be important in regulating CBF. The cerebral autoregulation maintained CBF down to arterial pressures of around 55 mm Hg. Below this level, CBF was significantly correlated with perfusion pressure (p less than 0.01). Multiple small emboli with a hyperemic border zone could cause a brain hyperperfusion, as seen in our patients during bypass. Measurements of CBF during ECC hold promise as a guide toward safer cardiac operations.     

Pulsatile Versus Nonpulsatile Flow: No Difference in Cerebral Blood Flow or Metabolism during Normothermic Cardiopulmonary Bypass in Rabbits

Background: Although pulsatile and nonpulsatile cardiopulmonary bypass (CPB) do not differentially affect cerebral blood flow (CBF) or metabolism during hypothermia, studies suggest pulsatile CPB may result in greater CBF than nonpulsatile CPB under normothermic conditions. Consequently, nonpulsatile flow may contribute to poorer neurologic outcome observed in some studies of normothermic CPB. This study compared CBF and cerebral metabolic rate for oxygen (CMRO2) between pulsatile and nonpulsatile CPB at 37 degrees Celsius.

Methods: In experiment A, 16 anesthetized New Zealand white rabbits were randomized to one of two pulsatile CPB groups based on pump systolic ejection period (100 and 140 ms, respectively). Each animal was perfused at 37 degrees Celsius for 30 min at each of two pulse rates (150 and 250 pulse/min, respectively). This scheme created four different arterial pressure waveforms. At the end of each perfusion period, arterial pressure waveform, arterial and cerebral venous oxygen content, CBF (microspheres), and CMRO2 (Fick) were measured. In experiment B, 22 rabbits were randomized to pulsatile (100-ms ejection period, 250 pulse/min) or nonpulsatile CPB at 37 degrees Celsius. At 30 and 60 min of CPB, physiologic measurements were made as before.

Results: In experiment A, CBF and CMRO2 were independent of ejection period and pulse rate. Thus, all four waveforms were physiologically equivalent. In experiment B, CBF did not differ between pulsatile and nonpulsatile CPB (72 plus/minus 6 vs. 77 plus/minus 9 ml *symbol* 100 g sup -1 *symbol* min1, respectively (median plus/minus quartile deviation)). CMRO2 did not differ between pulsatile and nonpulsatile CPB (4.7 plus/minus 0.5 vs. 4.1 plus/minus 0.6 ml Oxygen2 *symbol* 100 g sup -1 *symbol* min1, respectively) and decreased slightly (0.4 plus/minus 0.4 ml Oxygen2 *symbol* 100 g sup -1 *symbol* min1) between measurements.     

pH-Stat Management Reduces the Cerebral Metabolic Rate for Oxygen during Profound Hypothermia (17 degrees Celsius): A Study during Cardiopulmonary Bypass in Rabbits

Background: Greater cerebral metabolic suppression may increase the brain's tolerance to ischemia. Previous studies examining the magnitude of metabolic suppression afforded by profound hypothermia suggest that the greater arterial carbon dioxide tension of pH-stat management may increase metabolic suppression when compared with alpha-stat management.

Methods: New Zealand White rabbits, anesthetized with fentanyl and diazepam, were maintained during cardiopulmonary bypass (CPB) at a brain temperature of 17 degrees Celsius with alpha-stat (group A, n = 9) or pH-stat (group B, n = 9) management. Measurements of brain temperature, systemic hemodynamics, arterial and cerebral venous blood gases and oxygen content, cerebral blood flow (CBF) (radiolabeled microspheres), and cerebral metabolic rate for oxygen (CMRO2) (Fick) were made in each animal at 65 and 95 min of CPB. To control for arterial pressure and CBF differences between techniques, additional rabbits underwent CPB at 17 degrees Celsius. In group C (alpha-stat, n = 8), arterial pressure was decreased with nitroglycerin to values observed with pH-stat management. In group D (pH-stat, n = 8), arterial pressure was increased with angiotensin II to values observed with alpha-stat management. In groups C and D, CBF and CMRO2 were determined before (65 min of CPB) and after (95 min of CPB) arterial pressure manipulation.

Results: In groups A (alpha-stat) and B (pH-stat), arterial pressure; hemispheric CBF (44 plus/minus 17 vs. 21 plus/minus 4 ml *symbol* 100 g sup -1 *symbol* min sup -1 [median plus/minus quartile deviation]; P = 0.017); and CMRO2 (0.54 plus/minus 0.13 vs. 0.32 plus/minus 0.10 ml Oxygen2 *symbol* 100 g sup -1 *symbol* min sup -1; P = 0.0015) were greater in alpha-stat than in pH-stat animals, respectively. As a result of arterial pressure manipulation, in groups C (alpha-stat) and D (pH-stat) neither arterial pressure (75 plus/minus 2 vs. 78 plus/minus 2 mm Hg) nor hemispheric CBF (40 plus/minus 10 vs. 48 plus/minus 6 ml *symbol* 100 g sup -1 *symbol* min sup -1; P = 0.21) differed between alpha-stat and pH-stat management, respectively. Nevertheless, CMRO2 was greater in alpha-stat than in pH-stat animals (0.71 plus/minus 0.10 vs. 0.45 plus/minus 0.10 ml Oxygen2 *symbol* 100 g sup -1 *symbol* min sup -1, respectively; P = 0.002).     

Cerebral blood flow rate during cardiopulmonary bypass and optimal cerebral perfusion flow rate during separated brain perfusion--a clinical study

There is no established theory to determine the cerebral blood flow rate (CBF) during not only the standard cardiopulmonary bypass but during the cardiopulmonary bypass with separated brain perfusion. This study was carried out to answer the following questions. (1) what is the relationship during the cardiopulmonary bypass between CBF and systemic flow rate or blood pressure?. (2) what is the optimal flow rate to the innominate artery during the separated brain perfusion? Twenty-one patients were selected for this study, who were operated under the cardiopulmonary bypass with a standard roller pump and a membrane oxygenator under moderate hypothermia (nasopharyngeal temperature of 26-28 degrees C). Systemic flow rate was maintained between 40 and 70 ml/kg/min. CBF before the cardiopulmonary bypass was 30.6 +/- 5.5 ml/100 g brain/min, and increased to 33.8 +/- 8.9 ml/100 g brain/min during the cardiopulmonary bypass. CBF was proportional to systemic flow rate (r = 0.62, p less than 0.01) and showed poor association with blood pressure ranged from 35 to 94 mmHg. As for the relationship between innominate arterial and cerebral blood flow rate, CBF linearly followed the decrease of innominate arterial flow rate to below about 9 ml/kg/min, but showed almost no changes when innominate arterial flow rate was over 9 ml/kg/min. It was observed that cerebral oxygen consumption did not decrease significantly under moderate hypothermia (26-28 degrees C), as far as CBF of 25 ml/100 g brain/min was maintained. Based on the relationship between innominate arterial and cerebral blood flow rate, it was shown that the innominate arterial flow rate to provide CBF of 25 ml/100 g brain/min was 5.5 ml/kg/min.(ABSTRACT TRUNCATED AT 250 WORDS)     

异丙酚中对中低温体外循环脑氧合的影响

目的 观察异丙酚对中低温体外循环心肺转流术（CPB）期间脑氧合的影响。8方法 心内直视手术患者17例随机分为芬太尼组和异丙酚组,通过监测动脉、颈内静脉血和混合静脉血氧含量以及乳酸浓度,计算全身和脑动静脉氧含量差、氧摄取率和动静脉乳酸浓度差,分析异丙酚对CPB期间脑氧合的影响。结果 两组复温过程中动静脉氧含量差和氧摄取率均较低温时升高;异丙酚组在CPB过程中动脉－颈内静脉血氧含量差和脑的氧摄取率要高于芬太尼组（P＜0．05）,动脉－混合静脉血氧含量差和全身的氧摄取率两组差异无显著意义（P＞0．05）。CPB全过程中两组血乳酸浓度均进行性升高。结论 CPB期间应用异丙酚麻醉并不能明显改善脑氧合。CPB期间脑保护机理有其复杂性一面,不能仅停留于氧代谢平衡方面。     

Effect of rewarming speed during hypothermic cardiopulmonary bypass on cerebral pressure-flow relation

BACKGROUND: Cerebral blood flow is less dependent on arterial blood pressure during hypothermic cardiopulmonary bypass (CPB) compared to warm CPB. Fast rewarming has a more pronounced effect on cognitive performance in the elderly and causes an increased arterio-jugular oxygen content difference. We studied the effect of rewarming and rewarming speed on cerebral pressure-flow relation in adult patients undergoing elective coronary artery bypass surgery with mild hypothermic CPB. METHODS: Fifty patients were randomly assigned to either a slow rewarming strategy (0.24 degrees C/min) or a fast rewarming strategy (0.5 degrees C/min). Cerebral pressure-flow relation was assessed by a transcranial Doppler derived index for cerebral pressure-flow relation (Pressure-flow Index, PFI). The effect of rewarming speed on cerebral pressure-flow relation was assessed by comparing the absolute PFI value after rewarming between the two treatment groups. RESULTS: The mean PFI decreased significantly from 0.73 (standard deviation: 0.28) before rewarming to 0.54 (0.35) after rewarming in the slow rewarming group and from 0.63 (0.29) to 0.48 (0.30) in the fast rewarming group. Absolute PFI after rewarming was not significantly different (mean PFI difference = 0.06; 95% CI = - 0.13; 0.26) between both rewarming strategies. CONCLUSION: Rewarming from mild hypothermic CPB might result in pressure-dependent cerebral blood flow velocity but rewarming speed did not aggravate the effect of rewarming on pressure-flow dependency.     

Cerebral response to hemodilution during hypothermic cardiopulmonary bypass in adults.

We examined the cerebral response to changing hematocrit during hypothermic cardiopulmonary bypass (CPB) in 18 adults. Cerebral blood flow (CBF), cerebral metabolic rate for oxygen (CMRO2), and cerebral oxygen delivery (CDO2) were determined using the nitrous oxide saturation technique. Measurements were obtained before CPB at 36 degrees C, and twice during 27 degrees C CPB: first with a hemoglobin (Hgb) of 6.2 +/- 1.2 g/dL and then with a Hgb of 8.5 +/- 1.2 g/dL. During hypothermia, appropriate reductions in CMRO2 were demonstrated, but hemodilution-associated increases in CBF offset the reduction in CBF seen with hypothermia. At 27 degrees C CPB, as the Hgb concentration was increased from 6.2 to 8.5 g/ dL, CBF decreased. CDO2 and CMRO2 were no different whether the Hgb was 6.2 or 8.5 g/dL. In eight patients in whom the Hgb was less than 6 g/dL, CDO2 remained more than twice CMRO2. IMPLICATIONS: This study suggests that cerebral oxygen balance during cardiopulmonary bypass is well maintained at more pronounced levels of hemodilution than are typically practiced, because changes in cerebral blood flow compensate for changes in hemoglobin concentration.     

Beneficial effect of balloon-induced pulsatility on brain oxygenation in hypothermic cardiopulmonary bypass

BACKGROUND: Sufficient O2 delivery to meet the demand is an important factor for protecting the brain during cardiopulmonary bypass (CPB). This study was designed to investigate the influences of temperature, pulsatility of blood flow (intra-aortic balloon pump-induced) and flow rate during CPB on the cerebral oxygenation. METHODS: Patients were divided into five groups. Normothermia (36 degrees C): pulsatile (n=8, 2.5 L/min/m2), nonpulsatile (n=12, 2.5 L), and nonpulsatile perfusion (n=12, 2.8 L); hypothermia (30 degrees C): pulsatile (n=9, 2.5 L) and nonpulsatile perfusion (n=11, 2.5 L). The oxygen saturation (SjVO2), lactate and CPK-BB levels in the jugular venous blood were measured. RESULTS: In all of the normothermic groups, the SjVO2 value decreased during the CPB (p<0.1-0.01). No remarkable change was observed in the hypothermic groups, with the exception during the rewarming period in the nonpulsatile group. A higher SjVO2 and a lower frequency of SjVO2 values <50% were observed in the hypothermic pulsatile group, as compared with those in the normothermic groups (p<0.05). The levels of CPK-BB were nearly the same, however the levels of lactate were higher in the normothermic pulsatile and nonpulsatile (2.5 L) groups (p<0.05). CONCLUSIONS: We concluded that the hypothermic CPB was advantageous over normothermic CPB in regard to the SjVO2 levels and lactate production. The beneficial effect of intra-aortic balloon pump assist was only obtained in the hypothermic CPB.     

异丙酚在心脏瓣膜置换术中对氧供、氧耗、氧摄取率及氧合状态的影响

目的：观察异丙酚在心脏瓣膜置换术中对氧供（ＤＯ２）,氧耗（ＶＯ２）,氧摄取率（ＥＲＯ２）及氧合状态的影响。方法：２０例心脏瓣膜置换术病人随机分为两组。组麻醉诱导与维持用异丙酚,对照组用咪唑安定。观察体外循环（ＣＰＢ）期间ＤＯ２,ＶＯ２,ＥＲＯ２,混合静脉血氧饱和度（ＳＶＯ２）及动脉血乳酸（ＡＢＬ０的变化。结果：（１）组内各时点ＤＯ２无明显变化,复温后观察组ＥＲＯ２增加非常显著;降温开始及复温后观察     

Continuous monitoring of blood oxygen saturation of internal jugular vein as a useful indicator for selective cerebral perfusion during aortic arch replacement.

We continuously monitored blood oxygen saturation in the internal jugular vein during selective cerebral perfusion for aortic arch operations and evaluated its efficacy as an indicator of cerebral oxygen metabolism. The selective cerebral perfusion method was applied in 11 patients who underwent operations for aortic arch replacement. Blood oxygen saturation in the internal jugular vein was continuously monitored at the bulbus jugularis with a fiberoptic catheter during the operation. Perfusion flow of 500 ml/min was continued for 134.7 +/- 14.9 minutes under moderate hypothermia at 25 degrees C, and bilateral temporal arterial pressure was 40 to 60 mm Hg. Blood gas data were used to estimate oxygen consumption, oxygen extraction ratio, and lactate uptake in the cerebrum. No patients had postoperative cerebral complications. Cerebral oxygen consumption was 2.93 +/- 0.4 ml/min/100 gm under general anesthesia at 36 degrees C. While selective cerebral perfusion at 25 degrees C decreased consumption to 0.92 +/- 0.39 ml/min/100 gm, it fell to about 30% of its former value. Blood oxygen tension in the internal jugular vein showed no significant correlation with rectal temperature. Selective cerebral perfusion did not significantly affect cerebral lactate uptake. In contrast, blood oxygen saturation in the internal jugular vein was significantly affected by temperature and cerebral flow during selective cerebral perfusion, and blood oxygen saturation in the internal jugular vein correlated closely with cerebral oxygen extraction ratio (r = 0.91). Cerebral oxygen metabolism was thus well maintained, and continuous monitoring of blood oxygen saturation in the internal jugular vein was found to serve as a useful indicator under selective cerebral perfusion during operations for aortic arch replacement.     

Cerebral oxygen metabolism during total body flow and antegrade cerebral perfusion at deep and moderate hypothermia

The aim of this study is to evaluate the effect of temperature on cerebral oxygen metabolism at total body flow bypass and antegrade cerebral perfusion (ACP). Neonatal piglets were put on cardiopulmonary bypass (CPB) with the initial flow rate of 200 mL/kg/min. After cooling to 18°C (n = 6) or 25°C (n = 7), flow was reduced to 100 mL/kg/min (half‐flow, HF) for 15 min and ACP was initiated at 40 mL/kg/min for 45 min. Following rewarming, animals were weaned from bypass and survived for 4 h. At baseline, HF, ACP, and 4 h post‐CPB, cerebral blood flow (CBF) was measured using fluorescent microspheres. Cerebral oxygen extraction (CEO₂) and cerebral metabolic rate of oxygen (CMRO₂) were monitored. Regional cranial oxygen saturation (rSO₂) was continuously recorded throughout the procedure using near‐infrared spectroscopy. At 18°C, CBF trended lower at HF and ACP and matched baseline after CPB. CEO₂ trended lower at HF and ACP, and trended higher after CPB compared with baseline. CMRO₂ at ACP matched that at HF. Cranial rSO₂ was significantly greater at HF and ACP (P < 0.001, P < 0.001) and matched baseline after CPB. At 25°C, CBF trended lower at HF, rebounded and trended higher at ACP, and matched baseline after CPB. CEO₂ was equal at HF and ACP and trended higher after CPB compared with baseline. CMRO₂ at ACP was greater than that at HF (P = 0.001). Cranial rSO₂ was significantly greater at HF (P = 0.01), equal at ACP, and lower after CPB (P = 0.03). Lactate was significantly higher at all time points (P = 0.036, P < 0.001, and P < 0.001). ACP provided sufficient oxygen to the brain at a total body flow rate of 100 mL/kg/min at deep hypothermia. Although ACP provided minimum oxygenation to the brain which met the oxygen requirement, oxygen metabolism was altered during ACP at moderate hypothermia. ACP strategy at moderate hypothermia needs further investigation.     

异丙酚对中低温体外循环脑氧合的影响

目的观察异丙酚对中低温体外循环心肺转流术(CPB)期间脑氧合的影响。方法心内直视手术患者17例随机分为芬太尼组和异丙酚组,通过监测动脉、颈内静脉血和混合静脉血氧含量以及乳酸浓度,计算全身和脑动静脉氧含量差、氧摄取率和动静脉乳酸浓度差,分析异丙酚对CPB期间脑氧合的影响。结果两组复温过程中动静脉氧含量差和氧摄取率均较低温时升高;异丙酚组在CPB过程中动脉-颈内静脉血氧含量差和脑的氧摄取率要高于芬太尼组(P<0.05),动脉-混合静脉血氧含量差和全身的氧摄取率两组差异无显著意义(P>0.05)。CPB全过程中两组血乳酸浓度均进行性升高。结论CPB期间应用异丙酚麻醉并不能明显改善脑氧合。CPB期间脑保护机理有其复杂性一面,不能仅停留于氧代谢平衡方面。     

The effect of pump flow on cerebral oxygen metabolism during cardiopulmonary bypass

We evaluated effects of pump flow on cerebral metabolism using transcranial Doppler (TCD) during cardiopulmonary bypass (CPB) in 22 adult patients undergoing coronary artery bypass grafting. All the patients were anesthetized with high dose fentanyl. The pump flow was controlled with non-pulsatile roller pump at 2.2–2.5 L/min/m² in group L and 2.7–3.0 L/min/m² in group H under α-stat acid-base regulation. Pharyngeal temperature was kept at 31°C in steady CPB state. Mean velocity of middle cerebral artery (MCAV) was monitored with TCD fixed on the temple continuously. Cerebral oxygen consumption was estimated by relating the difference in oxygen content between arterial and venous (jugular bulb) blood (AVDO₂) to flow velocity. In group L, blood oxygen saturation of jugular bulb (SjO₂) was stable during hypothermic period, but decreased significantly during rewarming period. In group H, SjO₂ was significantly increased with cooling, but went down to preoperative level during rewarming period. Significant difference of SjO₂ between two groups was noticed in rewarming period (52.9 ± 10.0% in group L and 65.6 ± 11.8% in group H, p=0.0133). MCAV tended to decrease with cooling and increase with rewarming, but which was not significant change respectively. Relative cerebral metabolic rate for oxygen (rCMRO₂) was defined as the percent change of the product AVDO₂ and MCAV. In each group, rCMRO₂ was decreased with cooling and increased with rewarming significantly. Especially, rCMRO₂ right after CPB discontinued was increased 1.7 times in L group and 2.0 times in group H as much as that of steady state of CPB. It is suggested that cerebral metabolism should be decreased during cooling to 31°C of pharyngeal temperature, 2.2–2.5 l/min/m² of pump flow was adequate to keep SjO₂ stable. On the other hand, it is necessary to increase pump flow to 2.7–3.0 l/min/m² during rewarming period as cerebral oxygen metabolic demand becomes greater.     

Hyperglycemia during Hypothermic Canine Cardiopulmonary Bypass Increases Cerebral Lactate

Background: Hyperglycemia frequently occurs during cardiopulmonary bypass (CPB), although its direct effects on cerebral perfusion and metabolism are not known. Using a canine model of hypothermic CPB, we tested whether hyperglycemia alters cerebral blood flow and metabolism and cerebral energy charge.

Methods: Twenty anesthetized dogs were randomized into hyperglycemic (n - 10) and normoglycemic (n - 10) groups. The hyperglycemic group received an infusion of D50 W, and the normoglycemic animals received an equal volume of 0.9% NaCl. Both groups underwent 120 min of hypothermic (28 degrees Celsius) CPB using membrane oxygenators, followed by rewarming and termination of CPB. Cerebral blood flow (radioactive microspheres) and the cerebral metabolic rate for oxygen were measured intermittently during the experiment and brain tissue metabolites were obtained after bypass.

Results: Before CPB, the glucose-treated animals had higher serum glucose levels (534 plus/minus 12 mg/dL; mean plus/minus SE) than controls (103 plus/minus 4 mg/dL; P < 0.05), and this difference was maintained throughout the study. Cerebral blood flow and metabolism did not differ between groups at any time during the experiment. Sagittal sinus pressure was comparable between groups throughout CPB. Tissue high-energy phosphates and water contents were similar after CPB, although cerebral lactate levels were greater in hyperglycemic (37.2 plus/minus 5.7 micro mol/g) than normoglycemic animals (19.7 plus/minus 3.7 micro mol/g; P < 0.05). After CPB, pH values of cerebrospinal fluid for normoglycemic (7.33 plus/minus 0.01) and hyperglycemic (7.34 plus/minus 0.01) groups were similar.     

Pharmacologic EEG suppression during cardiopulmonary bypass: cerebral hemodynamic and metabolic effects of thiopental or isoflurane during hypothermia and normothermia

We have determined the effects of thiopental or isoflurane upon cerebral blood flow (CBF) and the cerebral metabolic rate for oxygen (CMRO2) when these agents are used in sufficient dose to attain a deep burst suppression pattern on the electroencephalogram (EEG) during hypothermic and normothermic cardiopulmonary bypass (CPB). Thirty-one patients undergoing coronary artery bypass graft surgery were anesthetized with fentanyl 0.1 mg X kg-1, and were randomly allocated to one of three groups: control (no further anesthetics during bypass and continuous EEG activity), thiopental treatment (EEG suppression), or isoflurane treatment (EEG suppression). Hypothermia (25-29 degrees C) was routinely induced at onset of nonpulsatile cardiopulmonary bypass. In the treatment groups, thiopental or isoflurane were used during bypass to achieve a deep burst suppression pattern. Cerebral blood flow and cerebral metabolic rate for oxygen were determined during hypothermia and upon rewarming to normothermia (37 degrees C). Pharmacologic EEG suppression with either isoflurane or thiopental was associated with lower cerebral metabolic rate than control values during both hypothermic and normothermic bypass. However, only thiopental-induced EEG suppression was associated with lower cerebral blood flow than control. Cerebral blood flow during isoflurane-induced EEG suppression was similar to control values in spite of the reduced cerebral metabolic rate.