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.     

Effects of hemodilution and phenylephrine on cerebral blood flow and metabolism during cardiopulmonary bypass

OBJECTIVE: Hypotension resulting from hemodilution on cardiopulmonary bypass is often treated by pressor (eg, phenylephrine) infusion. The effect of phenylephrine on cerebral blood flow (CBF) in this setting is not clear. It was hypothesized that phenylephrine might decrease CBF. MEASUREMENTS and MAIN RESULTS: Six different radioactively labeled microspheres (15 microm) were used to measure CBF at 6 time points (T) in 9 pigs (mean body weight 11.3 +/- 1.2 kg): T1 baseline before bypass (mean arterial pressure [MAP] 76 +/- 5 mmHg), T2 on mildly hypothermic CPB (34 degrees C, pump flow 100 mL/kg/min, hematocrit 30%, MAP 79 +/- 7 mmHg), T3 after moderate hemodilution with crystalloid (hematocrit 20%, resulting MAP 62 +/- 6 mmHg), T4 after phenylephrine administration to increase MAP to baseline values (hematocrit 20%), T5 after severe hemodilution (hematocrit 10%, resulting MAP 41 +/- 4 mmHg), and T6 after phenylephrine administration to normalize MAP (hematocrit 10%). In addition, blood flow to liver, small bowel and skeletal muscle, and pH of jugular venous blood were measured at each time point. After institution of CPB, the CBF (mL/min/100 g tissue) increased significantly to 53 +/- 9 (baseline levels 44 +/- 8, T1 v T2, p = 0.03). Hemodilution resulted in significant increases in CBF on CPB to 65 +/- 9 and 90 +/- 9 at hematocrit 20% and hematocrit 10%, respectively (T2 v T3, p = 0.03; T3 v T5, p = 0.01) and a progressive fall in jugular venous pH. At each level of hemodilution, phenylephrine resulted in an additional increase in CBF (T4, 74 +/- 8; T6, 108 +/- 12; T3 v T4, p = 0.04; T5 v T6, p = 0.01) but did not improve jugular venous pH. Changes in liver blood flow after hemodilution and vasopressor injection showed a similar pattern to CBF. However, the blood flow to small bowel and skeletal muscle increased with hemodilution but decreased significantly with phenylephrine administration. CONCLUSIONS: Phenylephrine redirects blood flow from the bowel and muscle to the brain and liver. Hemodilution increases CBF and pressor administration further increases CBF by elevating perfusion pressure. Maintenance of a higher hematocrit on CPB increases MAP and should decrease the need for vasopressor administration.     

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.     

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.     

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.     

Cerebral hemodynamics in neonates and infants undergoing cardiopulmonary bypass and profound hypothermic circulatory arrest: assessment by transcranial Doppler sonography

Profound hypothermic circulatory arrest (PHCA) is followed by a transient period of increased intracranial pressure and a longer period of neurophysiologic dysfunction. To investigate the effect of cardiopulmonary bypass (CPB) with PHCA on cerebral hemodynamics, we used transcranial Doppler sonography to measure cerebral blood flow velocity in 10 neonates and infants before and after PHCA. Cerebral blood flow velocity was compared before and after PHCA during normothermic cardiopulmonary bypass at the same mean arterial pressure, central venous pressure, hematocrit, and arterial carbon dioxide tension. Cerebral blood flow velocity decreased exponentially with decreasing nasopharyngeal temperature before PHCA (P less than 0.05) and remained decreased after PHCA during normothermic CPB, compared with values for normothermic CPB before PHCA (P less than 0.005). During normothermic CPB after PHCA, the modified cerebral vascular resistance (mm Hg.cm.s-1) was increased above values for normothermic CPB before PHCA (P less than 0.05). The results of this study suggest that the observed increase in intracranial pressure during PHCA is not caused by increased cerebral perfusion, but rather that cerebral perfusion is reduced in response to a decreased demand for cerebral metabolic oxygen.     

The effect of hematocrit on cerebral blood flow velocity in neonates and infants undergoing deep hypothermic cardiopulmonary bypass.

Varying degrees of hemodilution are used during deep hypothermic cardiopulmonary bypass. However, the optimal hematocrit (Hct) level to ensure adequate oxygen delivery without impairing microcirculatory flow is not known. In this prospective, randomized study, cerebral blood flow velocity in the middle cerebral artery was measured using transcranial Doppler sonography in 35 neonates and infants undergoing surgery with deep hypothermic cardiopulmonary bypass. Patients were randomized to low Hct (aiming for 20%) or high Hct (aiming for 30%) during cooling on cardiopulmonary bypass (CPB). Systolic (V(s)), mean (Vm), and diastolic (Vd) cerebral blood flow velocity, as well as pulsatility index (PI = [V(s) - Vd]/Vm) and resistance index (RI = [V(s) - Vd]/V(s)) were recorded at six time points: postinduction, at cannulation, after 10 min cooling on CPB, rewarmed to 35 degrees C on CPB, immediately off CPB, and at skin closure. Vm was significantly lower in the high Hct group compared with that in the low Hct group during cooling (P < 0.01). Postinduction, the high Hct group demonstrated significantly lower Vd immediately off CPB (P < 0.01) and significantly lower Vm and V(s) at skin closure (P < 0.001). We conclude that there is an inverse relation between hematocrit and cerebral blood flow velocity during deep hypothermic cardiopulmonary bypass in neonates and infants. Implications: There is an inverse relation between hematocrit and cerebral blood flow velocity during deep hypothermic cardiopulmonary bypass in neonates and infants. Further studies correlating Hct and cerebral blood flow velocity with cerebral metabolic rate and neurologic outcome are necessary to determine the optimal Hct during deep hypothermic cardiopulmonary bypass.     

Avoidance of hemodilution during selective cerebral perfusion enhances neurobehavioral outcome in a survival porcine model.

INTRODUCTION: The ideal hematocrit (HCT) level during hypothermic selective cerebral perfusion (SCP)--to ensure adequate oxygen delivery without excessive perfusion--has not yet been determined. METHODS: Twenty pigs (26.0+/-2.6 kg) were randomized to low or high HCT management. The cardiopulmonary bypass (CPB) circuit was primed with crystalloid in the low HCT group (21+/-1%), and with donor blood in the high HCT group (30+/-1%). Pigs were cooled to 20 degrees C and SCP was carried out for 90 min. During rewarming, whole blood was added in the low HCT group and crystalloid in the high HCT group to produce equivalent HCT levels by the end of the procedure. Using fluorescent microspheres and sagittal sinus sampling, cerebral blood flow (CBF) and oxygen metabolism (CMRO2) were assessed at baseline, after cooling, at two points during SCP (30 and 90 min), and at 15 min and 2 h post-CPB. In addition, a range of physiological and metabolic parameters, including intracranial pressure (ICP), were recorded throughout the procedure. The animals' behavior was videotaped and assessed blindly for 7 days postoperatively (maximum score=5). RESULTS: HCT levels were equivalent at baseline, 2 h post-CPB, and at sacrifice, but significantly different (p<0.0001) during cooling and SCP. Mean arterial pressure, pH and pCO2, and CMRO2 were equivalent between groups throughout. ICP was similar in the two groups throughout cooling, SCP, and rewarming, but was significantly higher in the low HCT animals after the termination of CPB. CBF was similar at baseline, but thereafter markedly higher in the low HCT group. Neurobehavioral performance was significantly better in the high HCT animals (median score 3.5 vs 4.5 on day 3, and 4.5 vs 4.75 on day 7, p=0.003). CONCLUSIONS: Higher HCT levels for SCP produced a significantly superior functional outcome, suggesting that the higher CBF with a lower HCT may be injurious, possibly because of an increased embolic load.     

Response of cerebral blood flow to phenylephrine infusion during hypothermic cardiopulmonary bypass: influence of PaCO2 management

Twenty-eight adult patients anesthetized with fentanyl, then subjected to hypothermic cardiopulmonary bypass (CPB), were studied to determine the effect of phenylephrine-induced changes in mean arterial pressure (MAP) on cerebral blood flow (CBF). During CPB patients managed at 28 degrees C with either alpha-stat (temperature-uncorrected PaCO2 = 41 +/- 4 mmHg) or pH-stat (temperature-uncorrected PaCO2 = 54 +/- 8 mmHg) PaCO2 for blood gas maintenance received phenylephrine to increase MAP greater than or equal to 25% (group A, n = 10; group B, n = 6). To correct for a spontaneous, time-related decline in CBF observed during CPB, two additional groups of patients undergoing CPB were either managed with the alpha-stat or pH-stat approach, but neither group received phenylephrine and MAP remained unchanged in both groups (group C, n = 6; group D, n = 6). For all patients controlled variables (nasopharyngeal temperature, PaCO2, pump flow, and hematocrit) remained unchanged between measurements. Phenylephrine data were corrected based on the data from groups C and D for the effect of diminishing CBF over time during CPB. In patients in group A CBF was unchanged as MAP rose from 56 +/- 7 to 84 +/- 8 mmHg. In patients in group B CBF increased 41% as MAP rose from 53 +/- 8 to 77 +/- 9 mmHg (P less than 0.001). During hypothermic CPB normocarbia maintained via the alpha-stat approach at a temperature-uncorrected PaCO2 of approximately equal to 40 mmHg preserves cerebral autoregulation; pH-stat management (PaCO2 approximately equal to 57 mmHg uncorrected for temperature, or 40 mmHg when corrected to 28 degrees C) causes cerebrovascular changes (i.e., impaired autoregulation) similar to those changes produced by hypercarbia in awake, normothermic patients.     

Pediatric physiologic pulsatile pump enhances cerebral and renal blood flow during and after cardiopulmonary bypass

Controversy over benefits of pulsatile flow after pediatric cardiopulmonary bypass (CPB) continues. Our study objectives were to first, quantify pressure and flow waveforms in terms of hemodynamic energy, using the energy equivalent (EEP) formula, for direct comparisons, and second, investigate effects of pulsatile versus nonpulsatile flow on cerebral and renal blood flow, and cerebral vascular resistance during and after CPB with deep hypothermic circulatory arrest (DHCA) in a neonatal piglet model. Fourteen piglets underwent perfusion with either an hydraulically driven dual-chamber physiologic pulsatile pump (P, n = 7) or a conventional nonpulsatile roller pump (NP, n = 7). The radiolabeled microsphere technique was used to determine the cerebral and renal blood flow. P produced higher hemodynamic energy (from mean arterial pressure to EEP) compared to NP during normothermic CPB (13 +/- 3% versus 1 +/- 1%, p < 0.0001), hypothermic CPB (15 +/- 4% versus 1 +/- 1%, p < 0.0001) and after rewarming (16 +/- 5% versus 1 +/- 1%, p < 0.0001). Global cerebral blood flow was higher for P compared to NP during CPB (104 +/- 12 ml/100g/min versus 70 +/- 8 ml/100g/min, p < 0.05). In the right and left hemispheres, cerebellum, basal ganglia, and brainstem, blood flow resembled the global cerebral blood flow. Cerebral vascular resistance was lower (p < 0.007) and renal blood flow was improved fourfold (p < 0.05) for P versus NP, after CPB. Pulsatile flow generates higher hemodynamic energy, enhancing cerebral and renal blood flow during and after CPB with DHCA in this model.     

Cerebral blood flow and metabolism during cardiopulmonary bypass with special reference to effects of hypotension induced by prostacyclin

Cerebral blood flow and metabolism of oxygen, glucose, and lactate were studied in 43 patients undergoing aortocoronary bypass. Twenty-five patients received prostacyclin infusion, 50 ng per kilogram of body weight per minute, during cardiopulmonary bypass (CPB), and 18 patients served as a control group. Regional cerebral blood flow (CBF) was studied by intraarterially injected xenon 133 and a single scintillation detector. Oxygen tension, carbon dioxide tension, oxygen saturation, glucose, and lactate were measured in arterial and cerebral venous blood. Mean arterial blood pressure decreased during hypothermia and prostacyclin infusion to less than 30 mm Hg. The regional CBF was, on average, 22 (standard deviation [SD] 4) ml/100 gm/min before CPB. It increased in the control group during hypothermia to 34 (SD 12) ml/100 gm/min, but decreased in the prostacyclin group to 15 (SD 5) ml/100 gm/min. It increased during rewarming in the prostacyclin group. After CPB, regional CBF was about 40 ml/100 gm/min in both groups. The cerebral arteriovenous oxygen pressure difference decreased more in the control group than in the prostacyclin group during hypothermia. The cerebral metabolic rate of oxygen decreased in both groups from approximately 2 ml/100 gm/min to about 1 ml/100 gm/min during hypothermia, increased again during rewarming, and after CPB was at the levels measured before bypass in both groups. There was no difference between the groups in regard to glucose and lactate metabolism.     

Prevention of cerebral hyperthermia during cardiac surgery by limiting on-bypass rewarming in combination with post-bypass body surface warming: a feasibility study

Cerebral hyperthermia is common during the rewarming phase of cardiopulmonary bypass (CPB) and is implicated in CPB-associated neurocognitive dysfunction. Limiting rewarming may prevent cerebral hyperthermia but risks postoperative hypothermia. In a prospective, controlled study, we tested whether using a surface-warming device could allow limited rewarming from hypothermic CPB while avoiding prolonged postoperative hypothermia (core body temperature <36 degrees C). Thirteen patients undergoing primary elective coronary artery bypass grafting surgery were randomized to either a surface-rewarming group (using the Arctic Sun thermoregulatory system; n = 7) or a control standard rewarming group (n = 6). During rewarming from CPB, the control group was warmed to a nasopharyngeal temperature of 37 degrees C, whereas the surface-warming group was warmed to 35 degrees C, and then slowly rewarmed to 36.8 degrees C over the ensuing 4 h. Cerebral temperature was measured using a jugular bulb thermistor. Nasopharyngeal temperatures were lower in the surface-rewarming group at the end of CPB but not 4 h after surgery. Peak jugular bulb temperatures during the rewarming phase were significantly lower in the surface-rewarming group (36.4 degrees C +/- 1 degrees C) compared with controls (37.7 degrees C +/- 0.5 degrees C; P = 0.024). We conclude that limiting rewarming during CPB, when used in combination with surface warming, can prevent cerebral hyperthermia while minimizing the risk of postoperative hypothermia[corrected].     

Cerebral pressure-flow velocity relationship during hypothermic cardiopulmonary bypass in neonates and infants.

To examine the effect of temperature on the relationship between cerebral perfusion pressure (CPP) and cerebral blood flow velocity (CBFV) and the effect of low-flow cardiopulmonary bypass (CPB) on cerebral perfusion, we studied 25 neonates and infants ranging from 3 to 210 days of age at three nasopharyngeal temperature (NPT) ranges during cardiopulmonary bypass. Pressure-flow velocity relationships were studied during normothermic (NPT = 36-37 degrees C), moderate hypothermic (NPT = 23-25 degrees C), and profound hypothermic (NPT = 14-20 degrees C) CPB. A transcranial Doppler monitor was used to obtain CBFV, which was measured in the M1 segment of the middle cerebral artery. The CBFV was used as an index of cerebral perfusion. Anterior fontanel pressure (AFP) was subtracted from mean arterial pressure (MAP) to calculate CPP in mm Hg. Nasopharyngeal temperature, PaCO2, and hematocrit were controlled during the study period. Arterial blood gases were analyzed at 37 degrees C, uncorrected for body temperature (alpha-stat acid-base management). The CBFV measurements were made over a range of CPP from 6 to 90 mm Hg. Using nonlinear regression analysis, we showed that cerebral pressure-flow velocity autoregulation was present during normothermic CPB (r2 = 0.68). Autoregulation became pressure-passive, using linear regression analysis, during moderate hypothermic CPB (r2 = 0.33) and profound hypothermic CPB (r2 = 0.69). Cerebral blood-flow velocity was not detectable at a mean (+/- SD) CPP of 9 (+/- 2) mm Hg induced by the low-flow CBP state but became apparent when CPP was increased to 13 (+/- 1) mm Hg (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Factors and their influence on regional cerebral blood flow during nonpulsatile cardiopulmonary bypass

In this study we examined the relationship of regional cerebral blood flow (CBF) to mean arterial pressure, systemic blood flow, partial pressure of arterial carbon dioxide (PaCO2), nasopharyngeal temperature, and hemoglobin during hypothermic nonpulsatile cardiopulmonary bypass (CPB). Regional CBF was determined by clearance of xenon 133 in 67 patients undergoing coronary bypass grafting procedures. There was a significant decrease in regional CBF (55% decrease) during CPB, with nasopharyngeal temperature and PaCO2 being the only two significant factors (p less than 0.05). In a subgroup of 10 patients, variation of pump flow between 1.0 and 2.0 L/min/m2 did not significantly affect regional CBF. We conclude that cerebral autoregulation is retained during hypothermic CPB. Under the usual conditions of CPB, variations in flow and pressure are not associated with important physiological or detrimental clinical affects.     

Correlation between cerebral and mixed venous oxygen saturation during moderate versus tepid hypothermic hemodiluted cardiopulmonary bypass

OBJECTIVE: This study was undertaken to compare cerebral oxygen saturation (RsO(2)) and mixed venous oxygen saturation (SvO(2)) in patients undergoing moderate and tepid hypothermic hemodiluted cardiopulmonary bypass (CPB). DESIGN: Prospective study. SETTINGS: University hospital operating room. PARTICIPANTS: Fourteen patients undergoing elective coronary artery bypass graft surgery using hypothermic hemodiluted CPB. INTERVENTIONS: During moderate (28 degrees -30 degrees C) and tepid hypothermic (33 degrees -34 degrees C) hemodiluted CPB, RsO(2) and SvO(2) were continuously monitored with a cerebral oximeter via a surface electrode placed on the patient's forehead and with the mixed venous oximeter integrated in the CPB machine, respectively. MEASUREMENTS AND MAIN RESULTS: Mean +/- standard deviation of RsO(2), SvO(2), PaCO(2), and hematocrit were determined prebypass and during moderate and tepid hypothermic phases of CPB while maintaining pump flow at 2.4 L/min/m(2) and mean arterial pressure in the 60- to 70-mmHg range. Compared with a prebypass value of 76.0% +/- 9.6%, RsO(2) was significantly decreased during moderate hypothermia to 58.9% +/- 6.4% and increased to 66.4% +/- 6.7% after slow rewarming to tepid hypothermia. In contrast, compared with a prebypass value of 78.6% +/- 3.3%, SvO(2) significantly increased to 84.9% +/- 3.6% during moderate hypothermia and decreased to 74.1% +/- 5.6% during tepid hypothermia. During moderate hypothermia, there was poor agreement between RsO(2) and SvO(2) with a gradient of 26%; however, during tepid hypothermia, there was a strong agreement between RsO(2) and SvO(2) with a gradient of 6%. The temperature-uncorrected PaCO(2) was maintained at the normocapnic level throughout the study, whereas the temperature-corrected PaCO(2) was significantly lower during the moderate hypothermic phase (26.8 +/- 3.1 mmHg) compared with the tepid hypothermic phase (38.9 +/- 3.7 mmHg) of CPB. There was a significant and positive correlation between RsO(2) and temperature-corrected PaCO(2) during hypothermia. CONCLUSIONS: During moderate hypothermic hemodiluted CPB, there was a significant increase of SvO(2) associated with a paradoxic decrease of RsO(2) that was attributed to the low temperature-corrected PaCO(2) values. During tepid CPB after slow rewarming, regional cerebral oxygen saturation was increased in association with an increase with the temperature-corrected PaCO(2) values. The results show that during hypothermic hemodiluted CPB using the alpha-stat strategy for carbon dioxide homeostasis, cerebral oxygen saturation is significantly higher during tepid than moderate hypothermia.     

深低温低流量灌注降温期pH稳态对乳猪脑保护的研究

目的：在深低温低流量灌注模式下，观察快速降温期应用不同血所气管理对乳猪脑功能的影响。方法：12只乳猪随机均分成2组。alpha组：降 温期应用alpha稳态；pH组：降温期应用pH稳态。脑功能的监测包括：脑血流量、脑部温度、生化指标，并计算脑氧代谢参数和复温末脑水含量。结果：pH组降温末脑血流量和脑皮质ATP含量均高于alpha组（P＜0．05），而脑氧代谢率、乳酸含量和降温期脑部温差均低于alpha组（P＜0．05），两组脑水含量的差异无显著意义。结论：深低温低流量灌注快速降温期应用pH稳态管理血气，可提供更好的脑保护。     

Phenylephrine Increases Cerebral Blood Flow during Low-flow Hypothermic Cardiopulmonary Bypass in Baboons

Background: Although low-flow cardiopulmonary bypass (CPB) has become a preferred technique for the surgical repair of complex cardiac lesions in children, the relative hypotension and decrease in cerebral blood flow (CBF) associated with low flow may contribute to the occurrence of postoperative neurologic injury. Therefore, it was determined whether phenylephrine administered to increase arterial blood pressure during low-flow CPB increases CBF.

Methods: Cardiopulmonary bypass was initiated in seven baboons during fentanyl, midazolam, and isoflurane anesthesia. Animals were cooled at a pump flow rate of 2.5 l *symbol* min-1 *symbol* m-2 until esophageal temperature decreased to 20 degrees C. Cardiopulmonary bypass flow was then reduced to 0.5 l *symbol* min-1 *symbol* m-2 (low flow). During low-flow CPB, arterial partial pressure of carbon dioxide (PCO2) and blood pressure were varied in random sequence to three conditions: (1) PCO2 30-39 mmHg (uncorrected for temperature), control blood pressure; (2) PCO2 50-60 mmHg, control blood pressure; and (3) PCO2 30-39 mmHg, blood pressure raised to twice control by phenylephrine infusion. Thereafter, CPB flow was increased to 2.5 l *symbol* min-1 *symbol* m-2, and baboons were rewarmed to normal temperature. Cerebral blood flow was measured by washout of intraarterial133 Xenon before and during CPB.

Results: Phenylephrine administered to increase mean blood pressure from 23+/-3 to 46+/-3 mmHg during low-flow CPB increased CBF from 14+/-3 to 31+/-9 ml *symbol* min-1 *symbol* 100 g-1, P < 0.05. Changes in arterial PCO2 alone during low flow bypass produced no changes in CBF.     

Cerebral ischemia caused by obstructed superior vena cava cannula is detected by near-infrared spectroscopy

OBJECTIVE: Bicaval venous cannulation is being used with increasing frequency in neonates and infants to avoid circulatory arrest. However, superior vena cava (SVC) cannula obstruction may result in cerebral ischemia with no change in blood pressure or mixed venous O2 saturation. The authors hypothesized that near-infrared spectroscopy (NIRS) would allow noninvasive detection of SVC cannula obstruction. METHODS: Fifteen Yorkshire piglets (9.07 +/- 0.20 kg) underwent total cardiopulmonary bypass (CPB) (100 mL/kg/min, pH-stat strategy, hematocrit of 20%) with ascending aortic and bicaval cannulations. Femoral arterial and SVC pressure were monitored as well as mixed venous O2 saturation. NIRS monitoring of tissue oxygenation index (TOI) as well as oxyhemoglobin and deoxyhemoglobin (HHb) was undertaken. Animals were cooled to an esophageal temperature of 25 degrees C over 20 minutes. CPB flow was reduced to 50 mL/kg/min for 20 minutes. Animals then underwent a 60-minute study period of continuous CPB at 50 mL/kg/min with manipulation of the SVC cannula: group 1, open; group 2, partial occlusion; and group 3, complete occlusion. Animals were rewarmed to 37 degrees C at full flow with the SVC cannula open. Cerebral blood flow was assessed at onset of CPB, at end of cooling, at end of low flow, at end of SVC manipulation period, and at end of rewarming using radioactive microspheres. RESULTS: CBF decreased to 27.9 +/- 1.5 mL/min/100 g with complete occlusion (p < 0.01 v group 1: 39.7 +/- 1.9, group 2, 38.3 +/- 2.0 mL/min/100 g) with no change in arterial pressure or mixed venous saturation. There were also significant differences in cerebral oxygen delivery between group 3 and other groups (p < 0.01). SVC pressure increased to 19.5 +/- 4.5 and 32.5 +/- 3.1mmHg with partial and complete occlusion. NIRS indicated significant cerebral ischemia with a decrease in TOI (p < 0.05; group 3 v group 1 and 2) and an increase in HHb (p < 0.05; group 3 v group 1). At the end of the study, significant acidosis was found in group 3 compared with group 1 (p < 0.05). CONCLUSION: SVC cannula obstruction causes cerebral ischemia with no change in blood pressure or venous oxygen saturation. In view of the difficulties and risks of CVP monitoring in babies, it is recommended to use other monitoring modalities such as NIRS to assess adequacy of cerebral perfusion if bicaval cannulation is used in neonates and infants.     

S100B blood levels correlate with rewarming time and cerebral Doppler in pediatric open heart surgery

BACKGROUND: Brain hyperthermia, accompanying the rewarming phase of cardiopulmonary bypass (CPB), has been involved in the genesis of postoperative brain damage. Blood S100B levels are emerging as a marker of brain distress, and could offer a reliable monitoring tool at different times during and after open heart surgery. METHODS: Thirty-two patients undergoing repair of congenital heart disease with CPB and deep hypothermic circulatory arrest (DHCA) were monitored by S100B blood levels and middle cerebral artery Doppler velocimetry pulsatility index (MCA PI) before, during, and after surgical procedure at five predetermined time-points. RESULTS: Both S100B and MCA PI significantly increased, MCA PI values exhibiting a peak at the end of surgery time-point (p > 0.05), while S100B blood levels were increased at the end of CPB (p < 0.05). Multivariate analysis, with S100B levels measured at the end of CPB as dependent variable, showed a positive significant correlation with MCA PI (p = 0.04), with the CPB and the rewarming duration (p = 0.03 and p = 0.009, respectively). CONCLUSIONS: The present results show a significant correlation between a biochemical marker of brain damage and an index of increased cerebrovascular resistance, with higher levels during the rewarming CPB phase in pediatric open heart surgery.