Transcranial Doppler-estimated versus thermodilution-estimated cerebral blood flow during cardiac operations. Influence of temperature and arterial carbon dioxide tension

The ability of the noninvasive continuous transcranial Doppler technique to reflect changes in cerebral blood flow during cardiac operations was evaluated in seven adults. Middle cerebral artery blood flow velocity changes were compared with simultaneous thermodilution measurements of venous blood flow in the ipsilateral internal jugular vein during 11 preset stages of the procedure. Cerebral blood flow was varied by changes in arterial carbon dioxide tension and temperature. High-dose fentanyl-droperidol anesthesia and alpha-stat pH management were employed. To facilitate comparisons between the two methods, the individual awake values of middle cerebral artery flow velocity (45.1 +/- 3.3 cm/sec, mean +/- standard error of the mean) and jugular venous blood flow (382 +/- 37 ml/min) were normalized (100%). Cerebral metabolic rate for oxygen was calculated as the product of jugular arteriovenous oxygen content difference and middle cerebral artery flow velocity or jugular venous blood flow, respectively. The individual correlations between the two flow estimates varied between 0.76 and 0.87 (median 0.83), and the correlation of the combined data from all seven patients was 0.77 (p less than 0.0001). Variations in arterial carbon dioxide tension induced significant changes in the two flow estimates both during normothermia before cardiopulmonary bypass and at deep hypothermia (20 degrees C) during cardiopulmonary bypass. The significant arterial carbon dioxide tension changes had no significant effects either on Doppler- or thermodilution-estimated cerebral metabolic rate for oxygen. Deep hypothermia (20 degrees C) reduced Doppler- and thermodilution-estimated cerebral metabolic rate for oxygen to 22.0% +/- 3.9% and 20.6% +/- 6.9% of the awake levels, respectively. The study supports the validity of using middle cerebral arterial flow velocity changes as an estimate of changes in volume flow through the brain during cardiac operations.     

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.     

Effectiveness of the Cobra aortic catheter for dual-temperature management during adult cardiac surgery

OBJECTIVES: In animals the Cardeon Cobra catheter (Cardeon Corp, Cupertino, Calif) allows independent control of aortic arch and descending aortic temperatures and profoundly reduces cerebral embolization during bypass. This investigation describes the first clinical use of the device during adult cardiac surgery. The purpose of the study was to confirm that the Cobra catheter delivers adequate cerebral and systemic perfusion while providing simultaneous cerebral hypothermia and systemic normothermia during cardiopulmonary bypass. METHODS: In a prospective multicenter study the Cobra aortic catheter was placed in 20 adults undergoing cardiopulmonary bypass. Arch and corporeal temperatures, bypass flows, and arterial blood pressures were recorded intraoperatively. Jugular bulb and mixed venous oxygen saturation was used to assess the adequacy of cerebral and systemic perfusion. RESULTS: Surgeons at 3 institutions placed the Cobra catheter in patients undergoing coronary artery bypass grafting (n = 13), valve (n = 3), and combined valve-bypass (n = 4) operations. Mean total bypass flows of 2.1 +/- 0.2 L x min(-1) x m(-2) maintained mean arterial pressures in arch and descending aortic circulations of greater than 55 mm Hg. A mean differential of 4.3 degrees C between arch and descending aortic temperatures was established before crossclamp application, and a mean maximum temperature differential of 7 degrees C was established during bypass. A 2.4 degrees C temperature differential was maintained at crossclamp removal. Cerebral and systemic venous oxygen saturation remained greater than 65% during bypass. CONCLUSIONS: The Cobra device met all expectations for an arterial cannula with adequate perfusion to the arch and corporeal circulations. Dual perfusion with the Cobra catheter allows for independent temperature control during cardiopulmonary bypass with simultaneous cerebral hypothermia and systemic normothermia.     

Cerebral perfusion and metabolism during profound hypothermia in children. A study of middle cerebral artery ultrasonic variables and cerebral extraction of oxygen

Flow velocity of the right middle cerebral artery was studied in eight children during cardiac operations performed with profound hypothermia. Cerebral oxygen consumption was estimated by relating the difference in oxygen content between arterial and venous blood (jugular bulb) to flow velocity. In another six children, also during profound hypothermic procedures, the diameter of the middle cerebral artery was studied with an electronic echo-tracking instrument connected to a real-time ultrasound scanner. Flow velocity and estimated oxygen consumption decreased during cooling in proportion to the temperature decrease (r = 0.67, p less than 0.001, and r = 0.86, p less than 0.001, respectively), whereas the diameter was unaffected by temperature. At a nasopharyngeal temperature of 16.9 degrees +/- 1.9 degrees C flow velocity was reduced to 33.1% +/- 7.0% of the value obtained at 35 degrees C after induction of anesthesia. Correspondingly, the oxygen consumption decreased to 20.1% +/- 6.4%. The increase in oxygen consumption per 10 degrees C change in temperature was 3.6 (2.0 to 3.9) during surface cooling, 2.6 (1.9 to 2.7) during cardiopulmonary bypass cooling, and 2.7 (1.5 to 4.6) during rewarming. Flow velocity was not influenced by perfusion pressure during profound hypothermia within the range of 20 to 42 mm Hg (r = 0.14, p = 0.52) but was related to pump flow (r = 0.73, p less than 0.001). A pump flow down to 0.5 L/min/m2 was found to be adequate during stable profound hypothermia, as judged from the maintained high jugular bulb venous oxygen saturation (70% to 80%). It is concluded that flow velocity is reduced at hypothermia in proportion to the reduced metabolic rate, although modified by other factors that influence cerebral blood flow.     

Cerebral blood flow and oxygen metabolism during cardiopulmonary bypass with moderate hypothermic selective cerebral perfusion

Cerebral blood flow was measured using transcranial doppler during cardiopulmonary bypass in nine patients with selective cerebral perfusion for surgery of arch aorta (group S). For comparison, 11 adult open heart patients (group C) were also measured. The authors' selective cerebral perfusion at 28 degrees C resulted in moderate hypothermia and antegrade perfusion using independent pumps for three branches. Total flow in the three branches was 500 ml/min. A Labodop DP-100 doppler ultrasound velocimeter was used to measure middle cerebral arterial blood flow velocity. Hemoglobin concentration and oxygen saturation were also measured in arterial and jugular venous blood. The arteriovenous oxygen content difference (Ca-vO2) was calculated and multiplied by the middle cerebral arterial blood flow velocity value, which resulted in the cerebral metabolic rate for oxygen (CMRO2). The cerebral perfusion pressure of group S was lower than in group C, and the arterial carbon-dioxide tension (PaCO2) of group S was higher than in group C during cardiopulmonary bypass. Middle cerebral arterial blood flow velocity values of both groups remained constant before, during and after cardiopulmonary bypass. The CMRO2 decreased during cardiopulmonary bypass and showed no difference between the two groups. The changes in PaCO2 might be significant factors in the increase in cerebral blood flow during selective cerebral perfusion. This study supports the conclusion that, compared with our routine open heart surgery procedures, our selective cerebral perfusion procedures had the same cerebral blood flow and oxygen metabolism during cardiopulmonary bypass.     

Brain blood flow and metabolism do not decrease at stable brain temperature during cardiopulmonary bypass in rabbits.

Cerebral blood flow (CBF) during human hypothermic cardiopulmonary bypass has been reported to decrease with time, suggesting that progressive cerebral vasoconstriction or embolic obstruction may occur. We tested the hypotheses: 1) that observed CBF reductions were due to continued undetected brain cooling and 2) that CBF during cardiopulmonary bypass would be stable after achievement of constant brain temperature. Anesthetized New Zealand White rabbits underwent cardiopulmonary bypass (membrane oxygenator, centrifugal pump, bifemoral arterial perfusion) and were assigned to one of three bypass management groups based on perfusate temperature and PaCO2 management: group 1 (37 degrees C, n = 8); group 2 (27 degrees C, pH-stat, n = 9); and group 3 (27 degrees C, alpha-stat, n = 8). Systemic hemodynamics, and cerebral cortical, esophageal, and arterial perfusate temperatures were recorded every 10 min for the first hour of bypass and again at 90 min. CBF and masseter blood flow (radiolabeled microspheres) were determined at 30, 60, and 90 min of bypass, while the cerebral metabolic rate for oxygen (CMRO2) was determined at 60 and 90 min. Groups were comparable with respect to mean arterial pressure, central venous pressure, hematocrit, and arterial oxygen content throughout bypass. Cortical temperature was stable in normothermic (group 1) animals, and there was no significant change in CBF between 30 and 90 min of bypass: 68 +/- 18 versus 73 +/- 20 ml.100 g-1.min-1 (mean +/- SD). In the hypothermic groups (2 and 3), cortical temperature equilibration (95% of the total change) required 41 +/- 6 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cerebral blood flow response to changes in arterial carbon dioxide tension during hypothermic cardiopulmonary bypass in children

We examined the relationship of changes in partial pressure of carbon dioxide on cerebral blood flow responsiveness in 20 pediatric patients undergoing hypothermic cardiopulmonary bypass. Cerebral blood flow was measured during steady-state hypothermic cardiopulmonary bypass with the use of xenon 133 clearance methodology at two different arterial carbon dioxide tensions. During these measurements there was no significant change in mean arterial pressure, nasopharyngeal temperature, pump flow rate, or hematocrit value. Cerebral blood flow was found to be significantly greater at higher arterial carbon dioxide tensions (p less than 0.01), so that for every millimeter of mercury rise in arterial carbon dioxide tension there was a 1.2 ml.100 gm-1.min-1 increase in cerebral blood flow. Two factors, deep hypothermia (18 degrees to 22 degrees C) and reduced age (less than 1 year), diminished the effect carbon dioxide had on cerebral blood flow responsiveness but did not eliminate it. We conclude that cerebral blood flow remains responsive to changes in arterial carbon dioxide tension during hypothermic cardiopulmonary bypass in infants and children; that is, increasing arterial carbon dioxide tension will independently increase cerebral blood flow.     

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.     

Cerebral effects of anaesthesia and hypothermia

Cerebral blood flow, cerebral oxygen and glucose consumption, and cerebral lactate and pyruvate release were measured; spectral analysis of the EEG was recorded in 10 male patients who had coronary artery bypass surgery. The measurements were taken to evaluate the effects of fentanyl-midazolam anaesthesia during normothermia and during hypothermic nonpulsatile cardiopulmonary bypass at 26 degrees C venous blood temperature, when a temperature-corrected PaCO2-value of 5.3 kPa was maintained. Anaesthesia with fentanyl 7 micrograms/kg and midazolam 200 micrograms/kg as induction doses, followed by infusions of fentanyl 0.15 micrograms/kg/minute and midazolam 3 micrograms/kg/minute, was characterised by a decrease in fast-wave activity and an increase in high-amplitude, slow-wave activity in the EEG. There was also a decrease in cerebral blood flow (38%), oxygen consumption (22%) and glucose consumption (25%), while lactate and pyruvate production remained unchanged. Hypothermia of 26 degrees C venous blood temperature suppressed EEG almost completely and decreased oxygen and glucose consumption by a further 61% and 54%, respectively, with no changes in lactate and pyruvate production while cerebral blood flow increased by 145%. These results show that the effects of fentanyl-midazolam anaesthesia on cerebral metabolism are enhanced during hypothermic cardiopulmonary bypass while the influence of anaesthesia on cerebral blood flow is overshadowed by the practice of a temperature-corrected acid-base management.     

Warming during cardiopulmonary bypass is associated with jugular bulb desaturation.

The objective of this study was to characterize cerebral venous effluent during normothermic nonpulsatile cardiopulmonary bypass. Thirty-one (23%) of 133 patients met desaturation criteria (defined as jugular bulb venous oxygen saturation less than or equal to 50% or jugular bulb venous oxygen tension less than or equal to 25 mm Hg) during normothermic cardiopulmonary bypass (after hypothermic cardiopulmonary bypass at 27 degrees to 28 degrees C). Cerebral blood flow, calculated using xenon 133 clearance methodology, was significantly (p less than 0.005) higher in the saturated group (33.7 +/- 10.3 mL.100 g-1.min-1) than in the desaturated group (26.2 +/- 6.9 mL.100 g-1.min-1), whereas the cerebral metabolic rate for oxygen was significantly lower (p less than 0.005) in the saturated group (1.28 +/- 0.39 mL.100 g-.min-1) than in the desaturated group (1.52 +/- 0.36 mL.100 g-1.min-1) at normothermia. The arteriovenous oxygen difference at normothermia was lower in the saturated group (3.92 +/- 1.12 mL/dL) than in the desaturated group (5.97 +/- 1.05 mL/dL). Neuropsychological testing was performed in 74 of the 133 patients preoperatively and on day 7 postoperatively. There was a general decline in mean scores of all tests postoperatively in both groups with no significant difference between the groups. We conclude that cerebral venous desaturation represents a global imbalance in cerebral oxygen supply-demand that occurs during normothermic cardiopulmonary bypass and may represent transient cerebral ischemia. These episodes, however, are not associated with impared neuropsychological test performance as compared with the performance of patients with no evidence of desaturation.     

Retrograde cerebral perfusion through a superior vena caval cannula protects the brain.

Retrograde cerebral perfusion through a superior vena caval cannula is a new technique for protecting the brain during aortic arch operations. In mongrel dogs (n = 10; 13 to 15 kg) we have performed retrograde cerebral perfusion (300 mL/min) by infusing blood through a superior vena caval cannula with aortic and inferior vena caval drainage. We have measured the cerebral tissue blood flow, oxygen consumption, and carbon dioxide exudation during retrograde cerebral perfusion at normothermia (NT, 37 degrees C) and hypothermia (HT, 20 degrees C) and have compared these values with values obtained in dogs during cardiopulmonary bypass (1,200 mL/min). Cerebral tissue blood flow was measured by the hydrogen clearance method. During retrograde cerebral perfusion about 20% of the superior vena caval perfusate was returned through the aorta and the rest drained from the inferior vena cava. Cerebral vascular resistance during retrograde cerebral perfusion was lower than that during cardiopulmonary bypass (NT, 63.8 +/- 52.5 versus 126.9 +/- 58.4; HT, 28.4 +/- 32.8 versus 69.5 +/- 28.7 x 10(3) dynes.s.cm(-5). Retrograde cerebral perfusion provided half the cerebral tissue blood flow of cardiopulmonary bypass (NT, 14.7 +/- 6.4 versus 34.3 +/- 7.8; HT, 17.6 +/- 5.6 versus 37.2 +/- 10.6 mL/min). Retrograde cerebral perfusion also provided a third of the oxygen (NT, 4.4 +/- 2.1 versus 12.3 +/- 7.1; HT, 1.4 +/- 0.8 versus 4.2 +/- 1.3 mL/min) and discharged 20% of the carbon dioxide (NT, 0.24 +/- 0.08 versus 1.19 +/- 0.58; HT, 0.15 +/- 0.06 versus 0.51 +/- 0.17 mmol/min) when compared with cardiopulmonary bypass. Retrograde cerebral perfusion may reduce ischemic damage during interruption of cerebral blood flow.     

Differential perfusion: a new technique for isolated brain cooling during cardiopulmonary bypass

BACKGROUND: The purpose of this study was to determine the feasibility of differential perfusion of the aortic arch and descending aorta during cardiopulmonary bypass using a cannula designed for aortic segmentation. METHODS: Pigs weighing 57 kg (n = 8), underwent cardiopulmonary bypass using the dual lumen aortic cannula. An inflatable balloon separated proximal (aortic arch) and distal (descending aorta) ports. During differential perfusion, the aorta was segmented and the arch and descending aorta perfused differentially using parallel heat exchangers. Ability to independently control brain and body temperature, cardiopulmonary bypass flow rate and mean arterial blood pressure was determined. RESULTS: During differential perfusion cerebral hypothermia (27 degrees C) with systemic normothermia (38 degrees C) was established in 23 minutes. Independent control of arch and descending aortic flow and mean arterial blood pressure was possible. Analysis of internal jugular venous O2 saturation data indicated an increase in the ratio of cerebral O2 supply to demand during differential perfusion. CONCLUSIONS: A cannulation system segmenting the aorta allows independent control of cerebral and systemic perfusion. This device could provide significant cerebral protection while maintaining the advantages of warm systemic cardiopulmonary bypass temperatures.     

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)     

Bypass flow, mean arterial pressure, and cerebral perfusion during cardiopulmonary bypass in dogs

OBJECTIVE: To determine if normal cardiopulmonary bypass (CPB) pump flows maintain cerebral perfusion in the context of reduced mean arterial pressure at 33 degrees C. DESIGN: A prospective investigation. SETTING: Animal CPB research laboratory. PARTICIPANTS: Seven dogs that underwent CPB. INTERVENTIONS: Seven dogs underwent CPB at 33 degrees C using alpha-stat management and a halothane, fentanyl-midazolam anesthetic. Cerebral blood flow was measured using the sagittal sinus outflow technique. After control measurements at 70 mm Hg, cerebral physiologic values were determined under four conditions in random order: (1) mean arterial pressure of 60 mm Hg achieved by a reduction in pump flow, (2) mean arterial pressure of 60 mmHg determined by partial opening of a femoral arterial-to-venous reservoir shunt, (3) mean arterial pressure of 45 mm Hg by reduced pump flow, and (4) mean arterial pressure of 45 mm Hg by shunt. A 9F femoral arterial-to-venous reservoir shunt was controlled by a screw clamp. MEASUREMENTS AND MAIN RESULTS: Except for the controlled variables of mean arterial pressure and bypass flow, physiologic determinants of cerebral blood flow (temperature, PaCO2 and hematocrit) did not differ under any of the CPB conditions. Pump flow per se was not a determinant of cerebral perfusion. Cerebral blood flow and cerebral oxygen delivery did not differ with changes in pump flow if mean arterial pressure did not differ. Cerebral blood flow depended on mean arterial pressure under all pump flow conditions, however. CONCLUSIONS: Over the range of flows typical in adult CPB at 33 degrees C, pump flow does not have an effect on cerebral perfusion independent of its effect on mean arterial pressure. A targeted pump flow per se is not sufficient to maintain cerebral perfusion if mean arterial blood pressure is reduced.     

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.     

The effect of hypothermic cardiopulmonary bypass and total circulatory arrest on cerebral metabolism in neonates, infants, and children

Cardiopulmonary bypass management in neonates, infants, and children often requires the use of deep hypothermia at 18 degrees C with occasional periods of circulatory arrest and represents marked physiologic extremes of temperature and perfusion. The safety of these techniques is largely dependent on the reduction of metabolism, particularly cerebral metabolism. We studied the effect of hypothermia on cerebral metabolism during cardiac surgery and quantified the changes. Cerebral metabolism was measured before, during, and after hypothermic cardiopulmonary bypass in 46 pediatric patients, aged 1 day to 14 years. Patients were grouped on the basis of the different bypass techniques commonly used in children: group A--moderate hypothermic bypass at 28 degrees C; group B--deep hypothermic bypass at 18 degrees to 20 degrees C with maintenance of continuous flow; and group C--deep hypothermic circulatory arrest at 18 degrees C. Cerebral metabolism significantly decreased under hypothermic conditions in all groups compared with control levels at normothermia, the data demonstrating an exponential relationship between temperature and cerebral metabolism and an average temperature coefficient of 3.65. There was no significant difference in the rate of metabolism reduction (temperature coefficient) in patients cooled to 28 degrees and 18 degrees C. From these data we were able to derive an equation that numerically expresses a hypothermic metabolic index, which quantitates duration of brain protection provided by reduction of cerebral metabolism owing to hypothermic bypass over any temperature range. Based on this index, patients cooled to 28 degrees C have a predicted ischemic tolerance of 11 to 19 minutes. The predicted duration that the brain can tolerate ischemia ("safe" period of deep hypothermic circulatory arrest) in patients cooled to 18 degrees C, based on our metabolic index, is 39 to 65 minutes, similar to the safe period of deep hypothermic circulatory arrest known to be tolerated clinically. In groups A and B (no circulatory arrest), cerebral metabolism returned to control in the rewarming phase of bypass and after bypass. In group C (circulatory arrest), cerebral metabolism and oxygen extraction remained significantly reduced during rewarming and after bypass, suggesting disordered cerebral metabolism and oxygen utilization after deep hypothermic circulatory arrest. The results of this study suggest that cerebral metabolism is exponentially related to temperature during hypothermic bypass with a temperature coefficient of 3.65 in neonates infants and children. Deep hypothermic circulatory arrest changes cerebral metabolism and blood flow after the arrest period despite adequate hypothermic suppression of metabolism.     

Importance of acid-base strategy in reducing myocardial and whole body oxygen consumption during perfusion hypothermia

During induced hypothermia with cardiopulmonary bypass, acid-base management usually follows one of two strategies: the so-called ectothermic or alpha-stat strategy, in which the pH of the arterial blood increases 0.015 pH units for every degree Celsius decrease in body temperature, or the pH-stat strategy, in which pH remains 7.4 at all temperatures. It has been assumed that oxygen consumption decreases approximately equally during hypothermia with either strategy, although there are biochemical reasons to hypothesize that oxygen consumption would be better maintained with the alpha-stat strategy. We also hypothesized that venous oxygen tension would be lower with the more alkaline alpha-stat strategy than with the pH-stat acid-base strategy, because of the Bohr effect. We tested these hypotheses by placing 10 anesthetized immature domestic pigs on cardiopulmonary bypass. We measured whole body oxygen consumption and myocardial oxygen consumption. Control measurements were made at 37 degrees C. Then the animals were cooled to 27 degrees C and the measurements were repeated. The alpha-stat strategy (pH 7.554 +/- 0.020 at 27 degrees C) was used in five animals and five animals received pH-stat management (pH 7.409 +/- 0.012 at 27 degrees C). Whole body and myocardial oxygen consumption rate decreased in both groups, but more so in the alpha-stat animals than in the pH-stat animals. The unexpectedly high oxygen consumption in the pH-stat animals also resulted in a lower than expected venous oxygen tension. Thus the effect of hypothermia in reducing oxygen consumption was less pronounced with pH-stat acid-base management.     

The effects of deep hypothermic cardiopulmonary bypass and total circulatory arrest on cerebral blood flow in infants and children

Cardiopulmonary bypass management in infants and children involves extensive alterations in temperature, hemodilution, and perfusion pressure, with occasional periods of circulatory arrest. Despite the use of these biologic extremes of temperature and perfusion, their effects on cerebral blood flow are unknown. This study was designed to examine the relationship of mean arterial pressure and nasopharyngeal temperature to cerebral blood flow during deep hypothermic cardiopulmonary bypass (18 degrees to 22 degrees C) with and without periods of total circulatory arrest. Cerebral blood flow was measured before, during, and after deep hypothermic cardiopulmonary bypass using xenon clearance techniques in 25 children, aged 2 days to 60 months. Fourteen patients underwent repair with circulatory arrest. There was a highly significant correlation of cerebral blood flow with temperature during cardiopulmonary bypass (p = 0.007). During deep hypothermic bypass there was a significant association between cerebral blood flow and mean arterial pressure (p = 0.027). In infants undergoing repair with deep hypothermia alone, cerebral blood flow returned to prebypass levels in the rewarming phase of bypass. However, in patients undergoing repair with circulatory arrest, no significant increase in cerebral blood flow during rewarming or even after bypass was observed (p = 0.01). These data show that deep hypothermic cardiopulmonary bypass significantly decreases cerebral blood flow because of temperature reduction. Under conditions of deep hypothermia, cerebral pressure-flow autoregulation is lost. This study also demonstrates that cerebral reperfusion after deep hypothermia is impaired if the patient is exposed to a period of total circulatory arrest.     

Novel cerebral physiologic monitoring to guide low-flow cerebral perfusion during neonatal aortic arch reconstruction

OBJECTIVE: This study was undertaken to describe the combined measurement of cerebral blood flow velocity and cerebral oxygen saturation as a guide to bypass flow rate for regional low-flow perfusion during neonatal aortic arch reconstruction. METHODS: Data were prospectively collected from 34 patients undergoing neonatal aortic arch reconstruction with regional low-flow perfusion. Cerebral oxygen saturation and blood flow velocity were measured by near-infrared spectroscopy and transcranial Doppler ultrasonography, respectively, throughout cardiopulmonary bypass. After cooling to 17 degrees C to 22 degrees C, baseline values of cerebral oxygen saturation and blood flow velocity were recorded during full-flow bypass. Regional low-flow perfusion was instituted for aortic arch reconstruction, and bypass flow rate was adjusted to maintain cerebral oxygen saturations and blood flow velocities within 10% of baseline recorded during cold full-flow bypass. Cerebral oxygen saturations and blood flow velocities were recorded again after repair during full-flow hypothermic bypass. Bypass flow during regional low-flow perfusion was recorded, as were arterial pressure and blood gas data. One-way repeated measures analysis of variance was used to determine differences in values during regional low-flow perfusion relative to baseline and after perfusion. RESULTS: A mean bypass flow of 63 mL/(kg x min) was required to maintain cerebral oxygen saturations and blood flow velocities within 10% of baseline. Mean arterial pressure had a poor correlation with the required bypass flow rate (r(2) = 0.006 by linear regression analysis). Fourteen of 34 patients had a cerebral oxygen saturation of 95% during regional low-flow perfusion, placing them at risk for cerebral hyperperfusion if the cerebral oxygen saturation had been used alone to guide bypass flow. Pressure was detected in the umbilical or femoral artery catheter (mean 12 mm Hg) in all patients during regional low-flow perfusion. CONCLUSIONS: Cerebral blood flow velocity, as determined by transcranial Doppler ultrasonography, adds valuable information to cerebral oxygen saturation data in guiding bypass flow during regional low-flow perfusion. Its most important use may be prevention of cerebral hyperperfusion during periods with high near-infrared spectroscopic saturation values.     

Use of the Cobra catheter for targeted temperature management during cardiopulmonary bypass in swine

BACKGROUND: The purpose of this investigation was to determine whether temperatures of the aortic arch and descending aortic circulations could be controlled independently during cardiopulmonary bypass with a cannula possessing an endoaortic baffle (Cobra; Cardeon, Cupertino, Calif). METHODS: After Institutional Animal Care and Use Committee approval, 12 pigs weighing 60 kg were started on bypass through a sternotomy. A dual-lumen endoaortic cannula with a deployable baffle was used for arterial cannulation. Bypass was initiated at 37 degrees C, and control measurements were obtained. The baffle was then inflated with saline solution, segmenting blood flow along the greater and lesser curvatures of the aortic arch. Parallel heat exchangers were used to independently control temperature of the arch and descending aortic perfusates. Cerebral and systemic temperatures were recorded continuously. RESULTS: During cardiopulmonary bypass, mean flow and arterial pressure were maintained at 2.4 to 2.6 L x min(-1) x m(-2) and 60 to 70 mm Hg, respectively. With aortic flow distributed by the baffle, a 5 degrees C temperature differential between brain (30 degrees C) and body (35 degrees C) was established in a mean of 5 +/- 2 minutes. Mean brain and corporeal temperatures of 27 degrees C and 35 degrees C were then maintained over 60 minutes. Relative to control, internal jugular and inferior vena cava oxygen saturations increased during targeted temperature control with the device. CONCLUSIONS: The Cobra cannula allows for independent control of brain and body temperature while providing satisfactory hemodynamics. Application of this temperature management strategy may offer cerebral protection and the advantages of warm systemic bypass temperature.