Bypass Flow Rate Research Articles

ESTABLISHMENT OF FLOW ESTIMATION FOR AN IMPLANTABLE CENTRIFUGAL BLOOD PUMP

A less invasive and non thrombogenic flow estimation of an implantable centrifugal blood pump (ICBP) has been developed, which was derived from electric power consumption, the rotating speed of a motor, and blood viscosity presumed by hematocrit and body temperature. The power consumption and the rotating speed of the motor were measured by a wattmeter every 0.2 sec. Accuracy and stability of the estimated flow (EF) were investigated during in vitro and in vivo experiments. The EF was compared with a measured flow rate (MF) monitored by an electromagnetic flowmeter. During in vitro experiments, the EF and MF were measured at 79 operating points. The ICBP was driven in a closed mock loop filled with goat blood with hematocrit values of 21.5, 28, 34, and 42%. During in vivo experiments, the ICBP was implanted in the chest cavity of a goat and driven for 40 days with continuous estimation of the bypass flow rate. Blood was taken to determine hematocrit value several times a week. The temperature of the pleura away from the ICBP was measured every 15 min. A linear correlation between the EF and MF was observed, and the correlation coefficient between the EF and MF was 0.99 during in vitro examinations. An averaged error of the EF was 0.5 L/min, with the MF ranging from 2.3 to 8.1 L/min during in vivo experiments. In conclusion, flow estimation was established with good stability and accuracy in both in vitro and in vivo experiments.

Bypass flow and its role in leaching of raised beds under different land use types on an acid sulphate soil

A better understanding of leaching processes in raised beds is useful in assessing management options for acid sulphate soils. Field and laboratory studies were carried out to quantify the effects of soil physical properties and bypass flow on leaching processes of new, 1-year-old and 2-year-old raised beds for yam and pineapple cultivation in a Typic Sulfaquept in Tien Giang, Vietnam. The methylene blue staining technique was used to characterize the water-conducting pores in terms of number, stained area, and total pore perimeter at 10 cm depth intervals of six 1 × 1 m subplots. Undisturbed 20 cm X 25 cm soil cores taken from the raised beds were subjected to three 30 mm h −1 rains. Volume, aluminum and sulphate concentration of the outflows were monitored. Consolidation with time decreased the area and perimeter of water-conducting pores in 2-year-old pineapple beds to about a third, and bypass flow rate to about 80% of those in newly constructed beds. Consolidation did not affect macropore network geometry in yam beds because they were subjected to annual tillage and yam tubers were uprooted regularly. A13 + and SO 4 2− concentrations in the outflows of the newly constructed and 1-year-old raised beds were higher in pineapple, while those in 2-year raised beds were higher in yam.

Cerebral protection during moderate hypothermic circulatory arrest: Histopathology and magnetic resonance spectroscopy of brain energetics and intracellular pH in pigs

Objective: We evaluated the effect of antegrade and retrograde brain perfusion during moderate hypothermic circulatory arrest at 28º C. Methods: Phosphorus 31–magnetic resonance spectroscopy was used to follow brain energy metabolites and intracellular pH in pigs during 2 hours of ischemia and 1 hour of reperfusion. Histopathologic analysis of brain tissue fixed at the end of the experimental protocol was performed. Fourteen pigs were divided into two experimental groups subjected to antegrade ( n = 6) or retrograde ( n = 8) brain perfusion. Anesthesia ( n = 8) and hypothermic cardiopulmonary bypass groups (15º C, n = 8) served as control subjects. In the antegrade and retrograde brain perfusion groups, the initial bypass flow rate was 60 to 100 ml · kg -1 · min -1. In the antegrade group, the brain was perfused through the carotid arteries at a flow rate of 180 to 210 ml · min -1 during circulatory arrest at 28º C. In the retrograde group, the brain was perfused through the superior vena cava at a flow rate of 300 to 500 ml · min -1 during circulatory arrest at 28º C. Results: The intracellular pH was 7.1 ± 0.1 and 7.2 ± 0.1 in the anesthesia and hypothermic bypass groups, respectively. Brain intracellular pH and high-energy metabolites (adenosine triphosphate, phosphocreatine) did not change during the course of the 3.5-hour study. In the antegrade group, adenosine triphosphate and intracellular pH were unchanged throughout the protocol. In the retrograde perfusion group, the intracellular pH level decreased to 6.4 ± 0.1, and adenosine triphosphate and phosphocreatine levels decreased within the first 30 minutes of circulatory arrest and remained at low levels until the end of reperfusion. High-energy phosphates did not return to their initial levels during reperfusion. Histopathologic analysis of nine regions of the brain showed good preservation of cell structure in the anesthesia, hypothermic bypass, and antegrade perfusion groups. The retrograde perfusion group showed changes in all the regions examined. Conclusions: The study shows that moderate hypothermic circulatory arrest at 28º C with antegrade brain perfusion during circulatory arrest protects the brain but that retrograde cerebral perfusion at 28º C does not protect the brain. (J T HORAC C ARDIOVASC S URG 1996;112:1073-80)

Bypass Flow Across a Bend for Chemical Injection into Microirrigation Systems

The hydraulic characteristics of a bypass injector for chemical injection into microirrigation systems were evaluated. Bypass injectors operate on the basis of the pressure differential generated between the inner and outer walls at the midpoint of a bend. A general equation to determine the pressure head differential across a bend is presented along with the bypass and main flow discharge relationship. Bypass flow depends not only on the available pressure differential across the bend, but also on the bypass tubing size and length. An equation to predict the bypass flow as a function of the bend geometric characteristics and the available pressure differential is proposed. An equation for head loss caused by the entrance and exit of fluid through the by-pass inlet and outlet was developed. Bypass flow rates were increased up to 2.5 times by using crested nipples. The effect of size, shape, and length of the crests on bypass flow rate was also measured.

ドラフトチューブ付噴流層による微粉流動化

The flow characteristics of fine particles in a spouted bed with a draft tube have been studied by measuring the solid hold-up in the draft tube, the bypass flow rate in the annulus, and the solids circulation rate. It was found that the gas-solid flow in the draft tube was the transport bed at high total gas flow rates.The bypass flow rate in the annulus decreased with the total gas flow rate to a minimum and then increased with further increase in the flow rate.On the other hand, the solids circulation rate increased with the total gas flow rate to a maximum and then decreased with further increase in the flow rate.An empirical equation for the maximum solids circulation rate was proposed, which almost agreed with the measued ones.

Characteristics of Bypass Flow Across PVC Pipe Elbows

By-pass flow across 90 commercial PVC pipe elbows was investigated for the case of diameters ranging from 20 to 50 mm. Factors affecting bypass flow rate were clarified. The results of this study showed that bypass flow depends not only on the available pressure differential across the elbow, but also on the bypass inlet and outlet orifice size and the tubing length. An equation giving the bypass flow as a function of the main flow discharge and the ratio of the bypass orifice diameter to the elbow diameter is presented for the case of 5- and 10-mm inlet and outlet size. The bypass flow rate and main flow discharge relationship was found to be linear. The head loss caused by the entrance and exit of fluid through the bypass inlet and outlet was clarified for the case of 5- and 10-mm orifice size.

A 31P-magnetic resonance study of antegrade and retrograde cerebral perfusion during aortic arch surgery in pigs

To evaluate the effect of hypothermic circulatory arrest on brain metabolism, we used 31P-magnetic resonance spectroscopy to monitor brain metabolites in pigs during 2 hours of ischemia and 1 hour of reperfusion. Twenty-eight pigs were divided into five groups. Anesthesia (n = 5) and hypothermic cardiopulmonary bypass groups (n = 5) served as controls. In the circulatory arrest (n = 6), antegrade perfusion (n = 6), and retrograde (n = 6) brain perfusion groups, the bypass flow rate was 60 to 100 ml.kg-1.min-1. In the antegrade group, the brain was perfused via the carotid arteries at a blood flow rate of 180 to 200 ml.min-1 during circulatory arrest at 15 degrees C. In the retrograde group, the brain was perfused through the superior vena cava at a flow rate of 300 to 500 ml.min-1 during circulatory arrest at 15 degrees C. The intracellular pH was 7.1 +/- 0.1 and 7.3 +/- 0.1 in the anesthesia and hypothermic cardiopulmonary bypass groups, respectively. In the circulatory arrest group, the intracellular pH decreased to 6.2 +/- 0.1 and did not recover to its initial value (7.0 +/- 0.1) during reperfusion (p < 0.05 compared with the value obtained from the control groups at the corresponding time). Inorganic phosphate did not return to its initial level during reperfusion. In three animals in this group, levels of high-energy phosphates, adenosine triphosphate and phosphocreatine, recovered partially but did not reach the levels observed before arrest. In the group receiving antegrade perfusion, cerebral metabolites and intracellular pH were unchanged throughout the protocol. During circulatory arrest in the retrograde perfusion group the intracellular pH decreased to 6.4 +/- 0.1 and recovered fully during reperfusion (7.1 +/- 0.1). High-energy phosphates also returned to their initial levels during reperfusion. These studies show that deep hypothermic circulatory arrest with antegrade brain perfusion provides the best brain protection of the options investigated.

Cerebral blood flow is determined by arterial pressure and not cardiopulmonary bypass flow rate

During cardiopulmonary bypass, global hypoperfusion of the brain has been shown to result in ischemic insult and subsequent neurologic injury. Furthermore, outcome after focal cerebral ischemia depends on collateral circulation, which is determined by the parameters of global perfusion. We therefore measured cerebral blood flow during independent manipulations of arterial blood pressure and pump flow rate to determine which of these hemodynamic parameters regulates cerebral perfusion during cardiopulmonary bypass. Seven anesthesized baboons were placed on cardiopulmonary bypass and cooled to 28 degrees C. Pump flow rate and arterial blood pressure were altered in varied sequence to each of four conditions: (1) full flow (2.23 +/- 0.06 L.min-1.m-2, mean +/- standard deviation) at high pressure (61 +/- 2 mm Hg), (2) full flow (2.23 +/- 0.06 L.min-1.m-2) at low pressure (24 +/- 3 mm Hg), (3) low flow (0.75 L.min-1.m-2) at high pressure (62 +/- 2 mm Hg), and (4) low flow (0.75 L.min-1.m-2 at low pressure (23 +/- 3 mm Hg). During each of these hemodynamic conditions cerebral blood flow was measured by washout of intracarotid xenon. Cerebral blood flow was greater at high blood pressure than at low pressure during cardiopulmonary bypass both at low flow (34 +/- 8.3 versus 14.1 +/- 3.7 mL.min-1 x 100 g-1) and full flow (27.6 +/- 9.9 versus 16.8 +/- 3.7 mL.min-1 x 100 g-1) (p < 0.01). At comparable mean arterial blood pressures alteration of pump flow rate produced no changes in cerebral blood flow. These results indicate that cerebral blood flow during moderately hypothermic cardiopulmonary bypass is regulated by arterial blood pressure and not pump flow rate.

Cerebral Blood Flow Is Determined by Arterial Pressure and Not Cardiopulmonary Bypass Flow Rate

Cerebral Blood Flow Is Determined by Arterial Pressure and Not Cardiopulmonary Bypass Flow Rate

Effect of extracorporeal life support on cerebral blood flow, metabolism and electrophysiology in normothermic cats.

Recently, extracorporeal life support (ECLS) by venoarterial bypass perfusion has been recommended for the treatment of refractory respiratory and/or cardiac failure but the safety of this application for the brain is not yet established. Therefore, the effects of normothermic ECLS on cerebral blood flow, metabolism and electrophysiology were studied in cats with total arrest of cardiopulmonary circulation. An extracorporeal circulation (ECC) system, consisting of a roller pump, a membrane oxygenator and a heat exchanger, was connected to the circulation of cat by cannulae inserted via the jugular vein and femoral vessels. After 2 h ECLS brains were frozen in situ and investigated for changes in regional metabolism. During 2 h ECC hematocrit declined from 37 +/- 7% to 21 +/- 10% (means +/- SD, p < 0.05), cerebral blood flow decreased to 73 +/- 14% of control (p < 0.05) and cerebral oxygen delivery to 46 +/- 13% of control (p < 0.05) although arterial blood pressure and bypass flow rate did not change. Plasma lactate increased from 0.8 +/- 0.3 to 9.2 +/- 4.2 mumol/ml (p < 0.05), and brain tissue lactate from 2.3 +/- 0.9 to 10.6 +/- 2.7 mumol/g (p < 0.05). Hematocrit correlated positively with cerebral oxygen delivery (r = 0.86, p < 0.001). These data demonstrate that ECLS is associated with reduced cerebral oxygen delivery and may cause brain hypoxia despite normal blood pressure. This complication may contribute to the high incidence of neurological disturbances after prolonged ECLS.

Mechanisms of change in cardiac performance in infants undergoing extracorporeal membrane oxygenation

To determine whether changes in cardiac performance observed in infants early during extracorporeal membrane oxygenation are due to the underlying disease process, the partial bypass procedure, changes in cardiac loading conditions, or due to a direct myocardial effect of hypoxia-reoxygenation. Consecutive case series. Infants with meconium aspiration syndrome were studied at the end of their extracorporeal membrane oxygenation course (veno-arterial, n = 12; veno-venous, n = 8), after lung compliance and pulmonary arterial pressure returned toward normal and they were ready to have extracorporeal membrane oxygenation discontinued. Heart rate, mean arterial blood pressure, and load-dependent indices, and a load-independent index of cardiac performance were measured at increasing bypass flow rates (25, 50, 75, 100, 125 mL/kg/min). Heart rate, mean arterial blood pressure, and load-dependent indices of cardiac performance (left ventricular shortening fraction and stroke volume) were normal at 25 mL/kg/min bypass flow rate and did not change significantly with increasing bypass flow rates. The load-independent index of performance was within the normal range for infants and did not change significantly with increasing bypass flow rates. The previously described decreases in cardiac performance during extracorporeal membrane oxygenation are not due to the underlying disease process or the bypass procedure but are due to changes in loading conditions during partial bypass.

Extracorporeal CO 2 removal in a lung lavage model of respiratory failure

Extracorporeal life support (ECLS) is a recognized treatment for neonatal respiratory distress unresponsive to other forms of therapy. Variations of this technique are being developed in an effort to extend its applicability and safety. Extracorporeal CO 2 removal (ECCO 2R) is one such modification that requires blood flows of 20% to 50% of cardiac output and therefore lends itself to percutaneous venous cannulation. The authors evaluated ECCO 2R in conjunction with low-frequency ventilation, using a lung lavage-induced model of respiratory failure in rabbits. Six rabbits were lavaged an average of 9 times with 15 mL/kg Plasma-Lyte A at 37°C via an endotracheal tube. Incremental ventilatory changes were made during lavage, to an Fio 2 of 1.0, rate of 80, peak inspiratory pressure (PIP) of 37 cm H 2O, and positive end-expiratory pressure (PEEP) of 4 cm H 2O. Arterial blood gas values of Pa o 2 < 40 mm Hg and Pa co 2 > 60 mm Hg resulted, meeting our criteria for respiratory failure. The rabbits were placed on veno-venous ECCO 2R using a 0.8-m 2 hollow fiber oxygenatir and a commercially available double-lumen dialysis catheter. Blood flows of 10 to 20 mL/kg/min were used to manage CO 2 removal. A low-frequency ventilation technique was employed using an Fio 2 of 1.0 and a rate of 5 breaths per minute. PEEP was increased incrementally to maintain the Pa o 2 above 80 mm Hg. After initiation of ECCO 2R, the arterial Pa o 2 increased to 165 ± 109 mm Hg, with PEEP above 15 cm H 2O, and Pa co 2 decreased to 37 ± 5 mm Hg, with a bypass flow rate of 15 mL/kg/min. In all cases, when the animals were removed from bypass, arterial blood gas levels returned to postlavage values, and death ensued, confirming respiratory failure. These results demonstrate the utility of a lung lavage model of respiratory failure in rabbits. Whereas conventional ventilatory management failed, ECCO 2R, when combined with low-frequency ventilation, not only removed CO 2 but also significantly increased oxygenation.

Non-invasive monitoring of brain oxygen sufficiency on cardiopulmonary bypass patients by near-infra-red laser spectrophotometry.

A new portable high-performance apparatus for near-infra-red (NIR) laser spectrophotometry was developed to monitor the oxygenation state in the human brain. Three different wavelengths of 780, 805 and 830 nm of the NIR light illuminated the head using a fibre-optic bundle, and the transmitted or reflected light was detected by a photodiode placed on the forehead. The oxygenated Hb (oxy Hb), deoxygenated Hb (deoxy Hb) content and the Hb (blood) volume changes in the brain were continuously monitored in cardiopulmonary bypass (CPB) patients in the reflectance mode. In hypothermic CPB with selective brain perfusion, the brain bypass flow rates of 0.5 L min-1 at 24-25 degrees C and 0.3 L min-1 at 21-22 degrees C were confirmed as the safe lower limits by our NIR monitoring. During deep hypothermic circulatory arrest, the brain oxy Hb and blood volume decreased significantly. The result indicates that brain Hb oxygenation decreases significantly during circulatory arrest. In the adult patients group, during moderate hypothermic CPB, the brain blood oxygenation level was maintained roughly constant at mean arterial perfusion pressure of over 60 mm Hg, whereas below 50-55 mmHg a progressive decline in the brain Hb oxygenation was observed. These findings led us to conclude that non-invasive monitoring of brain oxygenation using NIR light can provide valuable data at the bed-side regarding tissue metabolism, and it can allow for the proper management of critical patients.

Clinical application of evoked spinal cord potentials elicited by direct stimulation of the cord during temporary occlusion of the thoracic aorta

Evoked spinal cord potentials elicited by direct stimulation of the cord were used to monitor spinal cord ischemia in 68 patients undergoing temporary occlusion of the thoracic aorta (29 thoracic nondissecting aortic aneurysms, 9 nondissecting thoracoabdominal aneurysms, and 30 dissecting aneurysms). "Immediate" postoperative paraplegia developed in three patients and "immediate" paraparesis developed in one, whereas "delayed" paraplegia developed in two others. During aortic crossclamping, four response patterns of the spinal cord potentials were obtained: (1) no change (n = 53), (2) change with return (n = 10), (3) change with inconsistent return (n = 2), and (4) change without return (n = 3). Neurologic complications occurred in 2%, 0%, 100% of these groups, respectively. Delayed paraplegia developed on the second postoperative day in only one patient with a false-negative result, and the potentials correlated well with this patient's clinical neurologic recovery. The aortic crossclamp time was significantly longer in the patients with "change with inconsistent return" and "change without return" than in the other two groups (p < 0.01). Femoral artery pressure and the cardiopulmonary bypass flow rate were also significantly lower in these groups than in the other two groups (p < 0.02 and p < 0.01, respectively). We conclude that intraoperative monitoring of direct spinal cord responses is useful for the early detection of spinal cord ischemia for assessing the efficacy of surgical countermeasures.

Fetal cardiac bypass: Improved placental function with moderately high flow rates

Prenatal correction of certain cardiac lesions with a poor prognosis may have advantages over postnatal repair. For this to be done, safe and effective support of the fetal circulation must be devised. Studies involving fetal cardiac bypass have demonstrated progressive fetal hypoxemia, hypercapnia, and acidosis, indicating placental dysfunction. We performed fetal cardiac bypass in 18 fetal lambs (126 to 140 days' gestation) to assess the effect of flow rate on fetal oxygenation and metabolism and function of the placenta as an in vivo oxygenator. Fetal cardiac bypass was done for a 30-minute study period at normothermia in all fetuses. During the study period the fetal aorta was cross-clamped and cold cardioplegia was administered to the heart so there was no fetal cardiac contribution to systemic output. Nine fetuses underwent studies at low flow rates (109 ± 20 mL · kg −1 · min −1) and 9 at higher flow rates (324 ± 93 mL · kg −1 · min −1). At the lower flow rate, mean aortic pressure, arterial pH, and oxygen tension decreased whereas carbon dioxide tension and lactate levels increased when compared with prebypass levels. At the higher flow rate mean aortic pressure, pH, oxygen tension, carbon dioxide tension, and lactate levels remained similar to prebypass levels during the 30-minute study period. When the animals were weaned from the bypass circuit after studies at high flow rates, arterial oxygen tension and pH decreased whereas carbon dioxide tension increased to levels similar to those in the low-flow group. We conclude that low fetal cardiac bypass flow rates (100 to 125 mL · kg −1 · min −1) are inadequate to maintain hemodynamics, oxygenation, CO 2 removal, and normal lactate levels when the placenta is used as an in vivo oxygenator. Higher flow rates 1300 to 400 mL · kg −1 · min −1) may limit these changes by improving placental perfusion and function during bypass. Despite high flow rates, placental dysfunction and fetal blood gas abnormalities still occur after fetal cardiac bypass.

Emissions of Metal and Organic Compounds from Cement Kilns Using Waste Derived Fuels

Research completed to date suggests that the use of waste as a substitute fuel in cement kilns is fundamentally sound in theory, and in a number of cases, has been demonstrated sound in practice. Analysis of the database generated in certificate of compliance tests clearly demonstrate that cement plants burning waste-derived fuel (WDF) can employ existing technology to meet and in some cases exceed current emission regulations. With the exception of cement kilns using raw materials with high levels of hydrocarbons, the emission of organics can be controlled by applying well-established good-combustion practices. Metals emissions from cement plants burning WDF can be predicted based upon input metal flow rates, provided that plant-specific information on the collection efficiency of air pollution control equipment and by-pass flow rates are available.

Effect on Liver and Splanchnic Circulation of Graded Flow Rates through Portal-Systemic Bypass during Acute Portal Occlusion

Using graded flow rates through a passive portalsystemic bypass during 2-hr portal occlusion in dogs, splanchnic, hemodynamic, and metabolic changes and liver insults were evaluated to determine critical bypass flow. To exclude effects caused by reduced circulating blood volume, constant preocclusion levels of cardiac output were maintained throughout the study by increasing intravenous infusion rates. With the decrease in bypass flow rates during portal occlusion, portal pressure increased, superior mesenteric arterial flow decreased, and hepatic arterial flow rapidly increased and these parameters maintained their individual levels during occlusion. These hemodynamic parameters recovered to nearly normal levels 3 hr after release of the portal clamp. Metabolic acidosis progressed with decreasing bypass flow, but portal potassium and inorganic phosphorus levels showed a significant rise only when there was no bypass. Portal levels of creatine phosphokinase BB, β-glucuronidase, and endotoxin did not show significant changes corresponding to bypass flow. The amounts of infusion required were 3 to 6 times the basal level (10 ml/kg/hr) during occlusion and 1.7 to 3 times after release in 30% or less bypass groups. Upon ceasing infusion 3 hr after release, dogs in the 10% or less bypass groups underwent circulatory insufficiency. Changes in total adenine nucleotide and energy charge ratio of the liver, postoperative changes in transaminase levels, and animal survival indicated that a 2-hr interruption of the portal flow, either sufficiently or insufficiently bypassed, caused only minimal insults of the liver. In conclusion, a bypass flow of 30% or more is the critical level for dogs to survive 2 hr of portal occlusion without adverse consequences.

Hepatic blood flow during cardiopulmonary bypass

To establish the effects of different modes of cardiopulmonary bypass on hepatic blood flow, with the aim of increasing understanding of the hemodynamic factors that may lead to hepatic dysfunction in patients after cardiopulmonary bypass. The anatomical and physiologic characteristics that are unique to the hepatic circulation are also reviewed, together with an account of the known specific effects on the liver of several hemodynamic stimuli that are commonly present during cardiopulmonary bypass. The entire world literature on the subjects of clinical and experimental cardiopulmonary bypass, extracorporeal circulation, blood gas tensions, hypothermia, hypotension, and hepatic blood flow was searched via Index Medicus, up to and including 1991. Data are presented only from those studies that employed sound cardiopulmonary bypass or blood flow measurement technique. For investigations on hypothermia, data for temperatures < 28 degrees C are not included. Details of one recent experimental study in the dog by the author are highlighted, in which the relative effects of pump flow rate, temperature, and type of perfusion (pulsatile or nonpulsatile) are presented; the data are critically compared and contrasted with those data obtained by earlier workers. Hepatic hemodynamics are characterized by a dual supply of blood from the hepatic artery, actively controlled within the liver, and the portal vein, principally regulated by prehepatic resistance vessels. Portal flow may modulate the hepatic arterial circulation through the "hepatic arterial buffer response." Hypotension caused by hemorrhage causes a decrease in portal blood flow but hepatic arterial flow is maintained by autoregulation and by the "buffer" response. Hypothermia (28 degrees C) has little effect on hepatic arterial flow, but portal flow may increase. During cardiopulmonary bypass, total liver blood flow is better maintained at a pump flow rate of 2.4 than at 1.2 L/min/m2. Perfusion at 28 degrees C causes an increase in portal flow and a slight decrease in hepatic arterial flow. Total hepatic blood flow is better preserved during cardiopulmonary bypass at 1.2 L/min/m2 by pulsatile than by nonpulsatile flow; however, no significant difference was noted between pulsatile and nonpulsatile perfusion at a bypass flow rate of 2.4 L/min/m2, particularly at 28 degrees C. During cardiopulmonary bypass, hepatic blood flow is better maintained by high pump flow than by low pump flow rates. Hypothermic cardiopulmonary bypass may benefit the hepatic circulation, although the additional advantages usually gained by the use of pulsatile perfusion may be partly lost when hypothermia is combined with a high pump flow rate.

Oesophageal thermal tube for intraoperative hypothermia in liver transplantation

In order to prevent the occurrence of major hypothermia during liver transplantation, with its deleterious effects on intraoperative cardiovascular activity and on postoperative graft functioning, this study evaluated the benefit of an oesophageal rewarmer, used during surgery, in addition to the usual methods of warming (OR temperature at 22 degrees C, rewarming of fluids and blood, heating mattress, heat and moisture exchanger). We compared 10 patients with an oesophageal rewarmer (OeR group) to 10 patients without (Control group). The anaesthetic procedure was similar in all cases. Rectal (RT) and pulmonary artery (PT) temperatures were recorded during the three phases of surgery (pre-anhepatic, anhepatic, postanhepatic phase) and their time course was analysed with non-parametric tests. The two groups were comparable with regard to duration of surgery, blood and fluid requirements and veno-venous bypass flow rate. The RT decreased similarly in both groups, but was significantly higher in the OeR group at peritoneum closure (P < 0.01). The PT was higher in the OeR group after onset of venous shunting (P < 0.05) and during the third phase of surgery (P < 0.01). Three incidents (one leakage and two herniations of the latex tube) occurred, without detrimental effects on the patients. It is concluded that the oesophageal heat exchanger allows better rewarming after revascularization of the graft, but is unable to prevent cardiac hypothermia at unclamping.

Effect of extracorporeal membrane oxygenation on cerebral hemodynamics in newborn lambs

Neurologic injury caused by changes in cerebral blood flow is a concern in infants treated with extracorporeal membrane oxygenation (ECMO). To investigate the hypothesis that cerebral hemodynamics would be influenced by bypass flow rates, eight ketamine anesthetized lambs (age range 1 to 8 days) had measurements of cerebral hemodynamics before and after institution of venoarterial bypass. Cerebral blood flow (whole brain and regional) were measured using the radiolabeled microsphere technique. A left carotid artery catheter (placed without sacrificing the vessel) reference sample was used for calculation of brain blood flows. Arterial and venous (sagittal sinus) oxygen contents and blood gases were measured across the cerebral circulation. Cerebral oxygen consumption, oxygen delivery, and fractional oxygen extraction were calculated. Measurements were made pre-ECMO and after placement on venoarterial ECMO at flow rates of 50 and 100 mL/kg/min. In addition, four other lambs were studied at a flow rate of 150 mL/kg/min, and seven additional animals were used to address methodologic issues with the microsphere technique in this model. Cerebral blood flow and oxygen delivery significantly decreased, compared with control at both flow rates of 100 and 50 mL/kg/min (87.8 +/- 5.2 vs. 36.4 +/- 6.2; and 35.1 +/- 5.8 mL/100 g/min; 5.82 +/- 0.37 vs. 2.14 +/- 0.35; and 2.19 +/- 0.39 microM/100 g/min, respectively). Cerebral fractional oxygen extraction increased significantly at a flow of 50 mL/kg, and cerebral oxygen consumption decreased significantly at a flow of 100 mL/kg. At a flow rate of 150 mL/kg/min, cerebral hemodynamic variables did not differ from control measurements. These findings demonstrate that changes in cerebral blood flow and cerebral oxygen metabolism can occur in the newborn lamb with the initiation of ECMO, and that these changes are dependent on bypass flow rate.