Numerical simulation of the hepatic circulation

Availability of donor livers and the relatively short preservation time limit the success of liver transplantation. The use of hypothermic machine perfusion could pave the way for expansion of the donor pool. To better define optimal settings of such a device, the feasibility of using a numerical simulation model of the hepatic circulation is determined. Hemodynamics in the hepatic arterial, portal venous and hepatic venous compartments of the hepatic vascular tree was modelled using an electrical analogue. Calculated pressure and flow profiles throughout the liver were in accordance with physiologic profiles in the total circulatory system. Comparison of calculated flow values with normal control values showed a discrepancy that was explained by inaccurate diameter input data. Until more precise methods for determining vascular dimensions become available, redefining vessel diameter makes the simulation model perfectly suitable for predicting influences of temperature and/or viscosity on hepatic hemodynamics and is thereby an excellent tool in defining optimal settings for our hypothermic liver perfusion system.     

The Groningen Hypothermic Liver Perfusion Pump: Functional Evaluation of a New Machine Perfusion System

To improve preservation of donor livers, we have developed a portable hypothermic machine perfusion (HMP) system as an alternative for static cold storage. A prototype of the system was built and evaluated on functionality. Evaluation criteria included 24 h of adequate pressure controlled perfusion, sufficient oxygenation, a maintained 0–4°C temperature and sterile conditions. Porcine livers were perfused with pump pressures that were set at 4 mmHg (continuous, portal vein) and 30/20 mmHg, at 60 BPM (pulsatile, hepatic artery). Control livers were preserved using the clinical golden standard: static cold storage. In the HMP group, pressure, flow and temperature were continuously monitored for 24 h. At time-points t = 0, 2, 4, 8, 12, and 24 h samples of University of Wisconsin machine preservation solution were taken for measurement of partial oxygen pressure (pO₂) and lacto-dehydrogenase. Biopsies in every lobe were taken for histology and electron microscopy; samples of ice, preservation solution, liver surface, and bile were taken and cultured to determine sterility. Results showed that temperature was maintained at 0–4°C; perfusion pressure was maintained at 4 mmHg and 30/20 mmHg for portal vein and hepatic artery, respectively. Flow was approximately 350 and 80 ml/min, respectively, but decreased in the portal vein, probably due to edema formation. Arterial pO₂ was kept at 100 kPa. Histology showed complete perfusion of the liver with no major damage to hepatocytes, bile ducts, and non-parenchymal cells compared to control livers.The machine perfusion system complied to the design criteria and will have to demonstrate the superiority of machine perfusion over cold storage in transplant experiments.     

Effects of hematocrit on oxygenation of the isolated perfused rat liver

The isolated perfused rat liver is used ubiquitously for metabolic and endocrine studies of hepatic function, yet few data are available regarding the inadequacy of the oxygenation of such preparations. Moreover, the isolated rat liver is usually deprived of its arterial supply and perfused via the hepatic portal vein with low-hematocrit or cell-free solutions. To investigate the efficacy of the oxygen supply, we determined the effect of hematocrit on the relation between oxygen consumption and perfusate flow. We then attempted to define a hematocrit at which hepatic oxygenation was maximal. Livers of male rats anesthesized with pentobarbital sodium were perfused via the portal vein with fresh canine red blood cells suspended in Krebs-Ringer-bicarbonate buffer. Perfusions were carried out at various flow rates, and the relation between perfusate flow and oxygen uptake was determined. At flow rates above 100 ml X min-1 X 100 g liver-1, oxygen uptake was independent of flow but below that value was flow limited, regardless of whether the hematocrit was 10, 20, or 40%. To determine the optimal hematocrit for hepatic oxygen uptake, hepatic portal venous and hepatic venous pressures were held at 10 and 0 mmHg, respectively. The hematocrit was lowered in steps from 80 to 10%. Blood flow increased exponentially as hematocrit fell while oxygen uptake increased to a maximum at approximately 20%. It is concluded that an hematocrit of approximately 20% provides the optimal combination of blood flow and oxygen-carrying capacity while maintaining physiological perfusion pressures, e.g., 10 mmHg.     

Artificial Circulation of the Liver: Machine Perfusion as a Preservation Method in Liver Transplantation

Due to the sharp increase in liver transplant candidates and the subsequent shortage of suitable donor livers, an extension of the current donor criteria is necessary. Simple cold storage, the current standard in organ preservation has proven to be insufficient to preserve extended criteria donor livers. Therefore a renewed interest grew toward alternative methods for liver preservation, such as hypothermic machine perfusion and normothermic machine perfusion. These “new” preservation methods were primarily assessed in rat models, and only a few clinically relevant large animal models have been described so far. This review will elaborate on these alternative preservation methods. Anat Rec, 291:735–740, 2008. © 2008 Wiley‐Liss, Inc.     

Ex vivo cardiac allograft preservation by continuous perfusion techniques

The current technique of cardiac preservation for clinical transplantation by infusion of cold cardioplegia and immersion of the heart in an isotonic saline bath at 4 degrees C limits safe tissue preservation time to 4 to 6 hours. The myriad of benefits to be gained by extending cardiac preservation time has prompted the search for alternatives to hypothermic immersion of the heart, the most promising of which involves techniques of coronary artery perfusion. Countless studies have shown the benefits of long-term storage of donor hearts by perfusion rather than the immersion technique. Continuous perfusion preservation has three basic advantages over simple immersion. Perfusion preservation with oxygen carrying solutions has the advantage of preventing ischemia, anaerobic metabolism, and reperfusion injury. Second, nutritional supplementation and provision of substrate can be more effectively delivered to myocardial cells. Third, continuous perfusion preservation effects the clearance of metabolic waste products from the coronary circulation. The composition of the ideal perfusion solution and optimal preservation conditions remain incompletely defined.     

Oxygen consumption during hypothermic and subnormothermic machine perfusions of porcine liver grafts after cardiac death

The use of grafts donated after cardiac death (DCD) would greatly contribute to the expansion of the donor organ pool. Machine perfusion (MP) is a promising technology to improve DCD liver grafts. Several perfusion technologies under various temperature and oxygenation conditions have been suggested and are still debated. It is important to confirm the relationship between oxygen consumption and organ conditions during MP. In this study, we analyzed oxygen consumption during oxygenated MP of porcine DCD liver grafts under different temperature conditions: hypothermic and subnormothermic. Grafts exposed to 60 min of warm ischemia were perfused for 4 h with a modified UW-gluconate perfusate under hypothermic (HMP) and subnormothermic conditions. Oxygen consumption, pressures of the portal vein and hepatic artery, and effluent enzymes were analyzed. Oxygen consumption was strongly related to the graft temperature during MP. Effluent enzyme level of the LDH were lower in the high oxygen consumption group than in the low oxygen consumption group during MP. In summary, we found that high oxygen consumption under subnormothermic temperature conditions has several advantages over HMP for DCD liver graft preservation.     

Recovery of cardiac function after standard hypothermic storage versus preservation with Peg-hemoglobin

Preservation of the heart for transplantation after infusion of cardioplegia and extirpation of a cardiac allograft results in an ischemic insult to the myocardium. This ischemic insult may lead to a loss of function in the transplanted heart. Hypothermic perfusion preservation with an oxygen hemoglobin carrying solution may avert ischemic injury and lead to improved recovery of cardiac function. The purpose of this study was to compare cardiac function after 8 hours of continuous hypothermic perfusion with a unique polyethylene-glycol-hemoglobin (PEG-Hb) solution to hearts preserved by 4 hours of hypothermic ischemic storage. Freshly extirpated hearts served as functional controls. The hearts of 26 anesthetized and intubated New Zealand white rabbits were harvested after cold cardioplegic arrest. Group I (n = 12) hearts were perfused with a PEG-Hb solution at 20 degrees C and 30 mm Hg for 8 hours. PO2 was maintained > or = 500 mm Hg. Group II (n = 7) hearts were preserved by cold ischemic storage for 4 hours at 4 degrees C. Group III (n = 7) were tested immediately after harvest. Left ventricular (LV) function was measured in the nonworking state at 15 minutes, 1 hour, and 2 hours after transfer to a standard crystalloid Langendorff circuit. Measurement of LV developed pressure, peak + dP/dt and -dP/dt revealed a superior trend between Group I and Group II hearts in comparison with freshly extirpated hearts. Heart rate was similar among all groups throughout testing (p = ns). Coronary blood flow was not significantly different between groups. Continuous perfusion preservation of rabbit hearts for 8 hours with PEG-Hb solution at 30 mm Hg and 20 degrees C yielded LV function that was similar to 4 hours of ischemic hypothermic storage. Furthermore, return of cardiac function after 8 hours of perfusion preservation using this PEG-Hb solution may be superior to that obtained in freshly extirpated hearts. These data suggest that some recovery of myocardial function may occur during perfusion preservation with this PEG-Hb solution after the ischemic insult of cardioplegic arrest. Continuous perfusion preservation using this PEG-Hb solution deserves further investigation in large animal transplant models.     

Flow competition between hepatic arterial and portal venous flow during hypothermic machine perfusion preservation of porcine livers

Hypothermic machine perfusion (HMP) is regarded as a better preservation method for donor livers than cold storage. During HMP, livers are perfused through the inlet blood vessels, namely the hepatic artery (HA) and the portal vein (PV). In previous HMP feasibility studies of porcine and human livers, we observed that the PV flow decreased while the HA flow increased. This flow competition restored either spontaneously or by lowering the HA pressure (PHA). Since this phenomenon had never been observed before and because it affects the HMP stability, it is essential to gain more insight into the determinants of flow competition. To this end, we investigated the influence of the HMP boundary conditions on liver flows during controlled experiments. This paper presents the flow effects induced by increasing PHA and by obstructing the outlet blood vessel, which is the vena cava inferior (VCI). Flow competition was evoked by increasing PHA to 55-70 mmHg, as well as by obstructing the VCI. Remarkably, a severe obstruction resulted in a repetitive and alternating tradeoff between the HA and PV flows. These phenomena could be related to intra-sinusoidal pressure alterations. Consequently, a higher PHA is most likely transmitted to the sinusoidal level. This increased sinusoidal pressure reduces the pressure drop between the PV and the sinusoids, leading to a decreased PV perfusion. Flow competition has not been encountered or evoked under physiological conditions and should be taken into account for the design of liver HMP protocols. Nevertheless, more research is necessary to determine the optimal parameters for stable HMP.     

Extended ex vivo myocardial preservation in the beating state using a novel polyethylene glycolated bovine hemoglobin perfusate based solution

Cardiac preservation for transplantation is generally limited by ischemic hypothermic storage of 4-6 hours. Earlier studies in the authors' laboratory have demonstrated that hypothermic perfusion preservation using a novel oxygen carrying hemoglobin solution may extend preservation times to 8 hours and decrease ischemic injury. The purpose of this study was to compare extended cardiac function after 12 and 24 hours of continuous hypothermic perfusion with a polyethylene glycolated bovine hemoglobin perfusate (PEG-Hb) solution to the clinical standard of hypothermic ischemic preservation. The hearts of 54 anesthetized and intubated New Zealand White rabbits were harvested after cold cardioplegic arrest. Group I (n = 12) hearts were perfused with a PEG-Hb solution at 20 degrees C and 30 mm Hg for 24 hours. Group II (n = 10) hearts were preserved similarly with PEG-Hb for 12 hours. Group III (n = 12) hearts were preserved for 8 hours with PEG-Hb; Group IV (n = 10) were preserved by cold ischemic storage for 4 hours at 4 degrees C; and Group V (n = 10) were tested after fresh extirpation. Left ventricular (LV) function was measured in the nonworking state at 15 minute, 1 hour, and 2 hour intervals after transfer to a standard crystalloid Langendorff circuit. Developed LV pressure at 0.5 ml LV volume was superior in Group II at early time points, yet it was similar in all preserved groups at 2 hours. +dP/dt(max) at 0.5 ml LV volume was consistent at all time points and greater in PEG-Hb preserved groups compared with Group V. -dP/dt(max) at 0.5 ml LV volume was significantly greater in Groups II and III compared with Group V initially (p < 0.05), but all were similar at the end of testing. Continuous perfusion preservation of rabbit hearts for time increments up to 24 hours with this novel PEG-Hb solution at 30 mm Hg and 20 degrees C yields LV function that is similar to 4 hours of ischemic hypothermic storage. Extended cardiac perfusion preservation with this PEG-Hb solution deserves further investigation in large animal transplant models.     

Comparison of Glutathione S-Transferase and Alanyl Aminopeptidase as Viability Markers in a Porcine NHBD Model

A critical shortage in the kidney donor pool has led to a renewed interest in non-heart beating donors (NHBDs). As warm ischaemic tissue injury is a characteristic of NHBD, it is therefore crucial to screen such kidneys for viability to disqualify kidneys that will never work, i.e. develop primary non-function. Viability screening is somewhat time-consuming, usually involving a combination of donor history, visual assessment of the donor kidney at procurement and monitoring during hypothermic machine preservation. Machine perfusion of NHBD kidneys enables physical characteristics to be evaluated by perfusion flow rate and pressure. Glutathione S-transferase (GST) is an established kidney injury biomarker, but its prolonged assay time may result in delays in any potential transplantation. Alanyl aminopeptidase (Ala-AP) has been developed as an alternative novel biomarker of ischaemic renal injury. A porcine NHBD model with differential ischaemic renal injury, using heparin ± streptokinase in the initial in situ flush solution, was developed to evaluate the two biomarker enzymes, GST and Ala-AP. Two groups of 16 porcine NHBD kidneys (heparin alone or heparin + streptokinase) were machine perfused for 4 h, with documentation of pressure, flow, resistance and temperature and samples of kidney perfusate were taken at hourly intrevals. Wedge sections of the kidneys were analysed at termination of perfusion. Histopathology of renal wedge sections, machine perfusion physical parameters and biomarkers demonstrated the differential renal ischaemic insult. The two biomarkers, GST and Ala-AP appear to be correlate with the degree of ischaemic injury and with each other. Mild ischaemic injury occurs in the streptokinase-treated group, with lower efflux of GST and Ala-AP. Assays for Ala-AP activity in humans are significantly shorter than the comparative assays for GST, enabling early decision making on viability. This can, therefore, facilitate transplantation of the potential NHBD kidney within the recommended 24-hour period of cardiac arrest of donor.     

Cardiac function after eight hour storage by using polyethylene glycol hemoglobin versus crystalloid perfusion

Efforts to extend myocardial preservation for transplantation by crystalloid perfusion have been limited by edema and compromised function. We hypothesized that hypothermic perfusion preservation with a polyethylene glycol (PEG) conjugated hemoglobin solution may extend preservation times. The purpose of this study was to compare cardiac function after continuous perfusion by using a hypocalcemic, normokalemic crystalloid perfusate with and without the addition of PEG-hemoglobin (Hb). The hearts of 20 anesthetized and ventilated New Zealand White rabbits were harvested after cold cardioplegic arrest. Group I (n = 10) hearts were continuously perfused with a hypocalcemic, normokalemic 3% bovine PEG-Hb solution at 20 degrees C and 30 mm Hg for 8 hours. Group II (n = 10) hearts were continuously perfused with an identical crystalloid solution without PEG-Hb for 8 hours under the same conditions as group I hearts. Cardiac function was measured with a left ventricular force transducer after transfer to a standard crystalloid Langendorff circuit at 37 degrees C and an aortic root pressure of 59 mm Hg. After 8 hours of perfusion preservation, heart rate was similar for groups I and II (p = not significant [NS]). Coronary blood flow after and during preservation was similar between PEG-Hb and crystalloid preserved hearts (p = NS). Left ventricular developed pressure, peak dP/dt, and peak -dP/dt were superior in hearts preserved with PEG-Hb. Percent water of total ventricular weight was 82.0% for group I and 81.6% for group II (p = NS). Continuous perfusion preservation of rabbit hearts for 8 hours with a hypocalcemic normokalemic PEG-Hb based solution at 30 mm Hg and 20 degrees C yields left ventricular function that is superior to perfusion with a similar crystalloid solution without PEG-Hb, despite similar myocardial edema and coronary flow. Extended cardiac perfusion preservation with this PEG-Hb based solution deserves further study, including comparison with traditional cardioplegic preservation solutions.     

The evaluation of synthetic oxygen carriers by perfusion of isolated rat liver.

The perfluorochemical FC-80 emulsion was added as oxygen carrier to a synthetic medium used for perfusion of isolated rat liver. The oxygen carrying function of FC-80 was evaluated from measurements of serum albumin synthesis and incorporation of 14C-lysine into the proteins in the medium. The effect of changes in flow rate of the medium on the evaluation of FC-80 was investigated. The results show that there is a correlation between the serum protein synthesis and the oxygen supply only when the liver is hypoxic. In hypoxic liver both the addition of FC-80 and the increase of the flow rate improve the oxygen supply which is indicated by increased serum protein synthesis. The effect of FC-80 on serum protein synthesis is different at different flow rates. For the evaluation of the oxygen carrying function of FC emulsions from the metabolic function of the liver such a flow rate and affluent PO2 are optimal in which the FC is a limiting factor for sufficient oxygen supply.     

Regulation of perfusion pressure during cardiopulmonary bypass using sevoflurane

In hypothermic cardiopulmonary bypass (CPB), various vasodilators are used to control the perfusion pressure. These agents, however, often decrease the pressure excessively, and the low perfusion pressure may persist until the end of CPB. In this study we evaluate the safety and characteristics of the regulation of perfusion pressure during CPB using a volatile anesthetic, sevoflurane which has an extremely low partition coefficient. Twenty adult patients who underwent cardiac surgery were studied. Sevoflurane was applied by a vaporizer inserted into the oxygenator gas supply line. Pump flows were fixed at 2.4 L/min/m2 during the hypothermic period. Sevoflurane concentration was adjusted to keep mean arterial pressure (MAP) between 40 and 70 mmHg during CPB. Hemodynamic and metabolic parameters were measured and compared to the group we previously treated with chlorpromazine. In all cases, MAP could be maintained adequately. In the sevoflurane group, systemic vascular resistance indices (SVRI) during the rewarming period and at the end of CPB were higher, and doses of norepinephrine needed at the end of CPB were significantly lower than in the chlorpromazine group. The regulation of perfusion pressure during CPB using sevoflurane was safe and could easily maintain adequate SVRI.     

Regulation of myocardial oxygen consumption by perfusion pressure in isolated fibrillating canine heart

In order to evaluate whether perfusion pressure or coronary flow affect myocardial oxygen metabolism, oxygen consumption of the isolated fibrillating blood-perfused canine heart was investigated at perfusion pressures of 100, 150, and 200 mm Hg. To obtain different coronary flow rates at a given coronary perfusion pressure, -adrenergic blockade by phenoxybenzamine (10 mg/kg b.w.) was applied, resulting in an increase in coronary flow and a decrease in myocardial oxygen extraction. Myocardial oxygen consumption was increased by elevation of perfusion pressure in both the control and phenoxybenzamine-pretreated group. At the same level of perfusion pressure there was no significant difference between the oxygen consumption of control and phenoxybenzaminepretreated preparations. It can be concluded that in the isolated fibrillating canine heart oxygen consumption is primarily regulated by perfusion pressure, and is independent from coronary blood flow.     

Maintenance of respiratory rhythm-generation by vascular perfusion with physiological saline in the isolated head of the carp

1) Vascular perfusion of the isolated head of the carp was carried out with physiological saline. Respiratory movement of the grill covers and respiratory discharges of cranial motor nerves were recorded in the preparation. 2) Blood vascular vessels supply the brain were anatomically investigated. Concentrations (electrolytes, O2, CO2, and glucose) and perfusing pressure of the saline were determined by reference to the useful information already available in fish physiology. 3) The movement of the operculum could be maintained in regular respiratory rhythm for more than 34 hr. Efferent discharges of cranial motor nerves also indicated that the respiratory rhythm generator functioned normally. Interruption of perfusate flow always immediately disturbed the opercular rhythm and resulted in respiratory arrest. 4) In conclusion, vascular perfusion is indispensable for the maintenance of breathing rhythm-generation in the isolated head of the carp; as the perfusate, the colloid-free physiological saline was found tenable for the long-term preservation of brain function in the carp.     

海藻糖对生物人工肝用人永生化肝细胞系C3A细胞低温的保存

背景：随着生物人工肝在临床上的应用,需要有一种方便、有效、实用的保护生物反应器装载细胞活力及功能的方法,设计有自主知识产权的生物人工肝用肝细胞的低温保护液迫在眉梢。 目的：验证海藻糖作为低温保护剂在4 ℃低温保存肝细胞的作用。 方法：采用C3A人永生化肝细胞系在不同低温保存液(DMEM培养液,乳酸钠林格氏液,加有不同浓度海藻糖的乳酸钠林格氏液,UW液)4 ℃保存24,48,72 h。复苏后行细胞活力、细胞损伤指标、氧自由基代谢相关指标等检测,并通过荧光双染法观察细胞凋亡情况。 结果与结论：不同低温保存液保存不同时间C3A细胞的活力、细胞损伤及细胞凋亡表现均有所不同,其中同时间点不同浓度海藻糖组均优于单用乳酸钠林格氏液组,但不如UW液组(P < 0.01)。而保存24 h的不同浓度海藻糖组及UW液组与乳酸钠林格氏液组相比差异无显著性意义(P > 0.05),提示海藻糖能有效减轻低温对肝细胞凋亡及缺血再灌注损伤的程度,可成为低温保存液中有效及重要的保护剂组分。     

自制移植肝低温保存液对氧自由基的影响

背景：在肝脏移植过程中,低温保存和缺血可导致肝脏产生氧自由基而损伤肝组织。 目的：研究自制器官保存液对低温保存大鼠肝脏氧自由基的影响,并与器官保存液的“金标准”UW液进行对比。 方法：16只9周龄SD大鼠随机分为实验组和对照组,每组8只。建立SD大鼠肝脏灌注模型,分别用自制器官保存液组和UW液组灌洗肝脏,取出肝脏置于4 ℃保存液中,于保存24,48,72 h后分别检测肝脏超氧化物歧化酶、一氧化氮合酶、总抗氧化能力和活性丙二醛水平。 结果与结论：自制器官保存液对大鼠肝脏低温保存各时点超氧化物歧化酶、一氧化氮合酶、总抗氧化能力活性及丙二醛含量与UW液组比较差异均无显著性意义(P > 0.05),表明自制器官保存液能够减轻缺血再灌注后氧自由基对大鼠肝脏的损伤。自制器官保存液对低温保存大鼠肝脏在减轻氧自由基损伤方面有较好的效果,与UW液相当。     

不同保存液对离体大鼠心脏保存效果的研究

目的:研究在冷缺血期3种液体对鼠心脏保存的效果。方法:将24只大鼠随机分为3组,每组8只。3组离体鼠心脏分别用Histidine-tryptophan-ketoglurate(HTK)液,UniversityofWisconsin(UW)液及St.ThomasⅡ(ST)液灌注停跳,并4℃冷冻保存6h。利用离体鼠心非循环式Langendorff灌流功能测定模型,测定左心室舒张期末压(LVEDP)、左心室发展压(LVDP)、左心室压力变化率(dp/dt_max,dp/dtmin)、冠脉流出量(CF),留取冠脉流出液检测心肌酶漏出量,并检测心肌线粒体三磷酸腺苷(ATP),取心肌标本测干湿比。结果:HTK组保存的心肌左心室功能恢复明显优于UW组(P＜0.05),UW组明显优于ST组(P＜0.05),LVEDP恢复HTK组和ST组无显著差异(P＞0.05),但均较UW组恢复好(P＜0.05)。保存后心肌ATP测定值HTK组高于UW组(P＜0.05),UW组高于ST组(P＜0.05)。3组心肌标本测干湿比结果无显著差异(P＞0.05)。结论:HTK液保存心肌效果最好,含有高钾的保存液对冠脉有损害。     

Oxygen uptake in saline-perfused rabbit heart is decreased to a similar extent during reductions in flow and in arterial oxygen concentration

In experiments reported in the literature, oxygen uptake in saline-perfused heart decreased after small reductions in arterial O₂ concentration (CaO₂) at constant perfusion flow. This may have resulted from the decrease in O₂ supply, but may also have been due to decreased O₂ demand caused by reduced perfusion pressure following hypoxic vasodilation (garden hose effect). We tested both possibilities in 8 isolated rabbit hearts, perfused with Tyrode solution at 37°C, perfusion pressure 94±4 mm Hg (mean ±SD). Vasodilation with 10 M adenosine in the perfusate prevented changes in perfusion pressure during hypoxia. Oxygen uptake decreased significantly by 5.8±2.1% for a 10% decrease inCaO₂ at constant flow, and by 4.4±1.8% per 10% decrease in flow at constantCaO₂. In both cases a 10% reduction in oxygen supply was applied and the decrease in oxygen uptake was not significantly different. The decrease in perfusion pressure during flow reduction did therefore not cause a detectable decrease in oxygen consumption via the garden hose effect in addition to the decrease caused by reduced oxygen supply. The data show that oxygen uptake in saline-perfused rabbit heart, at 37°C, is limited by O₂ supply.     

Hypothermia related changes in electrocortical activity at stepwise increase of intracranial pressure in piglets.

Recent experimental studies have demonstrated that mild hypothermia can be effective in the control of intracranial hypertension. However, investigations to analyze the effects of hypothermia on changes in brain oxygen metabolism and electrocortical activity caused by increased intracranial pressure (ICP) are lacking. We examined the effects of mild hypothermia on electrocorticogram (ECoG) in combination with measurement of regional cerebral blood flow (CBF) and estimation of brain oxygen metabolism during stepwise increase of ICP. For this purpose thirteen female piglets (14 days old, 4-5 kg b.w.) were anaesthetized and mechanically ventilated. An epidural balloon was gradually inflated in order to increase ICP to 25 mmHg, 35 mmHg and 45 mmHg every 30 minutes at adjusted mean arterial blood pressures (MAP). This procedure resulted in gradual cerebral perfusion pressure (CPP) reduction of about 70%, 50%, and 30% of baseline [baseline CPP: normothermia (NT) 80+/-3 mmHg; hypothermia (HT) 84+/-3 mmHg]. Control animals were maintained in a normothermic state (38.6+/-0.2 degrees C). HT animals were surface cooled and maintained at 31.9+/-0.1 degrees C. ECoG, regional CBF, cerebral oxygen delivery (cDO2) and the cerebral metabolic rate of oxygen (CMRO2) were estimated during the normothermic period, after hypothermic stabilization, and after the gradual CPP reductions. The baseline ECoG showed the typical delta-dominated frequency pattern for isoflurane anaesthesia. At the hypothermic level, a frequency shift was seen from delta activity towards the higher frequencies (theta- and alpha activity) and the total spectral power was significantly reduced (56+/-17% from baseline, p < 0.05). the cortical CBF decreased markedly to 67+/-10% (p < 0.05), whereas the medulla oblongata blood flow increased slightly. During controlled increase of ICP by regional mass expansion from epidural balloon inflation, we found at mild and moderate stages of ICP increase (25 and 35 mmHg) only minimal changes in the ECoG in hypothermic animals compared to the hypothermic baseline, whereas the ECoG in normothermic animals showed a marked decrease in frequency, amplitude and total spectral power. We conclude that mild hypothermia produces an arousal-like ECoG activity with marked frequency shift to alpha activity and a change from high to low voltage activity. Furthermore, the hypothermic brain showed a preserved neuronal function at moderate stages of ICP. Obviously, hypothermia improves the functional tolerance of the brain to impaired oxygen supply.