Brain oedema and blood-brain barrier permeability in pulsatile and nonpulsatile cardiopulmonary bypass

The Neurovascular Unit: Effects of Brain Insults During the Perinatal Period

The neurovascular unit (NVU) is a relatively recent concept in neuroscience that broadly describes the relationship between brain cells and their blood vessels. The NVU incorporates cellular and extracellular components involved in regulating cerebral blood flow and blood-brain barrier function. The NVU within the adult brain has attracted strong research interest and its structure and function is well described, however, the NVU in the developing brain over the fetal and neonatal period remains much less well known. One area of particular interest in perinatal brain development is the impact of known neuropathological insults on the NVU. The aim of this review is to synthesize existing literature to describe structure and function of the NVU in the developing brain, with a particular emphasis on exploring the effects of perinatal insults. Accordingly, a brief overview of NVU components and function is provided, before discussion of NVU development and how this may be affected by perinatal pathologies. We have focused this discussion around three common perinatal insults: prematurity, acute hypoxia, and chronic hypoxia. A greater understanding of processes affecting the NVU in the perinatal period may enable application of targeted therapies, as well as providing a useful basis for research as it expands further into this area.

Fluid distribution kinetics during cardiopulmonary bypass

OBJECTIVE

The purpose of this study was to examine the isovolumetric distribution kinetics of crystalloid fluid during cardiopulmonary bypass.

METHODS

Ten patients undergoing coronary artery bypass grafting participated in this prospective observational study. The blood hemoglobin and the serum albumin and sodium concentrations were measured repeatedly during the distribution of priming solution (Ringer's acetate 1470 ml and mannitol 15% 200 ml) and initial cardioplegia. The rate of crystalloid fluid distribution was calculated based on 3-min Hb changes. The preoperative blood volume was extrapolated from the marked hemodilution occurring during the onset of cardiopulmonary bypass. Clinicaltrials.gov: NCT01115166.

RESULTS

The distribution half-time of Ringer's acetate averaged 8 minutes, corresponding to a transcapillary escape rate of 0.38 ml/kg/min. The intravascular albumin mass increased by 5.4% according to mass balance calculations. The preoperative blood volume, as extrapolated from the drop in hemoglobin concentration by 32% (mean) at the beginning of cardiopulmonary bypass, was 0.6-1.2 L less than that estimated by anthropometric methods (p<0.02). The mass balance of sodium indicated a translocation from the intracellular to the extracellular fluid space in 8 of the 10 patients, with a median volume of 236 ml.

CONCLUSIONS

The distribution half-time of Ringer's solution during isovolumetric cardiopulmonary bypass was 8 minutes, which is the same as for crystalloid fluid infusions in healthy subjects. The intravascular albumin mass increased. Most patients were hypovolemic prior to the start of anesthesia. Intracellular edema did not occur.

On-pump coronary artery bypass graft surgery: biochemical, hormonal and cellular features

OBJECTIVE

The authors sought to assess biochemical, hormonal and cellular repercussions from use of cardiopulmonary bypass (CPB) in coronary artery bypass graft (CABG) surgery.

METHODS

Eighteen patients underwent on-pump CABG surgery. Mean time of CPB was 80.3 minutes. Hormonal, biochemical and cellular measurements were taken in some time points - preoperatively, immediately after coming off CPB, 24 and 48 hours postoperatively. Friedman and Wilcoxon tests were applied based on significance level of 5%.

RESULTS

There was activation and significant elevation of total leukocytes and neutrophils count over CPB, remaining this way up to 48 hours postoperatively. Total platelets count, in turn, was marked by relevant reduction immediately after coming off CPB as well as in two postoperative time points. Serum levels of total proteins and albumin, immediately after coming off CPB and also in two postoperative time points, were significantly decreased comparing with preoperative status. There was remarkable reduction of total T3, free T3 and total T4 particularly up to first 24 hours postoperatively.

CONCLUSION

In on-pump CABG surgery, inflammatory effects encompass activation of total leukocytes, neutrophils and platelets, reduction of serum level of total proteins and albumin and decreased thyroid hormones levels, especially within first postoperative 24 hours.

Is There a Future for Neuroprotective Agents in Cardiac Surgery?

This article gives an overview of neuroprotective drugs that were recently tested in clinical trials in cardiac surgery. Also, recommendations are given for successful translational research and considerations for management during cardiac surgery.

Cardiopulmonary bypass increases permeability of the blood-cerebrospinal fluid barrier

BACKGROUND

The integrity of the blood-cerebrospinal fluid (CSF) barrier during cardiopulmonary bypass (CPB) with hypothermic circulatory arrest (HCA) has not been systematically studied, especially in children. We tested the hypothesis that the blood-CSF barrier is disrupted by CPB.

METHODS

The study randomized 25 piglets (mean weight, 11 kg) to five groups (5 per group): anesthesia alone (control); CPB at 37 degrees C with full-flow (FF); CPB at 25 degrees C with very low flow (LF); and HCA at 15 degrees C and 25 degrees C. pH-stat strategy was applied during CPB. An epidural catheter was inserted into the cisterna magna for collection of CSF. CSF and blood samples were collected at seven points: after induction of anesthesia (baseline), at 10, 50 and 115 minutes after start of CPB, just before the end of CPB, and at 30 and 120 minutes after CPB. Albumin levels in CSF and plasma were measured to assess blood-CSF barrier integrity and the albumin ratio (CSF/plasma) was calculated (Q(Alb)).

RESULTS

In both HCA groups, the Q(Alb) was significantly higher than in the control and 37 degrees C FF groups (all p < 0.05), whereas Q(Alb) in the 37 degrees C group was not significantly different vs control.

CONCLUSIONS

The blood-CSF barrier is impaired by CPB with 1 hour of 15 degrees C or 25 degrees C HCA. Further investigations are needed to understand the behavior of the blood-CSF barrier during CPB and its role in neuroprotection.

Blood-brain barrier integrity in a rat model of emergency preservation and resuscitation

Emergency Preservation and Resuscitation (EPR) represents a novel approach to treat exsanguination cardiac arrest (CA) victims, using an aortic flush to induce hypothermia during circulatory arrest, followed by delayed resuscitation with cardiopulmonary bypass (CPB). The status of the blood-brain barrier (BBB) integrity after prolonged hypothermic CA is unclear. The objective of this study was to assess BBB permeability in two EPR models in rats, associated with poor outcome. Rats subjected to traumatic brain injury (TBI) and naïve rats served as positive and negative controls, respectively.

HYPOTHESIS

The BBB will be disrupted after TBI, but intact after prolonged hypothermic CA.

METHODS

Four groups were studied: (1) EPR-IC (ice cold)-75 min CA at 15 degrees C; (2) EPR-RT (room temperature)-20 min CA at 28 degrees C; (3) TBI; (4) sham. Rats in EPR groups were subjected to rapid hemorrhage, followed by CA. Rats in the TBI group had a controlled cortical impact to the left hemisphere. Naïves were subjected to the same anesthesia and surgery. 1h after insult, rats were injected with Evans Blue (EB), a marker of BBB permeability for albumin. Rats were sacrificed after 5h and EB absorbance was quantified in brain samples.

RESULTS

TBI produced an approximately 10-fold increase in EB absorbance in the left (injured) hemisphere vs. left hemisphere for all other groups (p=0.001). In contrast, EB absorbance in either EPR group did not differ from sham.

CONCLUSION

BBB integrity to albumin is not disrupted early after resuscitation from prolonged CA treated with EPR. Neuroprotective adjuncts to hypothermia in this setting should focus on agents that penetrate the BBB. These findings also have implications for deep hypothermic circulatory arrest.

Emboli formation rather than inflammatory mediators are responsible for increased cerebral water content after conventional and assisted beating-heart myocardial revascularization in a porcine model

BACKGROUND AND PURPOSE

Emboli and proinflammatory mediators are suspected of generating cerebral edema after coronary surgery. In contrast to cardiopulmonary bypass (CPB), off-pump coronary artery bypass surgery (OPCAB) reduces microemboli count and proinflammatory mediator release but carries the risk of hemodynamic instability. A microaxial blood pump can augment cardiac output.

METHODS

Coronary bypasses were constructed in pigs with CPB and cardioplegia (n=9), OPCAB (n=9), or blood-pump support CAB (n=9). Nine animals underwent sham operation. Embolus count was monitored and regional cerebral blood flow was assessed with microspheres in 21 brain specimens per animal (n=189 per group). Interleukins 6 and 8 and tumor necrosis factor-alpha concentrations were determined. These variables were studied before, during, and for 4 hours after surgery. Finally, cerebral water content was determined.

RESULTS

During CPB and blood-pump CAB, a significant number of emboli were counted in contrast to OPCAB and controls (P<0.05). During CPB, regional cerebral blood flow was affected (32 of 189) and showed reactive hyperemia except in 10 specimens after aortic cross-clamp release. This impairment persisted in 20 specimens. During and after OPCAB, regional cerebral blood flow remained nearly unchanged but showed low flow during (58 of 189) and after (35 of 189) the blood-pump run. A significant increase in proinflammatory mediators was observed only in the CPB group. CPB and blood-pump CAB significantly increased cerebral water content (P<0.05). A strong correlation between embolic load and cerebral water content was observed in all groups. No correlation between proinflammatory mediator release and cerebral water content was detected.

CONCLUSIONS

Emboli formation rather than inflammatory mediators are responsible for increased cerebral water content after conventional and assisted beating-heart myocardial revascularization.

A recovery model of minimally invasive cardiopulmonary bypass in the rat

This study was undertaken to develop a rodent (rat) model of cardiopulmonary bypass (CPB) that has been designed to mimic functionally the minimally invasive clinical setting. The circuit is similar to the clinical model in terms of its construction, configuration, material surface area to blood volume ratio, and priming volume to blood ratio. The overall priming volume was 10 mL. Thirty-six male Sprague-Dawley rats (422 +/- 32 g) were anaesthetized while maintaining spontaneous ventilation. Anticoagulation was achieved with heparin (500 IU/kg). Blood arterial pressure was monitored continuously. Normal central temperature was maintained throughout. Intermittent arterial blood gas levels also were monitored. All animals were cannulated in preparation for CPB; however, CPB, utilizing a double roller pump and a flow rate of 100 mL/kg/minute for 60 min, was initiated in only 18 animals, the remaining 18 animals acting as non-CPB controls (Sham). The animals were haemodynamically stable. After the operative procedure, the animals were allowed to recover from the anaesthesia and, after transfer to a recovery facility, were monitored for a period of 1 week. There were no differences between the groups in terms of blood gas analysis and blood pressure data; all animals survived the procedure and had an uneventful follow-up. Differences were found between the CPB animals and the Sham group in terms of TNFalpha used as a marker of inflammatory processes. This trend tends to support this model as an analogue for the clinical scenario for future studies of CPB-related inflammation. Overall, the CPB procedure was easy to perform and was associated with excellent survival. This recovery model is an effective tool to perform pathophysiological studies associated with minimally invasive CPB.

Arterio-jugular Differences in Serum S-100β Proteins in Patients Receiving Selective Cerebral Perfusion

PURPOSE

The early increase in serum S100beta after cardiopulmonary bypass (CPB) seems to be derived from an extracerebral source. To exclude contamination, we investigated the arterio-jugular differences in S100beta levels in patients receiving selective cerebral perfusion (SCP). We also evaluated the brain-protective effect of SCP by comparing the arterial S100beta levels with those in patients undergoing coronary artery bypass grafting (CABG).

METHODS

We measured arterial and jugular venous levels of S100beta in ten patients undergoing aortic arch repair with SCP for up to 12 h postoperatively (SCP group). We also measured arterial levels of S100beta in nine patients undergoing CABG (CPB group).

RESULTS

There was no incidence of hospital death or stroke. The arterial levels of S100beta in both groups were comparable and peaked just after the conclusion of CPB. The arterial and jugular venous levels of S100beta were almost equivalent. The arterio-jugular differences in S100beta levels were negligible, even in our SCP-group patient with postoperative delirium, who had a peak value three times higher than the other patients.

CONCLUSIONS

The arterio-jugular differences in S100beta did not clarify the origin of their increase. Thus, measuring the jugular venous levels of S100beta in patients without postoperative clinical neurological deterioration would be of little benefit. However, SCP seems to protect the brain against S100beta release as effectively as conventional CPB.

Increased Transcription Factor Expression and Permeability of the Blood Brain Barrier Associated With Cardiopulmonary Bypass in Lambs

BACKGROUND

The pathophysiology of neurocognitive dysfunction and developmental delay after cardiopulmonary bypass (CPB) in infants is not known. It is known that head trauma, stroke, and seizures cause dysfunction of the blood brain barrier (BBB) that is associated with increased inducible transcription factor gene expression in the cells of the barrier. The purpose of this study was to determine the effects of CPB and hypothermic circulatory arrest on expression of the transcription factor FOS and the function of the BBB in an infant animal model.

METHODS

Infant lambs (n = 36; 10-12 days) were exposed to 0, 15, 30, 60, or 120 minutes of normothermic (38 degrees C) CPB or 2 hours of hypothermic circulatory arrest at 16 degrees C. After terminating bypass 15 animals had their brains perfusion-fixed and removed for immunohistochemical analysis of expression of the transcription factor FOS. The other animals were perfused with fluorescent albumin to visualize the brain microvasculature. Brain sections were analyzed with a laser scanning confocal microscope.

RESULTS

Control animals (n = 6, sham operated and cannulated) exhibited normal vasculature with negligible leakage and no FOS protein expression in neurons or endothelial cells anywhere in the brain. Significant FOS expression in barrier-associated structures including the blood vessels, choroid plexus, and ependyma but not neurons occurred at all times on bypass. CPB caused leakage of fluorescent albumin from blood vessels in all animals. Two hours of normothermic CPB (n = 4) caused significant (p < 0.01) leakage in the cerebellum, cortex, hippocampus, and corpus callosum. Animals exposed to circulatory arrest experienced severe leakage throughout the brain (p < 0.001) and FOS expression in all cells.

CONCLUSIONS

These experiments indicate that the BBB is dysfunctional after all time points on normothermic CPB, BBB dysfunction is worsened by hypothermic circulatory arrest, and BBB dysfunction is associated with intense molecular activity within the barrier-forming cells. Dysfunction of the BBB may contribute to neurologic complications after heart surgery.

Factors influencing the change in cerebral hemodynamics in pediatric patients during and after corrective cardiac surgery of congenital heart diseases by means of full-flow cardiopulmonary bypass

BACKGROUND

The pathophysiology of hypoxic-ischemic brain injury in relation to extracorporeal circulation is multifactorial and can be interpreted, in part, as possible alteration in cerebral perfusion and inadequate oxygen delivery to the brain cells. The aim of this study was to evaluate influencing factors on the change in cerebral blood flow velocity (CBFV) patterns determined by transcranial Doppler sonography (TCD) in infants who undergo corrective cardiac surgery by means of full-flow cardiopulmonary bypass (CPB).

METHODS

Included in the study were 67 neonates, infants, and children with a median age of 4 months (0.1-70 months), median weight of 4.8 kg (2.5-18.8 kg), and with cyanotic and noncyanotic congenital heart disease (CHD), who underwent surgical correction of CHD by means of CPB [flow rate 144 +/- 47 ml/kg body weight (BW)] and the alpha-stat strategy. The patients were divided into three groups with respect to the minimum rectal temperature during perfusion: deep hypothermic CPB (<18 degrees C) n=18, moderate hypothermic CPB (22-35 degrees C) n=29, normothermic CPB (36 degrees C) n=20. Continuous determination of mean flow velocity (Vmean) in the middle cerebral artery (MCA) by TCD provided qualitative on-line information on cerebral perfusion. The pulsatility index (PI) was calculated in accordance with the formula: Maximum flow velocity - end - diastolic flow velocity/ Mean flow velocity and was used as a parameter for the qualitative assessment of cerebrovascular resistance after the end of CPB.

RESULTS

The Vmean was significantly increased 15 min after cross-clamping in the normothermic group (p=0.03) and decreased in the moderate hypothermic group (p=0.02) and deep hypothermic group (p=0.009). The postoperative Vmean values correlated significantly with age (r=0.79, p<0.0001), weight (r=0.75, p<0.0001), bypass time (r=-0.51, p=0.0006), and minimum rectal temperature (r=0.60, p=0.0001). Mean arterial pressure and hemoglobin concentration, but not pCO2, seem to significantly influence the change in Vmean after the termination of CPB (r=0.5, p=0.001; r=-0.55, p=0.002, respectively). In comparison with the values at the start of CPB, the Vmean was significantly decreased after the end of CPB in the hypothermic and moderate hypothermic groups and still significantly elevated in the normothermic group. The age-independent PI was increased after termination of bypass in all groups (p<0.05) and still slightly elevated after the end of operation in the hypothermic group (p=0.05).

CONCLUSIONS

The changes in CBFV patterns before, during, and after the termination of CPB were dependent on age, weight, perfusion pressure, and degree of hypothermia during CPB.

A recovery model of partial cardiopulmonary bypass in the rat

This study was undertaken to develop a recovery model of cardiopulmonary bypass (CPB) in rats. Twenty male Wistar rats (475-550 g) were anaesthetized, mechanically ventilated and the femoral vessels cannulated. The extracorporeal circulation circuit comprised a roller pump, a venous reservoir and a modified Capiox 308 paediatric membrane oxygenator. Priming consisted of 20 ml of fresh homologous blood and 15 ml of colloid. Anticoagulation was achieved with heparin (500 IU/kg). Blood gas analysis, blood pressure monitoring and survival studies were performed in CPB (n=10) and Sham (n=10) rats. Partial CPB was always easily established and was conducted at a flow rate of 100 ml/kg/min for 90 min Blood gas analysis and blood pressure data did not differ between the two groups. All CPB rats survived and the 3-week follow-up period remained uneventful. The rat model of CPB was easy to perform and was associated with excellent survival. This recovery model should allow us to study the pathophysiological processes underlying post-CPB multiple organ dysfunction.

Cerebral emboli and serum S100beta during cardiac operations

BACKGROUND

The glial protein S100beta has been used to estimate cerebral damage in a number of clinical settings. The purpose of this investigation was to determine the correlation between cerebral microemboli and S100beta levels during cardiac operations.

METHODS

Transcranial Doppler ultrasonography was used to measure emboli in the right middle cerebral artery. Emboli counts (n = 111) were divided into five time periods: (1) incision to aortic cannulation; (2) aortic cannulation to cross-clamp onset; (3) cross-clamp onset to cross-clamp release; (4) cross-clamp release to decannulation; and (5) decannulation to chest closure. The level of S100beta (n = 156) was measured at baseline, at the end of cardiopulmonary bypass, then 150 and 270 minutes after cross-clamp release.

RESULTS

The level of S100beta correlated with age, cardiopulmonary bypass time, cross-clamp time, and number of emboli at time period 2. Although cardiopulmonary bypass time was univariately associated with S100beta level, it became nonsignificant in a multivariable model that included age and cross-clamp time.

CONCLUSIONS

The correlation of S100beta level with emboli measured during cannulation (time period 2) supports the hypothesis that cannulation is a high-risk time period for cerebral injury.

Morphological damage to the central nervous system (CNS) following open heart surgery

We investigated 40 patients who had died following open heart surgery between January 1990 and May 1992. Between this time, 703 open heart procedures were carried out at the University Medical School of Debrecen, 2nd Department of Surgery's Cardiac Surgery Unit with a mortality rate of 8.3%. We studied each individual's clinical records, autopsy findings, and histology of defined portions of brain tissue. Clinical information used were: sex, age, clinical stage (according to New York Heart Association (NYHA) classification) at time of surgery, type of procedure performed, and duration of time of extracorporeal circulation. Autopsy records collected and studied focused on the macroscopic and microscopic alterations in the CNS. The most common morphological findings in the CNS for the 40 studies cases were, cerebral edema in all cases, reactive microgliosis and nuclear pyknosis in most cases, and in a minority of the cases focal microscopic hemorrhage or focal microscopic white emolition. Also found were, destructive brain hemorrhage and global cerebral necrosis. Of the 40 cases studied there was only one case in which brain death was diagnosed perioperatively, and which was clinically diagnosed to be caused by global cerebral hypoxia.

Cerebral physiology during cardiopulmonary bypass: pulsatile versus nonpulsatile flow

In summary, neurological injury continues to be a significant source of perioperative morbidity and mortality in cardiac surgical procedures. To reduce the incidence and/or severity of these complications, a better understanding of the cerebral physiology of CPB is needed. Specifically, it is important to determine how the conduct of CPB either contributes to or modifies the response of the brain to neurological insults. Our research indicates that, in the uninjured brain, nonpulsatile perfusion per se does not appear disadvantageous in terms of brain blood flow or oxygen metabolism at 27 degrees C. Conversely, pulsatile perfusion does not appear to confer any special benefits. Whether pulsatility might influence neurological outcome in the presence of an ischemic insult remains to be determined.

Swine as a model for cardiovascular research: improved cardiopulmonary bypass techniques

A large-animal model is essential for the assessment of functional parameters in cardiovascular surgical research. To date the canine model has been used successfully because of its availability and tolerance to cardiopulmonary bypass. However, because of decreased availability and increased cost, an alternative animal model is now needed. The swine model has been used in experimental cardiac procedures, but complications during cardiopulmonary bypass have presented a formidable challenge. These complications include enormous fluid shifts from the vascular bed, increased metabolic acidosis, and marked hemoglobinuria. To eliminate these deleterious complications within the swine model, a number of technical alterations were achieved. The priming solution used for the extracorporeal circuit was altered to consist of 1000 mL lactated Ringer's solution. 500 mL 20% mannitol, 500 mL 6% dextran in 5% detrose solution. 50 mEq sodium bicarbonate, and 10,000 IU heparin. The extracorporeal circuit employed the use of membrane oxygenation. Three different blood flow rates (150, 175, and 200 mL/kg min-1) were studied. We conclude that the optimum blood flow rate for cardiopulmonary bypass in swine is in the range of 175-200 mL/kg min-1. Membrane oxygenation results in less damage to blood during cardiopulmonary bypass. The asanguinous hyperosmolar priming solution is beneficial for cardiopulmonary bypass in swine to greatly reduce fluid shifts, prevent metabolic acidosis, and eliminate hemoglobinuria.