Cardiopulmonary bypass and long-term neurocognitive dysfunction in the rat

Neurologic and neurocognitive complications after cardiac surgery with cardiopulmonary bypass (CPB) have been reported repeatedly. To better understand its etiology and design protective strategies, an appropriate animal model may prove useful. Although impaired short-term neurocognitive function has been recently demonstrated after CPB in rats, the demonstration of persistent long-term neurocognitive changes would be more relevant from a clinical perspective. We hypothesized that CPB results in long-term impairment of neurocognitive performance in rats. Male rats were exposed to either 60 min of normothermic non-pulsatile CPB, using a roller-pump and a neonatal membrane oxygenator, or to cannulation only (sham animals). Long-term neurocognitive function was assessed at 4 to 7 weeks after CPB (Can test), and again after 12 weeks (Morris water maze) in both operated groups and in a non-operated control group, followed by histologic evaluation of the hippocampus. In separate groups of CPB and sham animals, we also measured TNF-alpha and IL-6 in plasma. There were no significant differences in long-term neurocognitive performance or histological outcome between the three groups. Cytokine patterns were also similar in both operated groups. We conclude that CPB did not appear to cause long-term neurocognitive dysfunction in this model of CPB in young healthy rats. The lack of long-term deficits may be due to the absence of clinically important etiologic factors such as atheromatous and gaseous embolization in this model. Similar cytokine patterns in both operated groups suggest that surgical trauma rather than exposure of blood to extra-corporeal circuit was probably responsible for the inflammatory response.

Comparison of three commercially available hollow fiber oxygenators: gas transfer performance and biocompatibility

The new generation of oxygenators have improved blood flow pathways that enable reduction in priming volume and, thus, hemodilution during cardiopulmonary bypass (CPB). We evaluated three oxygenators and two sizes of venous reservoirs in relation to priming volume, gas transfer, and blood activation. To compare priming volume, gas transfer, and biocompatibility of three hollow fiber oxygenators and two different size venous reservoirs, 60 patients were randomly allocated in groups to undergo cardiopulmonary bypass. In each group, an oxygenator with a different surface area and priming volume was used: 1.8 m2 and 220 ml (group 1, n = 23), 2.2 m2 and 290 ml (group 2, n = 20), and 2.5 m2 and 270 ml (group 3, n = 17). In groups 1 and 3, a large soft shell (1900 ml) venous reservoir was used, whereas in group 2, a smaller soft shell (600 ml) venous reservoir was used. Gas transfer was assessed by calculating the oxygen transfer rate for each group and per square meter for each oxygenator group. Partial arterial oxygen pressure (paO2) and partial arterial carbon dioxide pressure (paCO2) between the groups were assessed with forward stepwise regression analysis. Biocompatibility was evaluated through measurement of platelet numbers, complement activation products (C3b/c), coagulation (thrombin anti-thrombin III complex), and fibrinolysis (plasmin anti-plasmin complex). No differences were found in oxygen transfer rate per group. However, when correcting the oxygen transfer rate for surface area, group 1 demonstrated a higher oxygen transfer rate compared with group 2 (p < 0.05) at an FiO2 of 40 and 60% and compared with group 3 at an FiO2 of 60 and 70%. The regression analysis showed that the average arterial PO2 was the highest in group 3, i.e., 79.2 mm Hg higher than in group 1 (p < 0.001) and 73.5 mm Hg higher than in group 2 (p < 0.001). Group 3 also had the lowest average arterial pCO2, 0.57 mm Hg lower than in group 1 (p = 0.004) and 0.81 mm Hg lower than in group 2 (p < 0.001). During CPB, platelet numbers decreased significantly in all groups (p < 0.001), without differences between the groups. C3b/c levels increased in all groups during CPB. At cessation of CPB the C3b/c level in group 2 (398 nmol/L(-1)) was significantly higher compared to group 1(251 nmol/L(-1); p < 0.05) and group 3 (303 nmol/L(-1); p < 0.05). Thrombin anti-thrombin III complexes and plasmin anti-plasmin complex complexes increased during CPB to significantly high levels at cessation of CPB, but there were no differences between the groups. The oxygenator with the smallest surface area and lowest priming volume (group 1) had the highest oxygen transfer rate per square meter and showed the least blood damage, as depicted by complement activation. The oxygenator with the largest blood contact surface area and improved geometric configuration (group 3) showed the lowest oxygen transfer rate per square meter. However, this oxygenator elevated oxygen partial pressure the most and reduced carbon dioxide partial pressure the most. In group 2, where a smaller venous reservoir was used, the highest blood activation was observed.

Phosphorylcholine coating of extracorporeal circuits provides natural protection against blood activation by the material surface

OBJECTIVE

The aim of this study is to evaluate the use of a new coating, mimicking the outer cell membrane, in paediatric cardiac surgery.

METHODS

Two groups of ten patients with a body weight below 8 kg, undergoing elective cardiac operations for different congenital anomalies, were prospectively enrolled in this study. In one group the whole extracorporeal circuit, including the cannulas, was coated with phosphorylcholine (PC). In the second group the same circuit was used without coating. Platelet activation (thromboxane B2 (TXB2), beta-thromboglobulin (betaTG)), activation of the coagulation system (F1+2), leukocyte activation (CD11b/CD18) and terminal complement activation (TCC) were analyzed pre-cardiopulmonary bypass (CPB), at 15, 60 min of CPB, at the end of CPB, 20 min post CPB and at postoperative day 1 and 6.

RESULTS

No statistical differences were found for F1+2 and CD11b/CD18. After onset of CPB mean levels of TCC remained stable in the PC group whereas an increase was observed in the control group. During CPB betaTG values in both groups increased to a maximum at the end of CPB. Within groups the increase in betaTG levels during CPB was statistically significant (P<0.05) from baseline in the control group starting from 60 min of CPB whereas no statistical difference was observed in the PC group. After the start of CPB TXB2 mean levels increased to 405+/-249 pg/ml in the PC group vs. 535+/-224 pg/ml in the control group. After this initial increase there was a small decline in the PC group with further increase. This was in contrast to the control group were TXB2 levels further increased up to a mean of 718+/-333 pg/ml at the end of CPB (P=0.016).

CONCLUSIONS

Phosphorylcholine coating had a favourable effect on blood platelets, which is most obvious after studying the changes during cardiopulmonary bypass. A steady increase of TXB2 and betaTG was observed in the control group, whereas plateau formation was observed in the phosphorylcholine group. Clinically, this effect may contribute to reduced blood loss and less thromboembolic complications. Complement activation is lower in the coated group.

Pressure drop, shear stress, and activation of leukocytes during cardiopulmonary bypass: a comparison between hollow fiber and flat sheet membrane oxygenators

The membrane oxygenator is known to be superior to the bubble oxygenator, but little information is available about the difference between the hollow fiber and flat sheet membrane oxygenators with regard to pressure drop, shear stress, and leukocyte activation. In this study, we compared these 2 types of membrane oxygenators in patients undergoing cardiopulmonary bypass (CPB) surgery with special focus on leukocyte activation and pressure drop across the oxygenators. Plasma concentration of elastase, a marker indicating leukocyte activation, increased to 593+/-68% in the flat sheet oxygenator group versus 197+/-42% in the hollow fiber oxygenator group (p<0.01) at the end of CPB compared to their respective baseline concentrations before CPB. Pressure drop across the oxygenator was significantly higher in the flat sheet group than in the hollow fiber group throughout the entire period of CPB (p<0.01). High pressure drop across the oxygenator as well as the calculated shear stress was positively correlated with the release of elastase at the end of CPB (r = 0.760, p<0.01, r = 0.692, p<0.01). However, this positive correlation existed in the flat sheet oxygenator but not in the hollow fiber oxygenator. Clinically, both membrane oxygenators have satisfactory performance in O2 and CO2 transfer. These results suggest that a higher pressure drop across the flat sheet oxygenator is associated with more pronounced activation of leukocytes in patients undergoing cardiopulmonary bypass.

Induced cell trauma during in vitro perfusion: a comparison between two different perfusion systems

The purpose of this study was to compare blood cell activation during in vitro long-term perfusion using 2 parallel in vitro extracorporeal membrane oxygenation (ECMO) systems. We compared two substantially different perfusion systems, an assistance respiratoire extra corporelle (AREC) system on one hand, containing an AREC pump, silicon tubing, and a hollow-fiber oxygenator, and a centrifugal pump system, on the other hand, containing a Biomedicus centrifugal pump, PVC tubing, and a membrane oxygenator. We measured the platelet count using an automated blood cell counter. Platelet activation was evaluated using a flow cytometric technique for the platelet membrane expression of glycoproteins and ELISA for the plasma concentration of beta-thromboglobulin (beta-TG), a platelet specific protein released into the blood upon platelet activation. The neutrophil count was assayed using an automated blood cell counter and the plasma concentration of cytokines using an ELISA kit. A significant difference between the two systems was observed in terms of the platelet membrane expression of glycoprotein (GP)Ib (p=0.0001) and GPIIb/IIIa (p=0.0037), indicating a lower degree of platelet activation in the AREC system. The concentration of neutrophils was significantly lower in the centrifugal system (p=0.002) compared to the AREC system. The neutrophil membrane expression of CD11b was significantly lower (p=0.0067) in the AREC system, indicating a lower degree of neutrophil activation compared to the centrifugal pump system. A significantly lower degree of hemolysis, as expressed by plasma hemoglobin, was observed in the AREC pump system (p=0.0491). In conclusion, lower degrees of the platelet membrane expression of GPIb and GPIIb/IIIa and of the neutrophil membrane expression of CD11b were observed in the AREC system, indicating a lower degree of platelet and neutrophil activation in this system. No significant difference between the two systems as to the plasma concentration of interleukin (IL)-1beta, IL-6, or IL-8 could be recorded. Further studies are warranted to specify the role of each individual component of the two systems.

Complement activation during major surgery: the effect of extracorporeal circuits and high-dose aprotinin

OBJECTIVE

To assess the in vivo contribution to complement activation of an extracorporeal circuit and the use of high-dose aprotinin during major surgery.

DESIGN

Sequential samples were obtained from 8 patients undergoing thoracic surgery, 20 patients undergoing orthotopic liver transplantation (OLT) using venovenous bypass, and 19 patients undergoing cardiac surgery using cardiopulmonary bypass (CPB).

INTERVENTION

The latter two groups were part of a randomized controlled trial of high-dose aprotinin.

MEASUREMENTS

Total complement activation was measured with the hemolytic complement activity and the C3 activation-specific marker, C3d antigen.

MAIN RESULTS

Complement activation did not occur during thoracic surgery. During OLT, C3d antigen levels, expressed as mean +/- standard deviation (SD), were elevated from baseline at skin closure (8.6 +/- 2.5 v 13.0 +/- 5.2 mg/L; p = 0.0082). During cardiac surgery, C3d antigen levels increased 10 minutes after the start of CPB (pre-CPB, 8.0 +/- 1.9 v 14.2 +/- 3.1 mg/L; p = 0.0001) and remained at greater than baseline values postoperatively (8.0 +/- 1.9 v 11.8 +/- 2.3 mg/L; p = 0.002). There was no difference in complement activation in those receiving high-dose aprotinin during OLT or cardiac surgery. Complement activation during cardiac surgery using extracorporeal circulation occurred to a greater extent than during OLT and thoracic surgery. Complement activation during cardiac surgery or OLT was not attenuated by the use of high-dose aprotinin.

Biocompatibility of three different membrane oxygenators: effects on complement, neutrophil and monocyte activation

The inflammatory reaction of extracorporeal circuits can be assessed by measuring complement activation and the release of activation markers of leucocytes. The purpose of this study was to compare three commercially available membrane oxygenators with respect to complement (C3a), granulocyte (lactoferrin) and monocyte (interleukin-6, IL-6) activation. Thirty patients undergoing cardiac surgery were randomly assigned to undergo cardiopulmonary bypass (CPB) with one of the following oxygenators: a polypropylene hollow-fibre membrane (group 1; 2.2 m2), a polypropylene flat-sheet membrane (group 2; 3.1 m2) or a silicone envelope membrane (group 3, 3.5 m2). In all patients, a significant increase in C3a in plasma occurred during CPB with peak levels after the administration of protamine sulphate. In blood samples taken before aortic crossclamp release, at the end of CPB, and 20 min after protamine administration C3a was significantly lower in group 1 than in the other two groups. Lactoferrin increased significantly during CPB in all patients without a significant difference between the groups. IL-6 did not increase during CPB, but raised significantly after 4 h in the intensive care unit in all groups. Moreover, IL-6 was significant lower in group 1 than group 3. The data suggest that the polypropylene hollow-fibre membrane oxygenator, i.e. the oxygenator with the smallest surface area, is more biocompatible than the other types, probably because of a smaller contact surface area.

Extracorporeal circulation, hemocompatibility, and biomaterials

BACKGROUND

Performance of a majority of cardiac surgical procedures requires the use of extracorporeal circulation. Contact of the patients' blood with the nonendothelial surface of the cardiopulmonary bypass circuit is responsible for several, potentially harmful systemic reactions.

METHODS

The patients' response to extracorporeal circulation is reviewed briefly. The interactions between patient and circuit are discussed not only as they relate to blood-material contact, but also from a mechanical and rheologic standpoint. The theoretic benefits of the newer, more hemocompatible materials are presented, along with a review of published clinical experience with heparinized cardiopulmonary bypass circuits.

RESULTS

The response to extracorporeal circulation extends far beyond a simple derangement of hemostasis. This inflammatory response is strongly influenced by the rheologic design of the circuit and by the physical and chemical properties of the surface. Heparinized circuits decrease inflammation, but the clinical benefits of this reduction remain unclear, except for extended cardiopulmonary support. The safe use of these circuits requires full heparinization and does not reduce allogeneic transfusions.

CONCLUSIONS

Clinicians are still in the search of the ideal material and the ideal extracorporeal circuit design. Newer, heparinized materials offer real but limited clinical benefits.

Low extracorporeal priming volumes for infants: a benefit?

An extracorporeal circuit consisting of an oxygenator especially designed for neonatal use and appropriately sized tubing, with an average total priming volume of 205 ml, was used on 80 infants undergoing cardiac surgery for congenital heart-disease. The priming volume and foreign surface area of the circuit were determined. The influence of low priming volumes on the use of blood products and the management of cardiopulmonary bypass was studied. No whole blood or platelets were used in this study. The mean volume of packed red blood cells used over the hospital stay was 202 +/- 67 ml. The mean volume of fresh frozen plasma (FFP) used until the second postoperative day was 62 +/- 72 ml. The mean total blood loss until the second postoperative day was 15.8 +/- 9.2 ml/kg. The priming volume of the extracorporeal circuit was 62% lower than values commonly reported in the literature. The low priming volume had a strong influence on the use of platelets and FFP and to a lesser extent on the use of packed red blood cells.