F De Somer

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.

Phosphorylcholine coating offers natural platelet preservation during cardiopulmonary bypass

Return of blood activated by tissue factor is the main culprit for triggering the coagulation cascade. When this activated blood is diverted from the cardiopulmonary bypass (CPB) circuit, it becomes possible to evaluate the effect of surface treatment on platelet and complement activation. Twenty adult patients undergoing elective coronary artery bypass grafting (CABG) were randomly assigned either to a control group ( n = 10) or to a group in which the CPB circuit was completely coated with phosphorylcholine ( n = 10). Plasma concentrations of platelet factor 4 (PF4), β-thromboglobulin (βTG), C3, C3d, C4, TCC, thrombin generation, haptoglobin and free haemoglobin, as well as blood loss, were measured. No significant differences between the two groups were found for haemolysis and thrombin generation. The mean total release of PF4 and βTG during CPB was 9338± 17303 IU/ml/CPB and 3790± 4104 IU/ml/CPB in the coated group versus 22192± 13931 IU/ml/CPB ( p = 0.011) and 8040± 3986 IU/ml/CPB ( p = 0.005) in the control group. Blood loss was 30% less in the coated group compared to the control group. Phosphorylcholine coating appears to have a favourable effect on blood platelets, which is most obvious after studying the changes during CPB. Clinically, this effect resulted in a 30% reduction in blood loss.

HYDRODYNAMICAL COMPARISON OF AORTIC ARCH CANNULAE

The high velocity of blood flow exiting aortic arch cannulae may erode atherosclerotic material from the aortic intima causing non-cardiac complications such as stroke, multiple organ failure and death. Five 24 Fr cannulae from the Sarns product line (straight open tip, angled open tip with and without round side holes, straight and angled closed tip with four rectangular, lateral side holes), and a flexible cannula used at the University Hospital of Gent (straight open tip) are compared in an in vitro steady flow setup, to study the spatial velocity distribution inside the jet. The setup consists of an ultrasound Doppler velocimeter, mounted opposite to the cannula tip in an outflow reservoir. An elevated supply tank supplies steady flow of 1.3 L/min of water. Exit forces at various distances from the tip are calculated by integrating the assessed velocity profiles. The pressure drop across the cannula tip is measured using fluid filled pressure transducers. The four sidehole design provides the lowest exit velocity (0.85 versus 1.08 m/s) and force per jet (0.03 vs 0.15–0.20 N). The round sideholes are useless as less than 1% of the flow is directed through them. Furthermore, the use of angled tip cannulae is suggested because the force exerted on the aortic wall decreases the more the angle of incidence of the jet deviates from 90°. Pressure drop is the lowest for the 4 side hole design and highest for the open tip and increases when an angled tip is used.

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.

Can an oxygenator design potentially contribute to air embolism in cardiopulmonary bypass? A novel method for the determination of the air removal capabilities of neonatal membrane oxygenators.

At present, air handling of a membrane oxygenator is generally studied by using an ultrasonic sound bubble counter. However, this is not a quantitative method and it does not give any information on where air was entrapped in the oxygenator and if it eventually was removed through the membrane for gas exchange. The study presented here gives a novel technique for the determination of the air-handling characteristics of a membrane oxygenator. The study aimed at defining not only the amount of air released by the oxygenator, but also the amount of air trapped within the oxygenator and/or removed through the gas exchange membrane. Two neonatal membrane oxygenators without the use of an arterial filter were investigated: the Polystan Microsafe and the Dideco Lilliput. Although the air trap function of both oxygenators when challenged with a bolus of air was similar, the Microsafe obtained this effect mainly by capturing the air in the heat exchanger compartment while the Lilliput did remove a large amount of air through the membrane. In conclusion, the difference in trap function was most striking during continuous infusion of air. Immediate contact with a microporous membrane, avoidance of high velocities within the oxygenator, pressure drop, transit time and construction of the fibre mat all contribute to the air-handling characteristics of a membrane oxygenator.

Impact of oxygenator design on hemolysis, shear stress, and white blood cell and platelet counts

OBJECTIVE To determine whether relative pressure drop, shear stress, hemolysis, and white blood cell and platelet counts are influenced by different oxygenator designs. To compare the oxygenator results with the average shear stress over an arterial cannula. DESIGN Prospective; patients enrolled consecutively. SETTING University hospital. PARTICIPANTS Three groups of 12 adults patients, scheduled for routine cardiac surgery. INTERVENTIONS Each group was submitted to a different oxygenator design: group 1 to a high-pressure hollow-fiber membrane oxygenator (Sarns Turbo); group 2 to a medium-pressure hollow-fiber membrane oxygenator (Cobe optima); and group 3 to a flat-sheet membrane oxygenator (Cobe Duo). MEASUREMENTS AND MAIN RESULTS Although the investigated oxygenators have important differences in pressure drop and shear stress, no statistical differences were found in hemolysis generation or blood handling among the different groups. Actually, the study shows much higher shear stress levels over an average arterial cannula than over any of the evaluated oxygenators. CONCLUSIONS The pressure drop over an oxygenator does not correlate well with shear stress and hemolysis because the dimensions of the system (radius and length) must be included in the calculation of shear stress from pressure drop.

Clinical evaluation of a new hollow fibre membrane oxygenator

Twelve consecutive patients undergoing elective cardiac surgery were perfused with the Cobe Optima hollow fibre oxygenator. Gas transfer characteristics and blood handling were studied. The device had a maximum oxygen transfer of 315 ml/minute. The average shunt fraction was 4.5%, and was not influenced by blood-flow rate. Mean platelet count declined slightly to 91 % of the baseline at the end of the study period. Haemolysis was evaluated by monitoring serum-free haemoglobin, serum haptoglobin and serum haemopexin. The evolution was as follows: free haemoglobin increased from 14 ± 5 mg/100 ml to 85 ± 0.8/100 ml (p = 0.01) at the end of bypass; haptoglobin decreased from 1.33 ± 0.90 g/l to 0.89 ± 0.15 g/l (p = 0.01); and haemopexin decreased from 0.84 ± 0.13 g/I to 0.74 ± 0.15 g/l (p = nonsignificant). In all patients the residual capacity of serum haptoglobin to protect against haemolysis was satisfactory. All patients had an uneventful postoperative course.

Does contemporary oxygenator design influence haemolysis

Based on their design, all membrane oxygenators generate a certain resistance to flow. In clinical practice, this resistance is calculated by measuring both blood flow and the pressure drop over the oxygenator. Historically, some designs, such as the Kolobow spiral coil oxygenator and the Cobe flat sheet oxygenator, had quite a high pressure drop, but were, nevertheless, considered very haemocompatible. Today, both medium and low pressure drop oxygenators are commercially available. Based on physics and the existing literature, this review aims to investigate whether pressure drop by itself can be considered an independent factor of haemolysis.Based on their design, all membrane oxygenators generate a certain resistance to flow. In clinical practice, this resistance is calculated by measuring both blood flow and the pressure drop over the oxygenator. Historically, some designs, such as the Kolobow spiral coil oxygenator and the Cobe flat sheet oxygenator, had quite a high pressure drop, but were, nevertheless, considered very haemocompatible. Today, both medium and low pressure drop oxygenators are commercially available. Based on physics and the existing literature, this review aims to investigate whether pressure drop by itself can be considered an independent factor of haemolysis.

Nitric oxide donors attenuate increase in pressure drop across membrane oxygenators

We have evaluated the effect of nitric oxide (NO) on the pressure drop across a membrane oxygenator in one in vitro model and two in vivo models (using four dogs and five pigs). In all the experiments sodium nitroprusside (SNP) was used as a NO source, whereas gaseous NO was only used in the in vitro model. The drugs were given when the pressure drop or resistance across the device increased to at least twice the baseline values. In the in vitro model, both SNP and gaseous NO decreased the pressure drop to 75% of its peak value after 10 min and to 67% after 20 min. In the dog model, resistance decreased from 390 to 153 mmHg/l/min after 5 min and to 85 mmHg/l/min after 20 min for a baseline value of 75 mmHg/l/min. The initial resistance across the membrane oxygenator in the pig model increased from 6.6 ± 1.3 to 74 ± 38 mmHg/l/min. An infusion of 10 μg/kg/min SNP reduced the resistance to 16 ± 5 mmHg/l/min.

Feasibility of a pumpless extracorporeal respiratory assist device

BACKGROUND Our study evaluated the efficacy and feasibility of a pumpless respiratory assist device and determined its capacity for carbon dioxide removal. METHODS In five adult pigs the left femoral vein and artery were cannulated with a 20F cannula and connected to a low-pressure hollow-fiber artificial lung. After we had obtained baseline values of mean arterial pressure, cardiac output, and blood flow across the artificial lung, the mean arterial pressure was reduced 20% and 40% relative to baseline; in a second phase, it was raised 20% and 40. Cardiac output and artificial lung flow were simultaneously recorded. We determined the carbon dioxide removal capacity of the artificial lung by gradually increasing the arterial partial carbon dioxide tension of the animal. RESULTS An increase of 10 mm Hg in mean arterial pressure resulted in an increase of flow of 0.14 L/min. The mean pressure drop across the artificial lung was measured at 17 +/- 9 mm Hg. The shunt flow over the artificial lung varied between 14 and 25% of the cardiac output of the animal. Depending on inlet conditions, carbon dioxide removal by the artificial lung was between 62 +/- 22 mL/L/min and 104 +/- 25 mL/L/min. CONCLUSIONS A pumpless respiratory assist device can remove a significant proportion of the metabolic carbon dioxide production. However, adequate mean arterial pressure is mandatory to maintain sufficient flow across the device. The technique seems attractive because of its simplicity and can be used in acute lung injury in conjunction of apneic oxygenation for prolonged respiratory support.