Biocompatibility of standard and silica-free silicone rubber membrane oxygenators S. W. FOUNTAIN,
J. DUFFIN,
C. A. WARD,
H. OSADA,
B. A. MARTIN,
Departments of Surgery, Anaesthesia, and Mechanical Engineering Division of Thoracic Surgery at Toronto General Hospital, Toronto,
FOUNTAIN, S. W., J. DUFFIN, C. A. WARD, H. OSADA, B. A. MARTIN, AND J. D. COOPER. The biocompatibiZity of standard
and silica-free silicone rubber membraneoxygenators. Am. J. Physiol. 236(Z): H371-H375, 1979 or Am. J. Physiol.: Heart Circ. Physiol. 5(2): H371-H375, 1979.-Spiral coil membrane oxygenators made from either standard silicone rubber or silica-free silicone rubber were comparedusing three priming techniques. Standard priming, carbon dioxide priming, and denucleation priming, were employed with each type of device. Four-hour venovenous membraneoxygenator perfusionswere carried out on awake sheep anticoagulated with heparin. Virtually no differences were observed in any parameters measured between standard silicone rubber and filler-free silicone rubber membranes. Significantly greater platelet lossesoccurred during the first hour of perfusion with standard priming and with carbon dioxide priming than with denucleation priming, using either type of membrane. These experiments demonstrate that denucleation priming reduces platelet lossesduring extracorporeal membrane oxygenator perfusion, but that the use of filler-free silicone rubber does not improve the biocompatibility of the membrane. standard priming; CO, priming; denucleation priming; platelet losses
during extracorporeal membrane oxygenation (ECMO) depends on a number of incompletely understood factors. Two principal determinants, LOSS OF PLATELETS
however, are the type of membrane used and the priming technique employed. Silicone rubber is the material most commonly used in the construction of membrane oxygenators. Comparison of the properties of different silicone polymers and their performance with and without reinforcing silica filler has suggested that polydimethyl siloxane is the most favorable polymer and that the elimination of silica filler from the blood-contacting surface of the membrane further improves biocompatibility (7). Trapped air bubbles remaining on the surface of the membrane after priming have been strongly implicated in the process of platelet adhesion on subsequent exposure of the membrane to blood (11). The elimination of these gas nuclei from the surface of the membrane is a function of the priming technique used. With carbon dioxide (COz) priming, air is displaced by COz, which then dissolves more readily when the oxygenator is flushed with priming solution. In this study controlled experiments were conducted 0363-6135/79/0000-0000$01.25
Copyright
0 1979 the American
Physiological
AND J. D. COOPER
at the University of Toronto, Ontario 145G IL7, Canada
and
to compare commercially available filler-free silicone rubber (FFSR) membrane oxygenators and those made of standard silicone rubber (SSR) containing filler. This comparison was made using standard, COz, and denucleation priming techniques with each type of device. MATERIALS
AND METHOD$
The experimental. model has been previously described in detail (1, 4). Under ketamine sedation and local anesthesia, Suffolk sheep weighing approximately 30 kg were cannulated for venovenous perfusion using both common jugular veins. A Swan-Ganz (7F) catheter and a carotid arterial catheter were used to monitor pulmonary arterial and systemic blood pressures, respectively. After recovery from the anesthetic, the animal was connected to the perfusion circuit depicted in Fig. 1. A 2-h period of control ,perfusion was undertaken, during which time blood flow was diverted through the shunt ‘around the oxygenator. Flow was then directed through the membrane oxygenator for 4 h. fir institution of ECMO, flow was 500 ml/min for the first 20 min and 1 l/min thereafter. Gas flow through the oxygenator was maintained at 1 l/min of oxygen. At the time of cannulation, 1.25 mg/kg of heparin was given intravenously. This dose was repeated 30 min later and subsequently 0.5-l (mg/kg)/h was given; dosage was regulated according to the activated clotting time (ACT) as measured by the hemochron method (International Technidyne Corp., Edison, NJ). ACT was maintained between 400 and 600 s throughout the experiment. The perfusion circuit was primed with lactated Ringer solution containing 0.75 g/l of sodium bicarbonate and 40 mgll heparin. The volume of the circuit excluding the oxygenator was 900 ml, and its surface area was 0.18 m2. The circuit was constructed with silicone rubber tubing (Silastic, Dow Corning, Midland, MI) and a silicone rubber reservoir and bubble trap. Sections were connected by polycarbonate connectors (Cobe Laboratories, Denver, CO), New tubing and connectors were used for each experiment. A roller pump (Sarns, model 5M6002) in the occlusive mode was used to control the flow, The membrane oxygenators employed in this study were of the spiral-coil type, The oxygenators and both types of membrane were commercially manufactured (Sci-Med Life Systems, Minneapolis, MN) The SSR Society
H371
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H372
FOUNTAIN
supcrio/
vwa via
l
cavu h ft
FIG. 1. Schematic diagram of perfusion circuit. Shunt around the oxygenator was used during 2-h control perfusion.
membrane was cast in two layers of polymethyl vinyl siloxane containing reinforcing silica filler. The FFSR membrane was cast in two layers of polydimethyl siloxane, the first layer containing reinforcing silica filler and the second containing no filler. Both types had a surface area of 2.5 m2 and a priming volume of 300 ml. The three priming techniques used were as follows: Standard priming entailed filling the perfusion circuit and the oxygenator with the priming solution and circulating the priming solution through the oxygenator for 1 h. During this time the oxygenator was vigorously agitated manually to dislodge trapped air bubbles. The oxygenator was then excluded from the circuit in preparation for control perfusion. The CO, priming technique used in this study was designed to displace all air from microscopic crevices in the membrane surface by saturating the oxygenator with CO,. Figure 2 shows the connections for the CO, priming technique with tubing clamps isolating the oxygenator from the rest of the perfusion circuit. With clamp 1 closed, CO, was introduced into the blood phase through a filter (Cathivex 0.22-pm disposable filter), and its flow was adjusted to provide vigorous bubbling in the trap (l-3 l/min>. The vacuum pump was then activated and clamp 2 was adjusted so as to produce a vacuum of -200 mmHg. Next, clamp II was carefully opened until the vacuum had decreased to -150 mmHg. This adjustment was made slowly enough to ensure that bubbling was maintained in the trap and that CO, flow was adjusted if necessary. In this way CO, flowed through both blood and gas phases, and the 150 mmHg gradient from blood to gas phase facilitated removal of all air from the blood compartment. After 90 min of continuous CO, flushing, the oxygenator was filled with priming solution at point B. The
ET AL.
flow of priming solution was then controlled by varying the CO, flow. Vacuum on the gas phase was maintained during filling to keep the blood phase expanded. Vigorous agitation of the oxygenator was employed to prevent bubbles of CO, from being trapped. When the oxygenator was filled, the CO2 line was clamped shut at point A. At this time vacuum was disconnected by releasing clamp 1 and clamp 2 sequentially and, finally, by disconnecting the tubing from the outflow gas port. Carbon dioxide flow through the gas phase was maintained without vacuum until the oxygenator was inserted into the perfusion circuit, at which time oxygen was substituted for CO,. The denucleation priming technique has been previously described (9). In brief, the gas content of the priming solution was reduced by agitating it in a highvacuum chamber. The oxygenator was then placed in the high-vacuum chamber and filled with priming solution. The oxygenator was then removed from the vacuum chamber and the degassed priming solution circulated through the oxygenator with the gas phase attached to a high-vacuum pump. The pressures in the perfusion circuit both proximal and distal to the membrane oxygenator (Fig. 1) and the systemic and pulmonary arterial pressures were continuously monitored and displayed on a multichannel recorder (Litton Medical Products, Long Island, NY). Blood samples were withdrawn from the circuit, at a
FIG. 2. Schematic diagram of CO, priming technique. Tubing clamps isolate oxygenator from. perfusion circuit. Clamps 1 and 2 control gas flow through gas phase of the oxygenator. See text for details of priming technique.
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BIOCOMPATIBILITY
OF SILICONE
RUBBER
MEMBRANE
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OXYGENATORS
peared earlier with denucleation priming than with standard priming, using both types of membrane oxygenator. Flow resistance. The pressure gradient measured between the blood inflow and outflow ports of the oxygenator was determined at 4 min and at 16 min, as an indication of altered resistance to the flow through the oxygenator. The flow rate was constant at 500 ml/ min during this period, for all perfusions. Results are shown in Fig. 5. With the use of the SSR oxygenator, the increase in resistance was significantly less with denucleation priming than with either standard priming (P < 0.05) or CO, priming (P < 0.01). The increases Group 1: SSR membrane, standard prime, n = 5 in flow resistance for the FFSR oxygenator showed Group 2: FFSR membrane, standard prime, n = 3 similar results, with the least change in resistance Group 3: SSR membrane, CO, prime, n = 3 occurring in the denucleation primed group. Group 4: FFSR membrane, COz prime, n = 3 Hemodynamic changes. After insertion of the oxyGroup 5: SSR membrane, denucleation prime; n = 7 genator, pulmonary artery pressure rose at least twoGroup 6: FFSR membrane, denucleation prime, n = 3 fold in all experiments. The pulmonary artery pressure reached its highest value between 10 and 20 min after RESULTS oxygenator insertion and returned to normal within the first hour. Although there was some variation in the Platelets. Changes in platelet count are shown in Fig. of this response, no significant differences 3. Using SSR oxygenators, denucleation priming was magnitude were observed between any of the groups of experiassociated with significantly less reduction in circulatthat cardiac ing platelets than either standard priming or CO2 ments. We have previously demonstrated output falls slightly during the pulmonary vascular priming (P c 0.001 at 16 min). Similarly, with the response, and thus the elevated pulmonary pressure is FFSR oxygenator, early platelet losses were signifia reflection of the increased pulmonary vascular resistcantly reduced with denucleation priming compared ance. with either standard priming (P < 0.05 at 16 min) or CO2 priming (P < 0.01 at 16 min). An earlier recovery DISCUSSION of circulating platelets occurred, however, with CO2 priming than with standard priming. Although improvements in the design of silicone Although the priming technique used greatly affected rubber membrane oxygenators have reduced blood platelet losses, the membrane material did not. Thus trauma, platelet loss continues to be a problem during there was no statistically significant difference in the ECMO. These experiments were designed to isolate platelet losses between these two types of oxygenators some of the factors that affect platelet loss. as long as the same priming technique was applied to Most medical grade silicones are composed of polydiboth. methyl and polymethyl vinyl siloxane, compounded Leukocjtes. An abrupt but transient leukopenia was with finely divided silica particles in varying proporobserved in all experiments following oxygenator inser- tions, depending on the strength of material required tion (Fig. 4). This was of the same order of magnitude (2). Existing evidence suggests that polydimethyl siloxwith each priming technique, although recovery ap- ane is more biocompatible than the methyl vinyl poly-
point proximal to the oxygenator, before and at the end of the 2-h control perfusion period and at regular intervals after oxygenator insertion for the measurement of hematocrit and for leukocyte and platelet counts. Hematocrits were measured using the microcentrifuge method. Leukocyte and platelet counts were measured with a Coulter counter (Coulter Electronics, Hialeah, FL) as previously described (9). The leukocyte and platelet counts were corrected for changes in hematocrit caused by blood sampling and hemodilution from the priming solution. Six groups of experiments were performed.
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FIG. 3. Platelet count during ECMO (2 SE) for SSR (Zefl) and FFSR (right) oxygenators. Platelet count is expressed as percentage of base-line value obtained at end of control perfusion period. All counts were corrected for changes in hematocrit due to blood sampling and hemodilution.
Standard --co2 *-=---* Denucleation l
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H374
FOUNTAIN
ET AL.
FFSR
SSR
100 90 FIG. 4. Leukocyte count during ECMO (2 SE) for SSR (left) and FFSR (right) oxygenators. White count is expressed as percentage of base-line value obtained at end of control perfusion period. All counts were corrected for changes in hematocrit due to blood sampling and hemodilution.
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FFSR membranes used in our experiments were constructed of the more favorable, polydimethyl siloxane, failure to show better compatibility with these silicafree membranes cannot be attributed to the differences .0 in the type of polymer used. Our findings are in keeping I with those of other workers who have compared com/ mercially available SSR and FFSR membrane oxygen/ 1’ ators (12, 13). The experiments of Kolobow et al. (6, 7) / and those of Zap01 et al. (14), demonstrating apparent / superiority of FFSR membranes, used membranes that 1’ 1’ were specially cast and cured, and their results may 1’ mm .Hg 0 have been due as much to the casting technique and 1’ L/min *O” resulting physical properties of the surface as to the 1’ / chemical composition of the silicone rubber itself. / 0 Even though the material used did not affect memi@.... ........- .***............=..==~~ brane oxygenator performance in this study, priming procedure was an important factor in determining the 0/ compatibility of both-SSR and FFSR membranes.-Perfusions using both types of device, subjected to the denucleation priming procedure, resulted in signifiStandard --co2 cantly reduced platelet losses and significantly smaller =-=-=-aDenucleatlon rises in flow resistance across the oxygenator than those I using either standard or CO, priming procedures. HowI 4 16 4 16 ever, when both membrane materials were subjected to Minutes the denucleation priming procedure, the mean value of each of these compatibility parameters was slightly FIG. 6. Flow resistance across oxygenator. Pressure gradient between blood inflow and outflow ports of the oxygenator was meabetter for the SSR membranes. Because FFSR exhibits sured at 4 min and at 16 min after insertion of oxygenator. During a larger advancing contact angle when in contact with this time blood flow rate remained constant at 500 ml/min. the priming solution (144.5 _ + 1.3 degrees at 23°C) than does SSR (106.5 t 1.4 degrees at 23”C), it is more mer (7) and that construction of the membrane with a difficult for the priming solution to enter its surface irregularities (12). It is the entry of the priming solution thin coating of silica-free silicone rubber may f&her improve biocompatibility and reduce platelet losses by into such surface irregularities that is thought to be the elimination of silica particles from the blood-surface essential step in denucleation priming (9). Since both interface (6, 7, 14). The results of the present experitypes of membrane materials were subjected to the ments, however, fail to demonstrate any superiority for same priming procedure, it is possible that more gas FFSR over SSR membranes, in terms of platelet losses nuclei were left on the surface of the FFSR than on the or anv of the other narameters measured. Since the SSR membrane. This could account for the slightlv 400
1
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BIOCOMPATIBILITY
OF
SILICONE
RUBBER
MEMBRANE
OXYGENATORS
poorer performance of the FFSR membrane with denucleation priming. Such theoretical considerations might also apply to the CO2 priming technique. CO, displaces air bubbles from the membrane oxygenator surface, and is then, by virtue of its 20.fold greiter solubility in water, itself displaced by priming solution (5). This is not the same process as &nucleation, but the difference in contact angles between FFSR and SSR may explain, on the same basis, the slightly poorer performance of the FFSR membrane with this priming procedure. Using both types of device, CO, priming was associated with early platelet losses similar to those seen with standard priming. Recovery of circulating platelets was, however, more rapid-and at the end of the 4-h perfusion period, little difference in platelet numbers existed between the CO,-primed experiments and the denucleation-primed experiments. Studies of platelet function as well as numbers might have revealed continuing differences, but were not performed during these experiments. The acute leukopenia seen at the start of ECMO in all experiments is probably a manifestation of complement activation. Similar changes are seen on exposure of blood to cellophane hemodialysis membrane (3), and
H375
complement activation by the silicone rubber used in membrane oxygenator construction has also been demonstrated @).-The elevation in pulmonary artery pressure was originally thought to be associated with a platelet release phenomenon (1). However, recent investigations in our-laboratory suggest that this pulmonary vascular response may, in fact, also be related to complement activation (unpublished observations). In conclusion, no innate superiority was demonstrated in this study for either the silica-containing or the silica-free silicone rubber membrane surface. The priming technique used was the principal determinant of oxygenator performance both in terms of platelet losses and flow resistance across the oxygenatorI The authors are indebted to Mr. S. R. Gregory for technical assistance, and to Dr. Elizabeth Musclow and the Department of HematoIogy at the Mount Sinai Hospital, Toronto for assistance with hematological studies. This work was supported by the Ontario Heart Foundation Grant l-27, the Medical Research Council of Canada Grant MA-5953, and the Toronto General Hospital Foundation. Address reprint requests to J. D. Cooper, l-131 University Wing, Toronto General Hospital, Toronto, Ontario M5G lL7, Canada. Received 2 June 1978; accepted in final form 1 September 1978.
REFERENCES A., J. DUFFIN, M. F. X. GLYNN, AND J. D. COOPER. The effect of sulfinopyrazone on platelet and pulmonary responses to onset of membrane oxygenator perfusion. Trans. Am. Sot. Artifi Intern. Organs 22: 94-101,1976. 2. BRALEY, S. The chemistry and properties of the medical-grade silicones. J. Macromol. Sci. 3: 529444, 1970. 3. CRADDOCK, P. R., J. FEHE, K. L. BRIGHAM, R. S. KRONENBERG, ANb H. S. JACOB. Complement and leukocyte-mediated pulmonary dysfunction in hemodialysis. New Engl. J. Med. 269: 769-
1. BIREK,
9.
10.
774,1977. 4.
5. 8.
7.
8.
DUFFIN, J., B. MARTIN, AND J. D. COOPER.Control, monitor and alarm system for clinical application of a membrane oxygenator. Can. Anuesth. Sot. J. 23: 143-152,1976. KOM)BOW, T. The promise of the membrane artificial lung. Int. J. Artif. Organs 1: 15-20, 1978. KOLOBOW, T., E. W. STOOL, P. K. WEATHERSBY, J. PIERCE, F. HAYANO, -AND J. SUAUDEA~. Superior blood compatibility of silicone rubber free of silica filler in the membrane lung. Trans. Am. Sot. Artif. Intern. Organs 20: 269-277, 1974. KOL~BOW, T., T. A. TOMLINSON, AND J. E. PIERCE. Blood compatibility of methyl, methyl vinyl, methyl phenyl and trifluoropropylmethylvinyl silicone rubber without silica fillers in the spiral-coiled membrane lung. J. Biomed. Mater. Res. 11: 471481,1977. LmmAy, R. M., M. FRIESEN, J. COOPER, A. BIREK, K. SCOT& AND A. L. LION. Platelet-foreign surface interactions with
11.
12.
13.
14.
oxygenator membranes. Proc. Bioeng. Semin. Artif. Intern. Organs, GZasgow, 1976. OSADA, H., C. A. WARD, J. DUFFIN, J. M. NELEMS, AND J. D. COOPER. Microbubble elimination during priming improves biocompatibility of membrane oxygenators. Am. J. Physiol. 234: H646-H652, 1978 or Am. J. Physiol.: Heart Circ. Physiol. 3: H646-H652,1978. WARD, C. A., AND T. W. FOREST. On the relation between platelet adhesion and the roughness of a synthetic biomaterial. Ann. Biomed. Eng. 4: 184-207,1976. WARD, C. A., B. RUEGSEGGEGI, D. STANGA, AND W. ZINGG. Reduction in platelet adhesion to biomaterials by removal of gas nuclei. Trans. Am. Sot. Artif. Intern. Organs 20: 77-85,1974. YATES, W. G., AND R. N. S~HAAP. Performance evaluation of commercial membrane lungs during long-term cardiopulmonary bypass of animals. Utah Biomed. Test kb., 1977. (Summary report) YATES, W. G., R. N. SCHAAP, AND G. C. BAUMAN. Performance evaluation of filler-free Sci-Med membrane lungs during longterm cardiopulmonary bypass of animals. Utah Biomed. Test Lab., 1977. (Report No. HR-52943-3F) ZAPOL, W. M., S. BLOOM, A. CARVALHO, T. WONDERS, M. SKOSKIEWICZ, R. SCHNEIDER, AND M. SNIDER. Improved platelet economy using filler free silicone rubber in long term membrane lung perfusion. Trans. Am. Sot. Artif. Intern. Organs 21: 587591,1975.
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (130.070.008.131) on January 16, 2019.
J. DUFFIN,
C. A. WARD,
H. OSADA,
B. A. MARTIN,
Departments of Surgery, Anaesthesia, and Mechanical Engineering Division of Thoracic Surgery at Toronto General Hospital, Toronto,
FOUNTAIN, S. W., J. DUFFIN, C. A. WARD, H. OSADA, B. A. MARTIN, AND J. D. COOPER. The biocompatibiZity of standard
and silica-free silicone rubber membraneoxygenators. Am. J. Physiol. 236(Z): H371-H375, 1979 or Am. J. Physiol.: Heart Circ. Physiol. 5(2): H371-H375, 1979.-Spiral coil membrane oxygenators made from either standard silicone rubber or silica-free silicone rubber were comparedusing three priming techniques. Standard priming, carbon dioxide priming, and denucleation priming, were employed with each type of device. Four-hour venovenous membraneoxygenator perfusionswere carried out on awake sheep anticoagulated with heparin. Virtually no differences were observed in any parameters measured between standard silicone rubber and filler-free silicone rubber membranes. Significantly greater platelet lossesoccurred during the first hour of perfusion with standard priming and with carbon dioxide priming than with denucleation priming, using either type of membrane. These experiments demonstrate that denucleation priming reduces platelet lossesduring extracorporeal membrane oxygenator perfusion, but that the use of filler-free silicone rubber does not improve the biocompatibility of the membrane. standard priming; CO, priming; denucleation priming; platelet losses
during extracorporeal membrane oxygenation (ECMO) depends on a number of incompletely understood factors. Two principal determinants, LOSS OF PLATELETS
however, are the type of membrane used and the priming technique employed. Silicone rubber is the material most commonly used in the construction of membrane oxygenators. Comparison of the properties of different silicone polymers and their performance with and without reinforcing silica filler has suggested that polydimethyl siloxane is the most favorable polymer and that the elimination of silica filler from the blood-contacting surface of the membrane further improves biocompatibility (7). Trapped air bubbles remaining on the surface of the membrane after priming have been strongly implicated in the process of platelet adhesion on subsequent exposure of the membrane to blood (11). The elimination of these gas nuclei from the surface of the membrane is a function of the priming technique used. With carbon dioxide (COz) priming, air is displaced by COz, which then dissolves more readily when the oxygenator is flushed with priming solution. In this study controlled experiments were conducted 0363-6135/79/0000-0000$01.25
Copyright
0 1979 the American
Physiological
AND J. D. COOPER
at the University of Toronto, Ontario 145G IL7, Canada
and
to compare commercially available filler-free silicone rubber (FFSR) membrane oxygenators and those made of standard silicone rubber (SSR) containing filler. This comparison was made using standard, COz, and denucleation priming techniques with each type of device. MATERIALS
AND METHOD$
The experimental. model has been previously described in detail (1, 4). Under ketamine sedation and local anesthesia, Suffolk sheep weighing approximately 30 kg were cannulated for venovenous perfusion using both common jugular veins. A Swan-Ganz (7F) catheter and a carotid arterial catheter were used to monitor pulmonary arterial and systemic blood pressures, respectively. After recovery from the anesthetic, the animal was connected to the perfusion circuit depicted in Fig. 1. A 2-h period of control ,perfusion was undertaken, during which time blood flow was diverted through the shunt ‘around the oxygenator. Flow was then directed through the membrane oxygenator for 4 h. fir institution of ECMO, flow was 500 ml/min for the first 20 min and 1 l/min thereafter. Gas flow through the oxygenator was maintained at 1 l/min of oxygen. At the time of cannulation, 1.25 mg/kg of heparin was given intravenously. This dose was repeated 30 min later and subsequently 0.5-l (mg/kg)/h was given; dosage was regulated according to the activated clotting time (ACT) as measured by the hemochron method (International Technidyne Corp., Edison, NJ). ACT was maintained between 400 and 600 s throughout the experiment. The perfusion circuit was primed with lactated Ringer solution containing 0.75 g/l of sodium bicarbonate and 40 mgll heparin. The volume of the circuit excluding the oxygenator was 900 ml, and its surface area was 0.18 m2. The circuit was constructed with silicone rubber tubing (Silastic, Dow Corning, Midland, MI) and a silicone rubber reservoir and bubble trap. Sections were connected by polycarbonate connectors (Cobe Laboratories, Denver, CO), New tubing and connectors were used for each experiment. A roller pump (Sarns, model 5M6002) in the occlusive mode was used to control the flow, The membrane oxygenators employed in this study were of the spiral-coil type, The oxygenators and both types of membrane were commercially manufactured (Sci-Med Life Systems, Minneapolis, MN) The SSR Society
H371
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (130.070.008.131) on January 16, 2019.
H372
FOUNTAIN
supcrio/
vwa via
l
cavu h ft
FIG. 1. Schematic diagram of perfusion circuit. Shunt around the oxygenator was used during 2-h control perfusion.
membrane was cast in two layers of polymethyl vinyl siloxane containing reinforcing silica filler. The FFSR membrane was cast in two layers of polydimethyl siloxane, the first layer containing reinforcing silica filler and the second containing no filler. Both types had a surface area of 2.5 m2 and a priming volume of 300 ml. The three priming techniques used were as follows: Standard priming entailed filling the perfusion circuit and the oxygenator with the priming solution and circulating the priming solution through the oxygenator for 1 h. During this time the oxygenator was vigorously agitated manually to dislodge trapped air bubbles. The oxygenator was then excluded from the circuit in preparation for control perfusion. The CO, priming technique used in this study was designed to displace all air from microscopic crevices in the membrane surface by saturating the oxygenator with CO,. Figure 2 shows the connections for the CO, priming technique with tubing clamps isolating the oxygenator from the rest of the perfusion circuit. With clamp 1 closed, CO, was introduced into the blood phase through a filter (Cathivex 0.22-pm disposable filter), and its flow was adjusted to provide vigorous bubbling in the trap (l-3 l/min>. The vacuum pump was then activated and clamp 2 was adjusted so as to produce a vacuum of -200 mmHg. Next, clamp II was carefully opened until the vacuum had decreased to -150 mmHg. This adjustment was made slowly enough to ensure that bubbling was maintained in the trap and that CO, flow was adjusted if necessary. In this way CO, flowed through both blood and gas phases, and the 150 mmHg gradient from blood to gas phase facilitated removal of all air from the blood compartment. After 90 min of continuous CO, flushing, the oxygenator was filled with priming solution at point B. The
ET AL.
flow of priming solution was then controlled by varying the CO, flow. Vacuum on the gas phase was maintained during filling to keep the blood phase expanded. Vigorous agitation of the oxygenator was employed to prevent bubbles of CO, from being trapped. When the oxygenator was filled, the CO2 line was clamped shut at point A. At this time vacuum was disconnected by releasing clamp 1 and clamp 2 sequentially and, finally, by disconnecting the tubing from the outflow gas port. Carbon dioxide flow through the gas phase was maintained without vacuum until the oxygenator was inserted into the perfusion circuit, at which time oxygen was substituted for CO,. The denucleation priming technique has been previously described (9). In brief, the gas content of the priming solution was reduced by agitating it in a highvacuum chamber. The oxygenator was then placed in the high-vacuum chamber and filled with priming solution. The oxygenator was then removed from the vacuum chamber and the degassed priming solution circulated through the oxygenator with the gas phase attached to a high-vacuum pump. The pressures in the perfusion circuit both proximal and distal to the membrane oxygenator (Fig. 1) and the systemic and pulmonary arterial pressures were continuously monitored and displayed on a multichannel recorder (Litton Medical Products, Long Island, NY). Blood samples were withdrawn from the circuit, at a
FIG. 2. Schematic diagram of CO, priming technique. Tubing clamps isolate oxygenator from. perfusion circuit. Clamps 1 and 2 control gas flow through gas phase of the oxygenator. See text for details of priming technique.
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BIOCOMPATIBILITY
OF SILICONE
RUBBER
MEMBRANE
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OXYGENATORS
peared earlier with denucleation priming than with standard priming, using both types of membrane oxygenator. Flow resistance. The pressure gradient measured between the blood inflow and outflow ports of the oxygenator was determined at 4 min and at 16 min, as an indication of altered resistance to the flow through the oxygenator. The flow rate was constant at 500 ml/ min during this period, for all perfusions. Results are shown in Fig. 5. With the use of the SSR oxygenator, the increase in resistance was significantly less with denucleation priming than with either standard priming (P < 0.05) or CO, priming (P < 0.01). The increases Group 1: SSR membrane, standard prime, n = 5 in flow resistance for the FFSR oxygenator showed Group 2: FFSR membrane, standard prime, n = 3 similar results, with the least change in resistance Group 3: SSR membrane, CO, prime, n = 3 occurring in the denucleation primed group. Group 4: FFSR membrane, COz prime, n = 3 Hemodynamic changes. After insertion of the oxyGroup 5: SSR membrane, denucleation prime; n = 7 genator, pulmonary artery pressure rose at least twoGroup 6: FFSR membrane, denucleation prime, n = 3 fold in all experiments. The pulmonary artery pressure reached its highest value between 10 and 20 min after RESULTS oxygenator insertion and returned to normal within the first hour. Although there was some variation in the Platelets. Changes in platelet count are shown in Fig. of this response, no significant differences 3. Using SSR oxygenators, denucleation priming was magnitude were observed between any of the groups of experiassociated with significantly less reduction in circulatthat cardiac ing platelets than either standard priming or CO2 ments. We have previously demonstrated output falls slightly during the pulmonary vascular priming (P c 0.001 at 16 min). Similarly, with the response, and thus the elevated pulmonary pressure is FFSR oxygenator, early platelet losses were signifia reflection of the increased pulmonary vascular resistcantly reduced with denucleation priming compared ance. with either standard priming (P < 0.05 at 16 min) or CO2 priming (P < 0.01 at 16 min). An earlier recovery DISCUSSION of circulating platelets occurred, however, with CO2 priming than with standard priming. Although improvements in the design of silicone Although the priming technique used greatly affected rubber membrane oxygenators have reduced blood platelet losses, the membrane material did not. Thus trauma, platelet loss continues to be a problem during there was no statistically significant difference in the ECMO. These experiments were designed to isolate platelet losses between these two types of oxygenators some of the factors that affect platelet loss. as long as the same priming technique was applied to Most medical grade silicones are composed of polydiboth. methyl and polymethyl vinyl siloxane, compounded Leukocjtes. An abrupt but transient leukopenia was with finely divided silica particles in varying proporobserved in all experiments following oxygenator inser- tions, depending on the strength of material required tion (Fig. 4). This was of the same order of magnitude (2). Existing evidence suggests that polydimethyl siloxwith each priming technique, although recovery ap- ane is more biocompatible than the methyl vinyl poly-
point proximal to the oxygenator, before and at the end of the 2-h control perfusion period and at regular intervals after oxygenator insertion for the measurement of hematocrit and for leukocyte and platelet counts. Hematocrits were measured using the microcentrifuge method. Leukocyte and platelet counts were measured with a Coulter counter (Coulter Electronics, Hialeah, FL) as previously described (9). The leukocyte and platelet counts were corrected for changes in hematocrit caused by blood sampling and hemodilution from the priming solution. Six groups of experiments were performed.
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H374
FOUNTAIN
ET AL.
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100 90 FIG. 4. Leukocyte count during ECMO (2 SE) for SSR (left) and FFSR (right) oxygenators. White count is expressed as percentage of base-line value obtained at end of control perfusion period. All counts were corrected for changes in hematocrit due to blood sampling and hemodilution.
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FFSR membranes used in our experiments were constructed of the more favorable, polydimethyl siloxane, failure to show better compatibility with these silicafree membranes cannot be attributed to the differences .0 in the type of polymer used. Our findings are in keeping I with those of other workers who have compared com/ mercially available SSR and FFSR membrane oxygen/ 1’ ators (12, 13). The experiments of Kolobow et al. (6, 7) / and those of Zap01 et al. (14), demonstrating apparent / superiority of FFSR membranes, used membranes that 1’ 1’ were specially cast and cured, and their results may 1’ mm .Hg 0 have been due as much to the casting technique and 1’ L/min *O” resulting physical properties of the surface as to the 1’ / chemical composition of the silicone rubber itself. / 0 Even though the material used did not affect memi@.... ........- .***............=..==~~ brane oxygenator performance in this study, priming procedure was an important factor in determining the 0/ compatibility of both-SSR and FFSR membranes.-Perfusions using both types of device, subjected to the denucleation priming procedure, resulted in signifiStandard --co2 cantly reduced platelet losses and significantly smaller =-=-=-aDenucleatlon rises in flow resistance across the oxygenator than those I using either standard or CO, priming procedures. HowI 4 16 4 16 ever, when both membrane materials were subjected to Minutes the denucleation priming procedure, the mean value of each of these compatibility parameters was slightly FIG. 6. Flow resistance across oxygenator. Pressure gradient between blood inflow and outflow ports of the oxygenator was meabetter for the SSR membranes. Because FFSR exhibits sured at 4 min and at 16 min after insertion of oxygenator. During a larger advancing contact angle when in contact with this time blood flow rate remained constant at 500 ml/min. the priming solution (144.5 _ + 1.3 degrees at 23°C) than does SSR (106.5 t 1.4 degrees at 23”C), it is more mer (7) and that construction of the membrane with a difficult for the priming solution to enter its surface irregularities (12). It is the entry of the priming solution thin coating of silica-free silicone rubber may f&her improve biocompatibility and reduce platelet losses by into such surface irregularities that is thought to be the elimination of silica particles from the blood-surface essential step in denucleation priming (9). Since both interface (6, 7, 14). The results of the present experitypes of membrane materials were subjected to the ments, however, fail to demonstrate any superiority for same priming procedure, it is possible that more gas FFSR over SSR membranes, in terms of platelet losses nuclei were left on the surface of the FFSR than on the or anv of the other narameters measured. Since the SSR membrane. This could account for the slightlv 400
1
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BIOCOMPATIBILITY
OF
SILICONE
RUBBER
MEMBRANE
OXYGENATORS
poorer performance of the FFSR membrane with denucleation priming. Such theoretical considerations might also apply to the CO2 priming technique. CO, displaces air bubbles from the membrane oxygenator surface, and is then, by virtue of its 20.fold greiter solubility in water, itself displaced by priming solution (5). This is not the same process as &nucleation, but the difference in contact angles between FFSR and SSR may explain, on the same basis, the slightly poorer performance of the FFSR membrane with this priming procedure. Using both types of device, CO, priming was associated with early platelet losses similar to those seen with standard priming. Recovery of circulating platelets was, however, more rapid-and at the end of the 4-h perfusion period, little difference in platelet numbers existed between the CO,-primed experiments and the denucleation-primed experiments. Studies of platelet function as well as numbers might have revealed continuing differences, but were not performed during these experiments. The acute leukopenia seen at the start of ECMO in all experiments is probably a manifestation of complement activation. Similar changes are seen on exposure of blood to cellophane hemodialysis membrane (3), and
H375
complement activation by the silicone rubber used in membrane oxygenator construction has also been demonstrated @).-The elevation in pulmonary artery pressure was originally thought to be associated with a platelet release phenomenon (1). However, recent investigations in our-laboratory suggest that this pulmonary vascular response may, in fact, also be related to complement activation (unpublished observations). In conclusion, no innate superiority was demonstrated in this study for either the silica-containing or the silica-free silicone rubber membrane surface. The priming technique used was the principal determinant of oxygenator performance both in terms of platelet losses and flow resistance across the oxygenatorI The authors are indebted to Mr. S. R. Gregory for technical assistance, and to Dr. Elizabeth Musclow and the Department of HematoIogy at the Mount Sinai Hospital, Toronto for assistance with hematological studies. This work was supported by the Ontario Heart Foundation Grant l-27, the Medical Research Council of Canada Grant MA-5953, and the Toronto General Hospital Foundation. Address reprint requests to J. D. Cooper, l-131 University Wing, Toronto General Hospital, Toronto, Ontario M5G lL7, Canada. Received 2 June 1978; accepted in final form 1 September 1978.
REFERENCES A., J. DUFFIN, M. F. X. GLYNN, AND J. D. COOPER. The effect of sulfinopyrazone on platelet and pulmonary responses to onset of membrane oxygenator perfusion. Trans. Am. Sot. Artifi Intern. Organs 22: 94-101,1976. 2. BRALEY, S. The chemistry and properties of the medical-grade silicones. J. Macromol. Sci. 3: 529444, 1970. 3. CRADDOCK, P. R., J. FEHE, K. L. BRIGHAM, R. S. KRONENBERG, ANb H. S. JACOB. Complement and leukocyte-mediated pulmonary dysfunction in hemodialysis. New Engl. J. Med. 269: 769-
1. BIREK,
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DUFFIN, J., B. MARTIN, AND J. D. COOPER.Control, monitor and alarm system for clinical application of a membrane oxygenator. Can. Anuesth. Sot. J. 23: 143-152,1976. KOM)BOW, T. The promise of the membrane artificial lung. Int. J. Artif. Organs 1: 15-20, 1978. KOLOBOW, T., E. W. STOOL, P. K. WEATHERSBY, J. PIERCE, F. HAYANO, -AND J. SUAUDEA~. Superior blood compatibility of silicone rubber free of silica filler in the membrane lung. Trans. Am. Sot. Artif. Intern. Organs 20: 269-277, 1974. KOL~BOW, T., T. A. TOMLINSON, AND J. E. PIERCE. Blood compatibility of methyl, methyl vinyl, methyl phenyl and trifluoropropylmethylvinyl silicone rubber without silica fillers in the spiral-coiled membrane lung. J. Biomed. Mater. Res. 11: 471481,1977. LmmAy, R. M., M. FRIESEN, J. COOPER, A. BIREK, K. SCOT& AND A. L. LION. Platelet-foreign surface interactions with
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oxygenator membranes. Proc. Bioeng. Semin. Artif. Intern. Organs, GZasgow, 1976. OSADA, H., C. A. WARD, J. DUFFIN, J. M. NELEMS, AND J. D. COOPER. Microbubble elimination during priming improves biocompatibility of membrane oxygenators. Am. J. Physiol. 234: H646-H652, 1978 or Am. J. Physiol.: Heart Circ. Physiol. 3: H646-H652,1978. WARD, C. A., AND T. W. FOREST. On the relation between platelet adhesion and the roughness of a synthetic biomaterial. Ann. Biomed. Eng. 4: 184-207,1976. WARD, C. A., B. RUEGSEGGEGI, D. STANGA, AND W. ZINGG. Reduction in platelet adhesion to biomaterials by removal of gas nuclei. Trans. Am. Sot. Artif. Intern. Organs 20: 77-85,1974. YATES, W. G., AND R. N. S~HAAP. Performance evaluation of commercial membrane lungs during long-term cardiopulmonary bypass of animals. Utah Biomed. Test kb., 1977. (Summary report) YATES, W. G., R. N. SCHAAP, AND G. C. BAUMAN. Performance evaluation of filler-free Sci-Med membrane lungs during longterm cardiopulmonary bypass of animals. Utah Biomed. Test Lab., 1977. (Report No. HR-52943-3F) ZAPOL, W. M., S. BLOOM, A. CARVALHO, T. WONDERS, M. SKOSKIEWICZ, R. SCHNEIDER, AND M. SNIDER. Improved platelet economy using filler free silicone rubber in long term membrane lung perfusion. Trans. Am. Sot. Artif. Intern. Organs 21: 587591,1975.
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