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The swing-type membrane oxygenator
Journal Clinical
of Surgical Research and Laboratory
Volume 8 Number
THE
SWING-TYPE
Gas Exchange KIMIKO
6,
June 1968
MEMBRANE
Performance
KATSUHARA, AND
Investigation
M.D., SIGERU
OXYGENATOR
of the Swing
TATSUYA SAKAKIBARA,
VARIOUS WORKERS have sought a practical membrane lung to facilitate long-term extracorporeal support of potentially reversible respiratory distress resulting from heart or lung disease and for use during cardiac surgery [2-4, 10, 11, 12, 16, 17, 20, 211. However, because of the composition, thickness, and the degree of hydration of the membrane material, gas exchange in a membrane lung has two principal limitationsgas permeation through a plastic membrane and gas diffusion in the liquid phase. Gas transfer depends upon the gas tension difference between the two sides of the membrane. For example, if pure oxygen is used in the gas phase and the partial pressure of COz in the blood is assumed to be below 50 mm. Hg, a ratio of at least 12 to 1 exists in favor of oxygen in terms of pressure gradient. To confirm this partial pressure ratio and to provide an equal transmission of CO, and O2 the membrane should be twelve times more permeable to CO, than to 0,. From the Department of Surgery of Tokyo Women’s Medical College and the Japan Heart Institute, Tokyo, Japan. *Professor of the Department of Surgery and the Director of the Japan Heart Institute. This work was previously reported in part at the 4th Conference of the Japanese Artificial Organ and Transplantation Society, Nov. 2, 1966, Tokyo, Japan. Submitted for publication April 5, 1967.
it/lotion
System
YOKOSUKA,
M.D.,
M.D.*
Silicon rubber, which is much more permeable to gases than any other type of membrane tested, is only five times more permeable to CO, than to 0, [7, 221, This is still insufficient for the transfer of equal volume of oxygen and carbon dioxide. Even when this problem is reduced by agitation of blood [ 141, the use of a mechanical blood distributing system [ 151, and the prolongation of transit time of blood [l, 16, 181 oxygen transfer is still low. Difficulties in O2 exchange are particularly evident in attempts to use the high gas permeability of 1 mil methyl silicon [ 61. The purpose of this investigation was to determine whether or not the gas diffusion could be improved when the blood flow pattern is regulated by a swing motion system. MATERIALS
AND
METHOD
The basic design of the membrane lung was the same as that developed by Katsuhara and colleagues; it is completely unsupported, having low Ilow resistance and an integral blood distributing chamber in the upper part of the membrane envelope [9]. For the completely unsupported membrane internal support was provided by reinforcing Dacron mesh on one side of the membrane surface. 245
JOURNAL
OF
SURGICAL
RESEARCH
VOL.
8
NO.
TWO sheets of the membrane were stretched and attached to each other lengthwise along the edges by Silicon rubber gaskets. The membrane envelope was enclosed with a polyvinyl sheet into which the edge of Phycon membrane was sealed to provide preassembled sterile units, as indicated in Figure 1A and B. The Fuji Polymer Industrial Company of Tokyo, in collaboration with the authors, developed the pinhole-free 3 mil thick Phycon (silicon rubber) membrane. The Phycon membrane is reinforced with a Dacron mesh which forms a hexagonal configuration throughout the membrane (Fig. 2A and B. Two advantages of ,this hexagonal fabrication are that it provides for a larger gas exchange surface area (less masking effect) than a square pattern, and that it provides for transverse membrane tension which produces a uniform roughness on the membrane surface. This roughness promotes filming as well as agitation of the blood. The membrane envelopes, each one measuring 25 cm. wide and 70 cm. long to form a lung with a membrane area of 3.5 m.2, were mounted and bolted in parallel on a stainless steel plate. The stainless steel plate slanted downward to the reservoir or vent and was turned from side to side on a central axis at an angle of 20 ‘degrees. The swing motion rate of the plate was accomplished by connecting a motor pump which had a variablespeed drive. The assembled lung was connected with a reservoir, two roller pumps, a heat exchanger, and a bubble trap to complete the circuit. Connections were established by means of plastic tubing, as indicated in Figure 3.* Evaluation of gas exchange was carried out by providing a second lung (bubble oxygenator) for deoxygenation. A diagram of the circulation system is presented in Figure 4. A constant rate of blood flow inside the membrane oxygenator was maintained by a recirculating pump. The flow rate from the recirculating pump ex*The Medical Tokyo,
246
oxygenator was Instruments Japan.
manufactured Manufacturing
by
the Senko Company,
6,
JUNE
1968
ceeded the flow rate from the arterial pump by a ratio of 3 to 2. The circuit was primed with heparinized bovine blood and the blood temperature was maintained at 37°C. by incorporating a heat exchanger into a circuit. Oxygen was blown into each gas layer of the oxygenator with a flow rate of 1 liter per minute, as shown in Figure 1B. Venous blood from the deoxygenator was led to the recirculating pump by gravity and then pumped to the membrane oxygenator. Deoxygenation and carbon dioxide partial pressure approximating venous blood were obtained by passing from 5 to 10% CO2 in N2 through a bubble oxygenator [ 19, 231. In our trial experiments an attempt was made to regulate the deoxygenation of the blood. At a bypass %ow rate below 2.0 liters per minute, the small deoxygenator was used (Number 2); at the increased bypass %ow rate to 4.0 liters per minute the large deoxygenator was used (Number 1). After a steady state had been reached, samples were taken every 30 minutes from the blood entering and leaving the oxygenator, and oxygen saturation, p02, pCOa, and pH were determined. The oxygen transfer rate (cc./min./ m.2) was calculated for each individual experiment. Gas exchange performance of a single membrane unit (surface area of 0.35 m.“) was studied at varying swing motion rates (0, 20, 30, 50, and 60 times per min. ) , at a constant flow rate, and at a constant angle of inclination; gas exchange performance of a ten-unit membrane oxygenator (surface area of 3.5 m.2) was studied at varying %ow rates at a constant swing motion rate and at a constant angle.
RESULTS Gross inspection through the polyvinyl sheet revealed that blood was distributed more evenly than in a stationary film and flowed gently at an angle of less than 20 degrees. The flow pattern is shown in Figure 5. The swing motion rate of below 50 times per minute created a better filming of the
-.:!. I... -_ ::. -.. I.I:
A
--
=-
61s
OUTLET
PHVCOW MEMSRASE
--•EM6ll~NE
.P@LV
VW1
=SOXVGEN
B
6lMlD
SHEET
IllLET
OUTLEV
envelope enclosed by polyvinyl sheet into which the edge of membrane is Fig. 1. Phycon membrane sealed. (A) Entirely disposable Phycon membrane lung unit. (B) Basic design of membrane lung unit. Left: Cross section of one layer. Right: Front view.
247
B
A
Fig. 2. Comparison of masking effect of Phycon and Silastic membranes. (A) Phycon membrane reinforced with Dacron mesh forming hexagonal fabrication. (B) Silastic membrane embedded with Dacron mesh forming square fabrication.
blood flowing between the membrane surfaces as the unit was swung from side to side. At the same time, agitation was caused by the roughness of the membrane surfaces. The oxygen transfer rates of various swing motion rates are plotted in Figure 6. The rate of oxygen transfer increased as the swing motion rate increased from twenty to fifty times per minute, reaching a maximum rate
I Fig. 3. Swing-type membrane oxygenator, front view . Membrane envelopes are mounted in parallel and bolted to stainless steel plate which rocks on central axis. Motor pump is attached to bottom of plate. 248
I
1.P
Fig. 4. Circulation system. R.C.P., recirculating AI’., arterial pump, D.O.l, 0.0.2, dispuqb oxygenator, R, electromagnetic flowmeter, F.B., filling burette, R.P., re-pulsator, B.T., bubble trap, H.E., heat exchanger, R.V., reservoir.
KATSUHARA
of 92 cc. of oxygen per minute per square meter. Further increase in frequency or reduction of the swing motion rate to the the stationary state seemed to decrease oxygen transfer rate.
-6
Fig. 5. Flow pattern of blood as it entered from distributing chamber (B) through Phyeon tube (A), Blood was continuously refilming as unit rocked from side to side. pE 7.2-7.3 Hemoglobin
CrE:
16.Sm.
ET
AL.:
THE
SWING-TYPE
MEMBRANE
OXYGENATOR
The results in ten units of membrane showing the arterial and venous oxygen saturation, pC% p% pH, and oxygen transfer rate at varying flow rates are given in Table 1. The oxygen saturation and the oxygen transfer rate were sufficient for the flow rate from 2,000 cc. to 4,000 cc. per minute with a ten-unit membrane oxygenator ( surface area of 3.5 m.2). A normal arterialvenous ~0, difference of 60 mm. Hg and a normal pCOa difference of 6 mm. Hg were maintained up to a blood flow rate of 4,000 cc. per minute. The rate of oxygen transfer increased as the flow rate increased up to 4,000 cc. per minute, reaching a maximum rate of 224 cc. of oxygen per minute per 3.5 square meters. These results indicate that a ten-unit membrane oxygenator having an area of 3.5 m.2 is probably sufficient for basal adult human support and will provide balanced O2 and CO, exchange. The blood volume in the lung depended on the flow rate per membrane unit. The blood volume was 30 cc. at a flow rate of 200 cc. per minute per unit, 40 cc. at the flow rate of 350 cc. per minute per unit, and 60 cc. at the flow rate of 450 cc. per minute per unit. Thus, the 3.5 m.3 lung (ten units) oxygenating 4,000 cc. per minute of blood would have a volume of approximately 600 cc. Even though the membranes are not supported the volume changes only about 30% per minute. DISCUSSION
0
20
SXln8
lotion
30 Rates
50 ( Tlmss/min.
60 )
Oxygen transfer rate influenced by swing motion rate. The rate of blood flow was 350 cc. per minute per membrane unit (surface area of 0.35 m.2); an angle of inclination of 10 degreeswas employed for testing. Fig. 6.
A major factor in the satisfactory function of a membrane lung is the gas permeability of the membrane material. The l/8 mil Teflon (dry) membrane was for a time the membrane of greatest permeability that was available [ 10, 12, 211. Subsequently, Silastic reinforced with Dacron (silicon rubber) mesh was found to be remarkably compatible with blood and more permeable to respiratory gases than was the Teflon membrane [8, 111. However, although silicon rubber is about five times more permeable to carbon dioxide than to oxygen [7, 221, it is still inadequate for practical use. 249
Q
1.
72 69 77
3,500
44 41 48
42 41 40
41 41 38
PO2 mm. Hg
49 49 52
57
z:
61 65 57
PCO, mm. Hg
Oxygenator
Performance
7.4 7.3 7.3
7.3 7.3 7.3
PH 7.3 7.3 7.3
of a Ten-Unit
4,000 65 39 49 7.3 Note: Hemoglobin, gr. %: 15 gr. An angle of 20 d eg rees and a swing motion per 3.5 m.a membrane surface area.
70 68 67
O3 Saturation (%I 69 68 61
Membrane
Gas Exchange
Blood Entering
2,800
2,000
S%i!%!a (3.5 m.2)
Bypass Flow
Table
112 122 111
94 98 98
Oxygenator
7.3 7.4 7.3
7.4 7.3 7.4
38 7.3 were used. Oxygen transfer
42 37 41
36 41 41
35 35 45
PH 7.4 7.3 7.3
Area of 3.5 m.2)
PCO, mm. Hg
Membrane
(Surface
98 129 rate of 35 times per minute
128 117 113
98 95 93
126 126 117
PO2 mm. Hg
Leaving
Oxygenator
Blood O2 Saturation (%) 98 97 97
Membrane
224 rate (O.T.R.) was
154 182 182
146 151 146
O.T.R. (cc./min.) 116 116 144
KATSUHARA
Agitation, the major avenue available for improvement in gas exchange, has received the most attention recently. A vertical, gravity-fed lung was designed using l/S mil Teflon which has transverse corrugation to create agitation of the blood as it cascades down [4, 10, 121. Marx et al. [ 141 have reported the use of a Mylar screen sandwiched between the membrane surfaces which functioned as a mixing device. Another method used to enhance respiratory gas diffusion was the support of the membrane envelope on both sides by multiple point silicon rubber mats which maintained a constant thickness of the blood film [16, 181. The swing-type device, as employed by Crystal et al. [5] and the authors of this article, improves gas exchange performance by dispersing the blood throughout the membrane envelope and continuously refilming it over the membrane surfaces. The hexagonal Dacron mesh of the Phycon membrane provides a rough surface and has a minimal masking effect. The hexagonal pattern produces small eddies or opposing streams caused by the nonuniform flow on the gas exchange surface. As a result of this the oxygen transfer rate in a stationary state (0 times per minute) was quite high-62 cc. per minute per square meter. The oxygen transfer rates increased with swing motion rate, reaching 92 cc. per minute per square meter. The circuit for testing oxygenator performance has been used by investigators [13, 19, 231. Indeed, the gas exchange capacity of a practical clinical oxygenator can hardly be determined in the human patient. Furthermore, the usual laboratory animals are too small to be used for such determinations because of the difficulties involved in maintaining steady perfusion conditions over long periods.
ET
AL.:
MEMBRANE
OXYGENATOR
REFERENCES 1.
2.
3.
4.
5.
6.
7.
system incorPhycon mem-
SWING-TYPE
brane provides the most efficient oxygenation of blood and a more efficient carbon dioxide excretion for its size than those oxygenators previously described. The improvement in gas exchange efficiency was brought about by prolongation of transit time due to the continuous refilming of the blood on the inclined membrane surfaces and by the mixing produced by the roughness of the membrane surface. Experimentally the gas exchange was sufficient to oxygenate venous blood at flows ranging from 2,000 cc. to 4,000 cc. per minute per ten units (surface area of 3.5 m.y) of membrane. The results indicate that this newly designed artificial lung device should be suitable for perfusion in both adults and children.
CONCLUSIONS The swing-type oxygenator porating the Dacron reinforced
THE
Benvenuto, R., and Lewis, F. J. Influence of some physical factors upon oxygenation with a plastic membrane oxygenator. Surgery 46: 1099, 1959. Bramson, M. L., Osborn, J. J., Main, F. B., O’Brien, M. F., Wright, J. S., and Gerbode, F. A new disposable membrane oxygenator with integral heat exchange. J. Thorac. Cardiov. Surg. 50:391, 1965. Clowes, G. H. A., Jr., Hopkins, A. L., and Neville, W. E. An artificial lung dependent upon diffusion of oxygen and carbon dioxide through plastic membranes. J. Thorac. Surg. 32:630, 1956. Crescenzi, A. A., Hofstra, P. C., Dibenedetto, A., Sze, K. C., Foster, B. H., Glass, P., Claff, C. L., and Cooper, P. Development of a simplified membrane oxygenator. Trans. Amer. Sot. Artif. Intern. Organs 5:148, 1959. Crystal, D. K., Day, S. W., Wagner, C. L., Martinis, A. J., Owen, J. J., and Walker, P. E. Arch. A gravity-flow membrane oxygenator. Szrrg. (Chicago) 88: 122, 1964. Esmond, W. G., and Dibelius, N. R. Permselective ultra-thin disposable silicon rubber membrane blood oxygenator: A preliminary report. Trans. Amer. Sot. Artif. Intern. Organs 11:325, 1965. Galletti, P. hl., Snider, M. T., and SilbertAiden, D. Gas permeability of plastic membranes for artificial lungs. Med. Res. Engin. 5:20, 1966. 251
JOURNAL
OF
SURGICAL
RESEARCH
VOL.
8 NO.
Katsuhara, K., Fujikura, I., Miura, I., Sakai, Y., and Sakakibara, S. Evaluation of Clinical Trial of Silastic Membrane Oxygenator: A Progress Report. Digest of the 6th International Conference on Medical and Biological Engineering. P. 336, 1965. 9. Katsuhara, K., Fujikura, I., Obunai, Y., Miura, I., Yamaguchi, S., Tanaka, T., Ishihara, A., and Sakakibara of the membr~i~cal S approach on the study oxygenator. Clin. Surg. (Rinsho Geka) 21:1371, 1966. 10. Katsuhara, K., Kolff, W. J., and Taylor, H. P. Experiment with artificial blood oxygenation on newborn dogs. Amer. J. Obstet. Gynec. 8:608, 1964. 11. Kolobow, T., and Bowman, R. L. Construction and evaluation of an alveolar membrane artificial heart-lung. Trans. Amer. SOC. Artif. Intern. Organs 9:238, 1963. 12. Kylstra, J. A., Moulopolous, S. D., and Kolff, W. J. Further development of an ultra-thin Teflon membrane gas exchanger. Trans. Amer. Sm. Artif. Intern. Organs 7:355, 1961. 13. Levin, M. B., Theye, R. A., Fowler, W. S., and Kirkline, J. W. Performance of the stationary vertical-screen oxygenator ( Mayo-Gibbon). .I. Thorac. Cardiov. Surg. 39:417, 1960. 14. Marx, T. I., Baldwin, B. R., and Miller, D. R. Factors influencing oxygen uptake by blood in membrane oxygenators: Report of a study. Ann. Surg. 156:204, 1962. 15. Peirce, E. C., II. The membrane lung: A new
6, JUNE
8.
252
16.
17.
18.
19.
20.
21.
22.
23.
1968 multiple point support for Teflon film. Surgery 52~777, 1962. Peirce, E. C., II. A new concept in membrane support for artificial lungs. Trans. Amer. Sot. Artif. Intern. Organs 12:334, 1966. Peirce, E. C., II, Roger, W. K., and Dabbs, C. H. Clinical experience with membrane hmg used in conjunction with hypothermia. J. Tenn. Med. Ass. 54139, 1961. Prados, J. W., and Peirce, E. C., II. The influence of membrane permeability and of design on gas exchange in the membrane lung. Trans. Amer. Sot. Artif. Intern. Organs 6~52, 1960. Ratan, R. S., Bennett, G. F., Bollin, P. L., McAlpine, W. A., and Selman, hl. W. Experimental evaluation of a rotating membrane oxygenator. J. Thorac. Cardiov. Surg. 53:519, 1967. Sarin, C. L., SenGupta, A., Taylor, H. P., and Kolff, W. J. Further development of an artificial placenta with the use of membrane oxySurgery genator and venovenous perfusion. 60:754, 1966. SenGupta, A., Taylor, H. P., and Kolff, W. J. An artificial placenta designed to maintain life during cardiorespiratory distress. Trans. Amer. Sot. Artif. Intern. Organs 10:63, 1964. Snider, M. T. Studies on the permeability of Silicon rubber and tetrafluoroethylene membranes. Emory University M.S. Thesis, 1966. Williams, K. R., and Blustein, R. Evaluation of a disposable bubble oxygenator. J. Thorac. Cardiov. Surg. 50:35, 1965.
of Surgical Research and Laboratory
Volume 8 Number
THE
SWING-TYPE
Gas Exchange KIMIKO
6,
June 1968
MEMBRANE
Performance
KATSUHARA, AND
Investigation
M.D., SIGERU
OXYGENATOR
of the Swing
TATSUYA SAKAKIBARA,
VARIOUS WORKERS have sought a practical membrane lung to facilitate long-term extracorporeal support of potentially reversible respiratory distress resulting from heart or lung disease and for use during cardiac surgery [2-4, 10, 11, 12, 16, 17, 20, 211. However, because of the composition, thickness, and the degree of hydration of the membrane material, gas exchange in a membrane lung has two principal limitationsgas permeation through a plastic membrane and gas diffusion in the liquid phase. Gas transfer depends upon the gas tension difference between the two sides of the membrane. For example, if pure oxygen is used in the gas phase and the partial pressure of COz in the blood is assumed to be below 50 mm. Hg, a ratio of at least 12 to 1 exists in favor of oxygen in terms of pressure gradient. To confirm this partial pressure ratio and to provide an equal transmission of CO, and O2 the membrane should be twelve times more permeable to CO, than to 0,. From the Department of Surgery of Tokyo Women’s Medical College and the Japan Heart Institute, Tokyo, Japan. *Professor of the Department of Surgery and the Director of the Japan Heart Institute. This work was previously reported in part at the 4th Conference of the Japanese Artificial Organ and Transplantation Society, Nov. 2, 1966, Tokyo, Japan. Submitted for publication April 5, 1967.
it/lotion
System
YOKOSUKA,
M.D.,
M.D.*
Silicon rubber, which is much more permeable to gases than any other type of membrane tested, is only five times more permeable to CO, than to 0, [7, 221, This is still insufficient for the transfer of equal volume of oxygen and carbon dioxide. Even when this problem is reduced by agitation of blood [ 141, the use of a mechanical blood distributing system [ 151, and the prolongation of transit time of blood [l, 16, 181 oxygen transfer is still low. Difficulties in O2 exchange are particularly evident in attempts to use the high gas permeability of 1 mil methyl silicon [ 61. The purpose of this investigation was to determine whether or not the gas diffusion could be improved when the blood flow pattern is regulated by a swing motion system. MATERIALS
AND
METHOD
The basic design of the membrane lung was the same as that developed by Katsuhara and colleagues; it is completely unsupported, having low Ilow resistance and an integral blood distributing chamber in the upper part of the membrane envelope [9]. For the completely unsupported membrane internal support was provided by reinforcing Dacron mesh on one side of the membrane surface. 245
JOURNAL
OF
SURGICAL
RESEARCH
VOL.
8
NO.
TWO sheets of the membrane were stretched and attached to each other lengthwise along the edges by Silicon rubber gaskets. The membrane envelope was enclosed with a polyvinyl sheet into which the edge of Phycon membrane was sealed to provide preassembled sterile units, as indicated in Figure 1A and B. The Fuji Polymer Industrial Company of Tokyo, in collaboration with the authors, developed the pinhole-free 3 mil thick Phycon (silicon rubber) membrane. The Phycon membrane is reinforced with a Dacron mesh which forms a hexagonal configuration throughout the membrane (Fig. 2A and B. Two advantages of ,this hexagonal fabrication are that it provides for a larger gas exchange surface area (less masking effect) than a square pattern, and that it provides for transverse membrane tension which produces a uniform roughness on the membrane surface. This roughness promotes filming as well as agitation of the blood. The membrane envelopes, each one measuring 25 cm. wide and 70 cm. long to form a lung with a membrane area of 3.5 m.2, were mounted and bolted in parallel on a stainless steel plate. The stainless steel plate slanted downward to the reservoir or vent and was turned from side to side on a central axis at an angle of 20 ‘degrees. The swing motion rate of the plate was accomplished by connecting a motor pump which had a variablespeed drive. The assembled lung was connected with a reservoir, two roller pumps, a heat exchanger, and a bubble trap to complete the circuit. Connections were established by means of plastic tubing, as indicated in Figure 3.* Evaluation of gas exchange was carried out by providing a second lung (bubble oxygenator) for deoxygenation. A diagram of the circulation system is presented in Figure 4. A constant rate of blood flow inside the membrane oxygenator was maintained by a recirculating pump. The flow rate from the recirculating pump ex*The Medical Tokyo,
246
oxygenator was Instruments Japan.
manufactured Manufacturing
by
the Senko Company,
6,
JUNE
1968
ceeded the flow rate from the arterial pump by a ratio of 3 to 2. The circuit was primed with heparinized bovine blood and the blood temperature was maintained at 37°C. by incorporating a heat exchanger into a circuit. Oxygen was blown into each gas layer of the oxygenator with a flow rate of 1 liter per minute, as shown in Figure 1B. Venous blood from the deoxygenator was led to the recirculating pump by gravity and then pumped to the membrane oxygenator. Deoxygenation and carbon dioxide partial pressure approximating venous blood were obtained by passing from 5 to 10% CO2 in N2 through a bubble oxygenator [ 19, 231. In our trial experiments an attempt was made to regulate the deoxygenation of the blood. At a bypass %ow rate below 2.0 liters per minute, the small deoxygenator was used (Number 2); at the increased bypass %ow rate to 4.0 liters per minute the large deoxygenator was used (Number 1). After a steady state had been reached, samples were taken every 30 minutes from the blood entering and leaving the oxygenator, and oxygen saturation, p02, pCOa, and pH were determined. The oxygen transfer rate (cc./min./ m.2) was calculated for each individual experiment. Gas exchange performance of a single membrane unit (surface area of 0.35 m.“) was studied at varying swing motion rates (0, 20, 30, 50, and 60 times per min. ) , at a constant flow rate, and at a constant angle of inclination; gas exchange performance of a ten-unit membrane oxygenator (surface area of 3.5 m.2) was studied at varying %ow rates at a constant swing motion rate and at a constant angle.
RESULTS Gross inspection through the polyvinyl sheet revealed that blood was distributed more evenly than in a stationary film and flowed gently at an angle of less than 20 degrees. The flow pattern is shown in Figure 5. The swing motion rate of below 50 times per minute created a better filming of the
-.:!. I... -_ ::. -.. I.I:
A
--
=-
61s
OUTLET
PHVCOW MEMSRASE
--•EM6ll~NE
.P@LV
VW1
=SOXVGEN
B
6lMlD
SHEET
IllLET
OUTLEV
envelope enclosed by polyvinyl sheet into which the edge of membrane is Fig. 1. Phycon membrane sealed. (A) Entirely disposable Phycon membrane lung unit. (B) Basic design of membrane lung unit. Left: Cross section of one layer. Right: Front view.
247
B
A
Fig. 2. Comparison of masking effect of Phycon and Silastic membranes. (A) Phycon membrane reinforced with Dacron mesh forming hexagonal fabrication. (B) Silastic membrane embedded with Dacron mesh forming square fabrication.
blood flowing between the membrane surfaces as the unit was swung from side to side. At the same time, agitation was caused by the roughness of the membrane surfaces. The oxygen transfer rates of various swing motion rates are plotted in Figure 6. The rate of oxygen transfer increased as the swing motion rate increased from twenty to fifty times per minute, reaching a maximum rate
I Fig. 3. Swing-type membrane oxygenator, front view . Membrane envelopes are mounted in parallel and bolted to stainless steel plate which rocks on central axis. Motor pump is attached to bottom of plate. 248
I
1.P
Fig. 4. Circulation system. R.C.P., recirculating AI’., arterial pump, D.O.l, 0.0.2, dispuqb oxygenator, R, electromagnetic flowmeter, F.B., filling burette, R.P., re-pulsator, B.T., bubble trap, H.E., heat exchanger, R.V., reservoir.
KATSUHARA
of 92 cc. of oxygen per minute per square meter. Further increase in frequency or reduction of the swing motion rate to the the stationary state seemed to decrease oxygen transfer rate.
-6
Fig. 5. Flow pattern of blood as it entered from distributing chamber (B) through Phyeon tube (A), Blood was continuously refilming as unit rocked from side to side. pE 7.2-7.3 Hemoglobin
CrE:
16.Sm.
ET
AL.:
THE
SWING-TYPE
MEMBRANE
OXYGENATOR
The results in ten units of membrane showing the arterial and venous oxygen saturation, pC% p% pH, and oxygen transfer rate at varying flow rates are given in Table 1. The oxygen saturation and the oxygen transfer rate were sufficient for the flow rate from 2,000 cc. to 4,000 cc. per minute with a ten-unit membrane oxygenator ( surface area of 3.5 m.2). A normal arterialvenous ~0, difference of 60 mm. Hg and a normal pCOa difference of 6 mm. Hg were maintained up to a blood flow rate of 4,000 cc. per minute. The rate of oxygen transfer increased as the flow rate increased up to 4,000 cc. per minute, reaching a maximum rate of 224 cc. of oxygen per minute per 3.5 square meters. These results indicate that a ten-unit membrane oxygenator having an area of 3.5 m.2 is probably sufficient for basal adult human support and will provide balanced O2 and CO, exchange. The blood volume in the lung depended on the flow rate per membrane unit. The blood volume was 30 cc. at a flow rate of 200 cc. per minute per unit, 40 cc. at the flow rate of 350 cc. per minute per unit, and 60 cc. at the flow rate of 450 cc. per minute per unit. Thus, the 3.5 m.3 lung (ten units) oxygenating 4,000 cc. per minute of blood would have a volume of approximately 600 cc. Even though the membranes are not supported the volume changes only about 30% per minute. DISCUSSION
0
20
SXln8
lotion
30 Rates
50 ( Tlmss/min.
60 )
Oxygen transfer rate influenced by swing motion rate. The rate of blood flow was 350 cc. per minute per membrane unit (surface area of 0.35 m.2); an angle of inclination of 10 degreeswas employed for testing. Fig. 6.
A major factor in the satisfactory function of a membrane lung is the gas permeability of the membrane material. The l/8 mil Teflon (dry) membrane was for a time the membrane of greatest permeability that was available [ 10, 12, 211. Subsequently, Silastic reinforced with Dacron (silicon rubber) mesh was found to be remarkably compatible with blood and more permeable to respiratory gases than was the Teflon membrane [8, 111. However, although silicon rubber is about five times more permeable to carbon dioxide than to oxygen [7, 221, it is still inadequate for practical use. 249
Q
1.
72 69 77
3,500
44 41 48
42 41 40
41 41 38
PO2 mm. Hg
49 49 52
57
z:
61 65 57
PCO, mm. Hg
Oxygenator
Performance
7.4 7.3 7.3
7.3 7.3 7.3
PH 7.3 7.3 7.3
of a Ten-Unit
4,000 65 39 49 7.3 Note: Hemoglobin, gr. %: 15 gr. An angle of 20 d eg rees and a swing motion per 3.5 m.a membrane surface area.
70 68 67
O3 Saturation (%I 69 68 61
Membrane
Gas Exchange
Blood Entering
2,800
2,000
S%i!%!a (3.5 m.2)
Bypass Flow
Table
112 122 111
94 98 98
Oxygenator
7.3 7.4 7.3
7.4 7.3 7.4
38 7.3 were used. Oxygen transfer
42 37 41
36 41 41
35 35 45
PH 7.4 7.3 7.3
Area of 3.5 m.2)
PCO, mm. Hg
Membrane
(Surface
98 129 rate of 35 times per minute
128 117 113
98 95 93
126 126 117
PO2 mm. Hg
Leaving
Oxygenator
Blood O2 Saturation (%) 98 97 97
Membrane
224 rate (O.T.R.) was
154 182 182
146 151 146
O.T.R. (cc./min.) 116 116 144
KATSUHARA
Agitation, the major avenue available for improvement in gas exchange, has received the most attention recently. A vertical, gravity-fed lung was designed using l/S mil Teflon which has transverse corrugation to create agitation of the blood as it cascades down [4, 10, 121. Marx et al. [ 141 have reported the use of a Mylar screen sandwiched between the membrane surfaces which functioned as a mixing device. Another method used to enhance respiratory gas diffusion was the support of the membrane envelope on both sides by multiple point silicon rubber mats which maintained a constant thickness of the blood film [16, 181. The swing-type device, as employed by Crystal et al. [5] and the authors of this article, improves gas exchange performance by dispersing the blood throughout the membrane envelope and continuously refilming it over the membrane surfaces. The hexagonal Dacron mesh of the Phycon membrane provides a rough surface and has a minimal masking effect. The hexagonal pattern produces small eddies or opposing streams caused by the nonuniform flow on the gas exchange surface. As a result of this the oxygen transfer rate in a stationary state (0 times per minute) was quite high-62 cc. per minute per square meter. The oxygen transfer rates increased with swing motion rate, reaching 92 cc. per minute per square meter. The circuit for testing oxygenator performance has been used by investigators [13, 19, 231. Indeed, the gas exchange capacity of a practical clinical oxygenator can hardly be determined in the human patient. Furthermore, the usual laboratory animals are too small to be used for such determinations because of the difficulties involved in maintaining steady perfusion conditions over long periods.
ET
AL.:
MEMBRANE
OXYGENATOR
REFERENCES 1.
2.
3.
4.
5.
6.
7.
system incorPhycon mem-
SWING-TYPE
brane provides the most efficient oxygenation of blood and a more efficient carbon dioxide excretion for its size than those oxygenators previously described. The improvement in gas exchange efficiency was brought about by prolongation of transit time due to the continuous refilming of the blood on the inclined membrane surfaces and by the mixing produced by the roughness of the membrane surface. Experimentally the gas exchange was sufficient to oxygenate venous blood at flows ranging from 2,000 cc. to 4,000 cc. per minute per ten units (surface area of 3.5 m.y) of membrane. The results indicate that this newly designed artificial lung device should be suitable for perfusion in both adults and children.
CONCLUSIONS The swing-type oxygenator porating the Dacron reinforced
THE
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