- Email: [email protected]
Historical Perspective
SECTION XIII Surfactant
Historical Perspective
79
John A. Clements
As a first-time contributor to Fetal and Neonatal Physiology, I have wondered what motivation prompted the editors of this scholarly and compendious tome to invite me to prepare a historical and personal perspective for the section on pulmonary surfactant. Did they hope to lighten the intellectual load imposed on its readers? Were they fostering gender neutrality? (Mary Ellen Avery wrote the previous account.) Maybe they wished to avoid age discrimination by including an author in his tenth decade of life! Whatever the editors’ reasons, my principal credential for this task is an interest in the subject that goes back more than six decades and has required continuing education in the mysteries and delights of surface phenomena. I hope that this brief essay will convey a little of the pleasure and excitement of contemplating, investigating, and applying the concepts, materials, and techniques of physics and chemistry to pulmonary biology and therapeutics. The ideas and observations necessary to understand pulmonary surfactant were developing long before it was recognized as an important component of lung structure. These included the existence of surface tension or surface free energy of liquids and the properties of insoluble films at water surfaces. Other aspects, such as surfactant functions, morphology, chemical composition, metabolism, regulation, development in the fetus, and therapeutic use, have been elucidated much more recently and make up a large part of the relevant current literature. Many of these aspects are addressed in other chapters of this section. The idea of surface tension and its connection with the rise of liquids in capillary tubes predated correct explanations by many years (summarized by Hardy1). For instance, the members of the Accademmia del Cimento in Florence were interested in such phenomena. Their 1667 report was largely devoted to experiments in vacuo and included a demonstration that fluid rose in a capillary held in a vacuum. Boyle (of the gas laws) also showed the rise of liquids in capillary tubes (1682) but did not succeed with the vacuum experiment. In 1709 Hauksbee, Demonstrator of the Royal Society, London, showed that the height of liquid rise was the same in two tubes of the same internal diameter, but one had a wall 10 times as thick as the other. He reasoned, brilliantly, from this result that the attraction of the solid for the liquid is limited to the surface of the solid and is in a direction perpendicular to the sides of the cylindrical glass. This statement was remarkable because students of capillary attraction intuitively felt that a force parallel to the glass causes the liquid to rise (or fall). Jurin, Secretary of the Royal Society, showed in a lovely experiment (1718) that if water was drawn up into an inverted funnel whose stem had been pulled out to a capillary, the funnel remained full, even in a vacuum. He concluded that cohesion in the water suspended the lower part in the funnel from that in the capillary. He also surmised that the water particles were more strongly attracted by glass than by each other, and vice versa for mercury, explaining why water is raised and mercury is depressed in a glass tube.
Meanwhile, scientists in France were studying such phenomena, but they tended to describe them mathematically, though incorrectly, because they failed to take into account the double curvature of many of the liquid surfaces. It fell to Young (1805) and Laplace (1806) to correct this mistake and derive the equations necessary to relate the tension and the complex curvature to the pressure difference across various liquid surfaces. (We still use these formulas to calculate the mechanical effects of surface tension on the lungs.) Many leading scientists throughout the nineteenth century (Poisson, Dupré, van der Waals, Boltzmann, Maxwell, Kelvin, Raleigh, Gibbs) and in the twentieth century (Bakker, Guggenheim, Onsager, Kirkwood and Buff, Born, Ono and Kondo, and Defay and Prigogine) studied capillarity and greatly refined theories of intermolecular forces in relation to the origin of surface tension. Application of thermodynamic analysis led to the result that mechanical surface free energy is equal to surface tension times surface area, a relationship that became useful much later in the measurement of alveolar surface area. The liquids at the surfaces of the pulmonary airspaces, however, are not simple like those considered in the classical systems, and one has to take into account other components that modify the surface tension. Many materials dissolved or suspended in water spontaneously accumulate in the interface, and these are commonly called surfactants. If such materials are freely soluble in water, their partition between interface and aqueous phase at equilibrium is described by the Gibbs-Duhem equation, and it turns out that changes in surface area do not change the surface tension significantly. If these substances have minute solubility in water so that they are in effect locked into the interface after adsorption, changes in area of the surface cause large changes in surface tension. Lung surfactant acts in this way, and such behavior is critical to its ability to reduce surface tension and stabilize alveolar structure at low transpulmonary pressures and volumes.2 Just as the understanding of surface tension and its origin in intermolecular attractive forces evolved over several centuries, so too the investigation and theory of “insoluble” surface films required many years. For example, pouring oil on troubled waters to calm the waves was apparently known in ancient times. The earliest record of such surface films that I know of was from Pliny the Elder. He was a self-taught man of insatiable curiosity, who read voraciously and corresponded with prominent Roman scholars. He made notes on zoology, botany, agriculture, geology, mining, navigation, astronomy, and land warfare that are said to have occupied 160 volumes, which he boiled down to 37 books by the year 77 CE. Interestingly, Pliny included oil films on water in his writings.3 About 16 centuries later another self-taught man, Benjamin Franklin, then living in London, became curious about films on water. He performed the famous experiment of pouring oil onto the surface of the pond at Clapham Common, in a town
795
796
SECTION XIII — Surfactant
southwest of London. The day was windy and the water was rough. In a paper he read to the Royal Society in 1774, he wrote “the oil, though not more than a teaspoonful … spread amazingly and extended itself gradually till it reached the lee side, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass.”3 His data would have allowed him to calculate the thickness of the film, approximately one 10-millionth of an inch, or about the length of an oil molecule, but he did not report doing that. Moving along a century later, we find another self-taught person, Agnes Pockels, who kept house for her invalid parents in Braunschweig, Germany. Her natural curiosity was boundless. Denied entrance to university because she was a woman, she read the literature of physics and chemistry avidly and thoughtfully. The greasy films on cooking dishes annoyed others, but she found them captivating. She set up clean dishes filled to the brim with clean water, laid metal strips across the surface, applied known amounts of oils to the water between the strips, and measured the surface tension as she moved the barriers to decrease the area available to the oil film. This was a new method, and it gave her important results. She tried to publish them, but had no success. Frustrated, she sent a letter describing her findings to Lord Raleigh in England, whose papers on surface phenomena she had read. Her letter was written in German, which Raleigh didn’t understand. Fortunately, his wife was fluent in German and translated the letter for him. He saw the importance of Pockels’ results and sent the letter to Nature with a request that it be published. It was, in 1891.4 Pockels’ apparatus was the precursor of a device we now call the surface balance, an instrument often used in physical chemistry laboratories. With various improvements it has made possible the characterization of hundreds of insoluble interfacial films since Pockels invented it (Figure 79-1). The first time I saw a surface balance was in 1950 in the laboratory of Hans Trurnit. Hans worked in the Medical Laboratories of the Army Chemical Center in Maryland, where I was doing my national service in the Army Medical Corps. He was an expert on surface films of proteins. Quite by accident, we became
friends. Hans burned with a gem-like flame, and nothing seemed to please him more than telling me about his work. Although my main assignment was to improve the treatment of nerve gas casualties, I enjoyed learning about surface effects and adding to what Hans told me by studying relevant textbooks. My formal education had not included any mention of surface properties, and so, without knowing it at the time, I became another in a long line of self-taught students of surface films. The Medical Laboratories had a research contract with the Harvard School of Public Health to study lung edema caused by the war gas phosgene. The work was directed by Jere Mead, a brilliant pulmonary physiologist, and the results led him to wonder what effects the surface tension of the edema fluid and its foam bubbles had on mechanical properties of the lungs. As an Army officer, I was assigned an additional duty in 1951 to evaluate progress under the contract, and that meant visiting Jere’s laboratory and discussing his ideas and results. Again, without my invitation, surface phenomena invaded my life. This time I was better prepared to understand and analyze them. In 1952 Ted Radford, a member of Jere’s group, began another project that involved surface effects. Ted wanted to estimate the area of the alveolar surface by a physicochemical method to check histologic estimates. His idea was to calculate the surface free energy of the alveoli from air and saline pressure-volume measurements and divide it by an assumed surface tension to compute alveolar area (Figure 79-2). This method gave an area only one tenth of morphometric estimates, and Ted concluded that the morphologists were wrong.5 When I asked him to explain the discrepancy, he told me I would not understand it. That annoyed me, and I decided to analyze his results in detail. I made a calculation of the diffusion capacity of the lungs using his area value, and it came out far lower than measured diffusion values. His pressure-volume data seemed solid, but his assumption of a particular surface tension bothered me. So I turned the calculation backwards and used the pressure-volume data and morphologic information to compute surface tension. A trial Surface free energy proportional to area between curves
250
200
Volume (mL)
COMPRESSION
Maximum area
150
100
Minimum area
73 Surface tension (dynes/cm)
Saline Air
50
0
12 0 Area
A 12
Figure 79-1 Schematic diagram of a surface balance consisting of a trough containing water, a barrier to change the area available to a surface film at the air-water interface, a Wilhelmy plate dipping into the water surface, and a strain gauge to register the pull of surface tension on the plate. When an insoluble film occupies the water surface, reducing the area causes the surface tension to decrease as shown in the surface tension-area diagram.
0
2
4
6
8
10
12
14
Pressure (cm H2O) Figure 79-2 Relationships of transpulmonary pressure and volume when cat lungs are filled with air or with normal saline. The area between the air and saline curves during emptying is proportional to the mechanical surface free energy of the alveoli. (Data from Radford EP: Method for estimating respiratory surface area of mammalian lungs from their physical characteristics. Proc Soc Exp Biol Med 87:58–61, 1954.)
797
Figure 79-3 Comparison of surface tension-area relationship of lung extract measured in a surface balance with that of alveolar surfaces computed from pressure-volume data. When area is decreased, both surfaces show surface tension falling to very low values. Blood plasma in the surface balance shows relatively high surface tensions and little hysteresis, unlike the lung extract. (With permission from Clements JA: Surface tension of lung extracts. Proc Soc Exp Biol Med 95:170–172, 1957.) 50
Surface tension (dynes/cm)
calculation assuming a surface tension of 50 dynes/cm at maximum lung volume and a relative alveolar radius proportional to the cube root of volume gave this astonishing result: on deflation of the lung, computed surface tension fell from 50 dynes/cm to very low values, implying the presence of a unique surfactant in the alveoli. Then in 1955 Pattle reported that bubbles in pulmonary edema foam or squeezed from the cut surface of normal lungs contained “an insoluble protein layer which can abolish the tension of the alveolar surface.”6 I thought this conclusion was dubious, because the pressure-volume data of Radford5 and von Neergaard7 suggested higher surface tension values for the airspaces. Clearly, the next step was to demonstrate the surface tensionarea behavior of the hypothetical surfactant directly. That meant extracting it from the lungs and observing it in a surface balance. I knew how to do that, from discussions with Trurnit, from papers I had read, and from my previous biochemical research. After some technical problems were solved, it turned out (Figure 79-3) that the surface tension versus area relationships measured in the surface balance and calculated from pressure-volume and morphologic information agreed unbelievably well … EUREKA! I presented my results at a 1956 meeting of the American Physiological Society showing that lung extracts could lower surface tension to <10 dynes/cm when surface area was decreased and theorized that this effect could stabilize alveoli against collapse.2 The talk drew little attention and less interest. Despite that cold reception, I was sure that the ideas and data were important and wrote a short paper for Science, which was rejected. After a few months of misery, I asked a friend to introduce it to the Proceedings of the Society for Experimental Biology and Medicine, an unreviewed journal, where it appeared in May 1957.8 (Years later the Institute for Scientific Information announced that it had become a citation classic.) In December 1957 Mary Ellen Avery visited my laboratory. She had gone to Harvard for a fellowship in pediatrics with Clement Smith and in lung physiology with Jere Mead, because of her strong interest in respiratory diseases in infants, especially hyaline membrane disease. I showed her my methods and told her all I knew about lung surfactant and lung physiology. Back in Boston she and Jere Mead set up the methods, applied them to infant lungs, and showed that surfactant could not be found in infants with respiratory distress syndrome (RDS) who had died. Figure 79-4 shows data from the famous graph in their January 1959 paper9 demonstrating that fact, which launched a flood tide of research on lung surfactant and RDS. The relevant literature now contains over 15,000 publications, of which more than 500 were added in the last year. Such a mountain of research cannot be properly summarized in this brief historical perspective. It does seem worthwhile, though, to mention a few milestones in the development of surfactant treatment for RDS. It had been obvious from the start that if lack or dysfunction of lung surfactant caused pulmonary failure, this might be ameliorated by replacement therapy. Attempts in the 1960s failed10,11 for various reasons: the necessary composition and physical properties of substitutes were not fully known; doses and delivery methods were inadequate; nursery personnel lacked knowledge and methods of assessing the physiologic status of newborns, especially premature infants. Most of these problems were addressed in the ensuing decades, and surfactant treatment finally became feasible for RDS. In 1972 Enhorning and Robertson12 showed that instilling surfactant extracted from adult rabbits into the trachea of prematurely delivered rabbit fetuses enabled good expansion of the lung. Then in 1980 Fujiwara and colleagues13 reported that in a series of 10 infants with RDS, tracheal instillation of cow lung surfactant enriched in its main component (dipalmitoyl
Chapter 79 — Historical Perspective
RDS Controls
40
30
20
10
0 0
1000
2000
3000
4000
5000
Weight (gr) Figure 79-4 Avery and Mead measured the surface tension-area relationships of lung extracts from newborn infants who died with or without respiratory distress syndrome (RDS). They were not able to demonstrate surfactant in those with RDS and those with normal lungs who weighed about <1200 g. (Data from Avery ME, Mead J: Surface properties in relation to atelectasis and hyaline membrane disease. Am J Dis Child 97:517–523, 1959.)
phosphatidylcholine) expanded the lungs and improved oxygenation. This paper stimulated many randomized controlled trials over the next 10 years of a variety of surfactant preparations derived from animal lungs. The results have shown benefit for the most part and have led to widespread use of surfactant substitution in infants with RDS, as well as to evaluation of synthetic preparations containing surfactant protein B or peptides of similar structure. The current state of the art is well summarized in the report and recommendations of the American Academy of Pediatrics 2012-2013 Committee on the Fetus and Newborn: Surfactant Replacement Therapy for Preterm and Term Neonates with Respiratory Distress.14 In essence, the Committee endorses surfactant therapy for preterm infants with severe RDS after they are stabilized but also suggests that in appropriate cases one should consider immediate continuous positive airway pressure with subsequent surfactant administration if needed, rather than routine tracheal intubation and early surfactant dosing. Although the Committee’s recommendations illustrate that surfactant therapy now has an established place in the management of the newborn with RDS, they also imply that there is room for further improvement. The incidence of chronic lung disease (bronchopulmonary dysplasia) remains distressingly high among RDS survivors, and the causes of premature birth need continuing study.15 Clearly, we still have much to learn, and if past history is a reliable guide, we can expect application of fundamental concepts and methods of physics and chemistry to lead the way towards better understanding and care of the fetus and neonate.
REFERENCES 1. Hardy WB: Historical notes upon surface energy and forces of short range. Nature 109:375–378, 1922. 2. Clements JA: Dependence of pressure-volume characteristics of lungs on intrinsic surface-active material. Am J Physiol 187:592, 1956. 3. Gaines GL: Historical introduction. In Insoluble monolayers at liquid-gas interfaces, New York, 1966, Interscience Publishers. 4. Pockels A: Surface tension. Nature 43:437–439, 1891. 5. Radford EP: Method for estimating respiratory surface area of mammalian lungs from their physical characteristics. Proc Soc Exp Biol Med 87:58–61, 1954. 6. Pattle RE: Properties, function and origin of the alveolar lining layer. Nature 175:1125–1126, 1955. 7. Neergaard KV: Neue auffassungen über einen grundbegriff der atemmechanik. Die retraktionkraft der lunge, abhängig von der oberflächenspannung in den alveolen. Z Gesamte Med 66:1–22, 1929. 8. Clements JA: Surface tension of lung extracts. Proc Soc Exp Biol Med 95:170– 172, 1957. 9. Avery ME, Mead J: Surface properties in relation to atelectasis and hyaline membrane disease. Am J Dis Child 97:517–523, 1959. 10. Robillard E, Alarie Y, Dagenais Perusse P, et al: Microaerosol administration of synthetic beta-gamma-dipalmitoyl-L-alpha-lecithin in the respiratory distress syndrome. A preliminary report. Can Med Assoc J 90:55–57, 1964. 11. Chu J, Clements JA, Cotton EK, et al: Neonatal pulmonary ischemia. Part I: clinical and physiological studies. Pediatrics 40:709–782, 1967. 12. Enhorning G, Robertson B: Lung expansion in the premature rabbit fetus after tracheal deposition of surfactant. Pediatrics 50:58–66, 1972. 13. Fujiwara T, Maeta H, Chida S, et al: Artificial surfactant therapy in hyaline membrane disease. Lancet 1:55–59, 1980. 14. Polin RA, Carlo WA, Committee on Fetus and Newborn: Surfactant replacement therapy for preterm and term neonates with respiratory distress. Pediatrics 133:156–163, 2014. 15. Behrman RE, Adashi EY, Allen MC, Committee on Understanding Premature Birth and Assuring Healthy Outcomes: Preterm birth: causes, consequences, and prevention, Report Brief July 2006, Washington, DC, 2006, Institute of Medicine, National Academies of Science USA, National Academies Press.
Historical Perspective
79
John A. Clements
As a first-time contributor to Fetal and Neonatal Physiology, I have wondered what motivation prompted the editors of this scholarly and compendious tome to invite me to prepare a historical and personal perspective for the section on pulmonary surfactant. Did they hope to lighten the intellectual load imposed on its readers? Were they fostering gender neutrality? (Mary Ellen Avery wrote the previous account.) Maybe they wished to avoid age discrimination by including an author in his tenth decade of life! Whatever the editors’ reasons, my principal credential for this task is an interest in the subject that goes back more than six decades and has required continuing education in the mysteries and delights of surface phenomena. I hope that this brief essay will convey a little of the pleasure and excitement of contemplating, investigating, and applying the concepts, materials, and techniques of physics and chemistry to pulmonary biology and therapeutics. The ideas and observations necessary to understand pulmonary surfactant were developing long before it was recognized as an important component of lung structure. These included the existence of surface tension or surface free energy of liquids and the properties of insoluble films at water surfaces. Other aspects, such as surfactant functions, morphology, chemical composition, metabolism, regulation, development in the fetus, and therapeutic use, have been elucidated much more recently and make up a large part of the relevant current literature. Many of these aspects are addressed in other chapters of this section. The idea of surface tension and its connection with the rise of liquids in capillary tubes predated correct explanations by many years (summarized by Hardy1). For instance, the members of the Accademmia del Cimento in Florence were interested in such phenomena. Their 1667 report was largely devoted to experiments in vacuo and included a demonstration that fluid rose in a capillary held in a vacuum. Boyle (of the gas laws) also showed the rise of liquids in capillary tubes (1682) but did not succeed with the vacuum experiment. In 1709 Hauksbee, Demonstrator of the Royal Society, London, showed that the height of liquid rise was the same in two tubes of the same internal diameter, but one had a wall 10 times as thick as the other. He reasoned, brilliantly, from this result that the attraction of the solid for the liquid is limited to the surface of the solid and is in a direction perpendicular to the sides of the cylindrical glass. This statement was remarkable because students of capillary attraction intuitively felt that a force parallel to the glass causes the liquid to rise (or fall). Jurin, Secretary of the Royal Society, showed in a lovely experiment (1718) that if water was drawn up into an inverted funnel whose stem had been pulled out to a capillary, the funnel remained full, even in a vacuum. He concluded that cohesion in the water suspended the lower part in the funnel from that in the capillary. He also surmised that the water particles were more strongly attracted by glass than by each other, and vice versa for mercury, explaining why water is raised and mercury is depressed in a glass tube.
Meanwhile, scientists in France were studying such phenomena, but they tended to describe them mathematically, though incorrectly, because they failed to take into account the double curvature of many of the liquid surfaces. It fell to Young (1805) and Laplace (1806) to correct this mistake and derive the equations necessary to relate the tension and the complex curvature to the pressure difference across various liquid surfaces. (We still use these formulas to calculate the mechanical effects of surface tension on the lungs.) Many leading scientists throughout the nineteenth century (Poisson, Dupré, van der Waals, Boltzmann, Maxwell, Kelvin, Raleigh, Gibbs) and in the twentieth century (Bakker, Guggenheim, Onsager, Kirkwood and Buff, Born, Ono and Kondo, and Defay and Prigogine) studied capillarity and greatly refined theories of intermolecular forces in relation to the origin of surface tension. Application of thermodynamic analysis led to the result that mechanical surface free energy is equal to surface tension times surface area, a relationship that became useful much later in the measurement of alveolar surface area. The liquids at the surfaces of the pulmonary airspaces, however, are not simple like those considered in the classical systems, and one has to take into account other components that modify the surface tension. Many materials dissolved or suspended in water spontaneously accumulate in the interface, and these are commonly called surfactants. If such materials are freely soluble in water, their partition between interface and aqueous phase at equilibrium is described by the Gibbs-Duhem equation, and it turns out that changes in surface area do not change the surface tension significantly. If these substances have minute solubility in water so that they are in effect locked into the interface after adsorption, changes in area of the surface cause large changes in surface tension. Lung surfactant acts in this way, and such behavior is critical to its ability to reduce surface tension and stabilize alveolar structure at low transpulmonary pressures and volumes.2 Just as the understanding of surface tension and its origin in intermolecular attractive forces evolved over several centuries, so too the investigation and theory of “insoluble” surface films required many years. For example, pouring oil on troubled waters to calm the waves was apparently known in ancient times. The earliest record of such surface films that I know of was from Pliny the Elder. He was a self-taught man of insatiable curiosity, who read voraciously and corresponded with prominent Roman scholars. He made notes on zoology, botany, agriculture, geology, mining, navigation, astronomy, and land warfare that are said to have occupied 160 volumes, which he boiled down to 37 books by the year 77 CE. Interestingly, Pliny included oil films on water in his writings.3 About 16 centuries later another self-taught man, Benjamin Franklin, then living in London, became curious about films on water. He performed the famous experiment of pouring oil onto the surface of the pond at Clapham Common, in a town
795
796
SECTION XIII — Surfactant
southwest of London. The day was windy and the water was rough. In a paper he read to the Royal Society in 1774, he wrote “the oil, though not more than a teaspoonful … spread amazingly and extended itself gradually till it reached the lee side, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass.”3 His data would have allowed him to calculate the thickness of the film, approximately one 10-millionth of an inch, or about the length of an oil molecule, but he did not report doing that. Moving along a century later, we find another self-taught person, Agnes Pockels, who kept house for her invalid parents in Braunschweig, Germany. Her natural curiosity was boundless. Denied entrance to university because she was a woman, she read the literature of physics and chemistry avidly and thoughtfully. The greasy films on cooking dishes annoyed others, but she found them captivating. She set up clean dishes filled to the brim with clean water, laid metal strips across the surface, applied known amounts of oils to the water between the strips, and measured the surface tension as she moved the barriers to decrease the area available to the oil film. This was a new method, and it gave her important results. She tried to publish them, but had no success. Frustrated, she sent a letter describing her findings to Lord Raleigh in England, whose papers on surface phenomena she had read. Her letter was written in German, which Raleigh didn’t understand. Fortunately, his wife was fluent in German and translated the letter for him. He saw the importance of Pockels’ results and sent the letter to Nature with a request that it be published. It was, in 1891.4 Pockels’ apparatus was the precursor of a device we now call the surface balance, an instrument often used in physical chemistry laboratories. With various improvements it has made possible the characterization of hundreds of insoluble interfacial films since Pockels invented it (Figure 79-1). The first time I saw a surface balance was in 1950 in the laboratory of Hans Trurnit. Hans worked in the Medical Laboratories of the Army Chemical Center in Maryland, where I was doing my national service in the Army Medical Corps. He was an expert on surface films of proteins. Quite by accident, we became
friends. Hans burned with a gem-like flame, and nothing seemed to please him more than telling me about his work. Although my main assignment was to improve the treatment of nerve gas casualties, I enjoyed learning about surface effects and adding to what Hans told me by studying relevant textbooks. My formal education had not included any mention of surface properties, and so, without knowing it at the time, I became another in a long line of self-taught students of surface films. The Medical Laboratories had a research contract with the Harvard School of Public Health to study lung edema caused by the war gas phosgene. The work was directed by Jere Mead, a brilliant pulmonary physiologist, and the results led him to wonder what effects the surface tension of the edema fluid and its foam bubbles had on mechanical properties of the lungs. As an Army officer, I was assigned an additional duty in 1951 to evaluate progress under the contract, and that meant visiting Jere’s laboratory and discussing his ideas and results. Again, without my invitation, surface phenomena invaded my life. This time I was better prepared to understand and analyze them. In 1952 Ted Radford, a member of Jere’s group, began another project that involved surface effects. Ted wanted to estimate the area of the alveolar surface by a physicochemical method to check histologic estimates. His idea was to calculate the surface free energy of the alveoli from air and saline pressure-volume measurements and divide it by an assumed surface tension to compute alveolar area (Figure 79-2). This method gave an area only one tenth of morphometric estimates, and Ted concluded that the morphologists were wrong.5 When I asked him to explain the discrepancy, he told me I would not understand it. That annoyed me, and I decided to analyze his results in detail. I made a calculation of the diffusion capacity of the lungs using his area value, and it came out far lower than measured diffusion values. His pressure-volume data seemed solid, but his assumption of a particular surface tension bothered me. So I turned the calculation backwards and used the pressure-volume data and morphologic information to compute surface tension. A trial Surface free energy proportional to area between curves
250
200
Volume (mL)
COMPRESSION
Maximum area
150
100
Minimum area
73 Surface tension (dynes/cm)
Saline Air
50
0
12 0 Area
A 12
Figure 79-1 Schematic diagram of a surface balance consisting of a trough containing water, a barrier to change the area available to a surface film at the air-water interface, a Wilhelmy plate dipping into the water surface, and a strain gauge to register the pull of surface tension on the plate. When an insoluble film occupies the water surface, reducing the area causes the surface tension to decrease as shown in the surface tension-area diagram.
0
2
4
6
8
10
12
14
Pressure (cm H2O) Figure 79-2 Relationships of transpulmonary pressure and volume when cat lungs are filled with air or with normal saline. The area between the air and saline curves during emptying is proportional to the mechanical surface free energy of the alveoli. (Data from Radford EP: Method for estimating respiratory surface area of mammalian lungs from their physical characteristics. Proc Soc Exp Biol Med 87:58–61, 1954.)
797
Figure 79-3 Comparison of surface tension-area relationship of lung extract measured in a surface balance with that of alveolar surfaces computed from pressure-volume data. When area is decreased, both surfaces show surface tension falling to very low values. Blood plasma in the surface balance shows relatively high surface tensions and little hysteresis, unlike the lung extract. (With permission from Clements JA: Surface tension of lung extracts. Proc Soc Exp Biol Med 95:170–172, 1957.) 50
Surface tension (dynes/cm)
calculation assuming a surface tension of 50 dynes/cm at maximum lung volume and a relative alveolar radius proportional to the cube root of volume gave this astonishing result: on deflation of the lung, computed surface tension fell from 50 dynes/cm to very low values, implying the presence of a unique surfactant in the alveoli. Then in 1955 Pattle reported that bubbles in pulmonary edema foam or squeezed from the cut surface of normal lungs contained “an insoluble protein layer which can abolish the tension of the alveolar surface.”6 I thought this conclusion was dubious, because the pressure-volume data of Radford5 and von Neergaard7 suggested higher surface tension values for the airspaces. Clearly, the next step was to demonstrate the surface tensionarea behavior of the hypothetical surfactant directly. That meant extracting it from the lungs and observing it in a surface balance. I knew how to do that, from discussions with Trurnit, from papers I had read, and from my previous biochemical research. After some technical problems were solved, it turned out (Figure 79-3) that the surface tension versus area relationships measured in the surface balance and calculated from pressure-volume and morphologic information agreed unbelievably well … EUREKA! I presented my results at a 1956 meeting of the American Physiological Society showing that lung extracts could lower surface tension to <10 dynes/cm when surface area was decreased and theorized that this effect could stabilize alveoli against collapse.2 The talk drew little attention and less interest. Despite that cold reception, I was sure that the ideas and data were important and wrote a short paper for Science, which was rejected. After a few months of misery, I asked a friend to introduce it to the Proceedings of the Society for Experimental Biology and Medicine, an unreviewed journal, where it appeared in May 1957.8 (Years later the Institute for Scientific Information announced that it had become a citation classic.) In December 1957 Mary Ellen Avery visited my laboratory. She had gone to Harvard for a fellowship in pediatrics with Clement Smith and in lung physiology with Jere Mead, because of her strong interest in respiratory diseases in infants, especially hyaline membrane disease. I showed her my methods and told her all I knew about lung surfactant and lung physiology. Back in Boston she and Jere Mead set up the methods, applied them to infant lungs, and showed that surfactant could not be found in infants with respiratory distress syndrome (RDS) who had died. Figure 79-4 shows data from the famous graph in their January 1959 paper9 demonstrating that fact, which launched a flood tide of research on lung surfactant and RDS. The relevant literature now contains over 15,000 publications, of which more than 500 were added in the last year. Such a mountain of research cannot be properly summarized in this brief historical perspective. It does seem worthwhile, though, to mention a few milestones in the development of surfactant treatment for RDS. It had been obvious from the start that if lack or dysfunction of lung surfactant caused pulmonary failure, this might be ameliorated by replacement therapy. Attempts in the 1960s failed10,11 for various reasons: the necessary composition and physical properties of substitutes were not fully known; doses and delivery methods were inadequate; nursery personnel lacked knowledge and methods of assessing the physiologic status of newborns, especially premature infants. Most of these problems were addressed in the ensuing decades, and surfactant treatment finally became feasible for RDS. In 1972 Enhorning and Robertson12 showed that instilling surfactant extracted from adult rabbits into the trachea of prematurely delivered rabbit fetuses enabled good expansion of the lung. Then in 1980 Fujiwara and colleagues13 reported that in a series of 10 infants with RDS, tracheal instillation of cow lung surfactant enriched in its main component (dipalmitoyl
Chapter 79 — Historical Perspective
RDS Controls
40
30
20
10
0 0
1000
2000
3000
4000
5000
Weight (gr) Figure 79-4 Avery and Mead measured the surface tension-area relationships of lung extracts from newborn infants who died with or without respiratory distress syndrome (RDS). They were not able to demonstrate surfactant in those with RDS and those with normal lungs who weighed about <1200 g. (Data from Avery ME, Mead J: Surface properties in relation to atelectasis and hyaline membrane disease. Am J Dis Child 97:517–523, 1959.)
phosphatidylcholine) expanded the lungs and improved oxygenation. This paper stimulated many randomized controlled trials over the next 10 years of a variety of surfactant preparations derived from animal lungs. The results have shown benefit for the most part and have led to widespread use of surfactant substitution in infants with RDS, as well as to evaluation of synthetic preparations containing surfactant protein B or peptides of similar structure. The current state of the art is well summarized in the report and recommendations of the American Academy of Pediatrics 2012-2013 Committee on the Fetus and Newborn: Surfactant Replacement Therapy for Preterm and Term Neonates with Respiratory Distress.14 In essence, the Committee endorses surfactant therapy for preterm infants with severe RDS after they are stabilized but also suggests that in appropriate cases one should consider immediate continuous positive airway pressure with subsequent surfactant administration if needed, rather than routine tracheal intubation and early surfactant dosing. Although the Committee’s recommendations illustrate that surfactant therapy now has an established place in the management of the newborn with RDS, they also imply that there is room for further improvement. The incidence of chronic lung disease (bronchopulmonary dysplasia) remains distressingly high among RDS survivors, and the causes of premature birth need continuing study.15 Clearly, we still have much to learn, and if past history is a reliable guide, we can expect application of fundamental concepts and methods of physics and chemistry to lead the way towards better understanding and care of the fetus and neonate.
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