In the Classroom
Carbon Dioxide Flooding: A Classroom Case Study Derived from Surgical Practice Robert C. Kerber Department of Chemistry, SUNY at Stony Brook, Long Island, NY 11794-3400;
[email protected] Many students take introductory chemistry courses to meet the requirements for admission to various health profession degree programs. That being the case, instructors should take advantage of opportunities to create and maintain interest in the chemical principles by using examples from medical practice. In this article, I highlight a widespread surgical practice not (to my knowledge) previously described in the chemical education literature, and I describe its use in illustrating principles of gas density, solubility, acid–base reactions, and buffering. Carbon Dioxide Use in Open-Heart Surgery Open-heart surgery, for purposes of valve repair or replacement, repair of septal defects, aneurysms, and the like, has become a routine practice (at least for the surgical team, if not for the patient). During the surgery, the patient is maintained on a heart–lung machine while the heart is cooled to cause cessation of its pumping action. When the repair is complete, the heart must be closed and restarted as the heart– lung bypass is terminated. An inherent risk in this surgery is the difficulty of removing all of the air from the chambers of the heart, since any left behind will result in air bubbles (embolisms). These can cause fatal damage if allowed to circulate, especially to the brain. Various physical techniques for deaeration of the coronary cavities, including combinations of venting, shunting, repositioning the patient, and compression and massaging of the heart, have been used. Three factors have led in recent years to the widespread use of carbon dioxide flooding, first suggested in 1958 (1, 2). One factor is the increasing use of smaller incisions in “minimally invasive surgery”, which limit the surgeon’s ability to implement these physical techniques (3). Another is the development of transesophageal echocardiography (TEE), in which an ultrasound image is generated during the surgical intervention, from a probe located immediately beneath the heart in the esophagus. This technique allows direct visualization of air bubbles in the heart, even after thorough attempts at physical deaeration (4). The third factor is the documentation of decreased cognitive function in many postsurgical patients as the result of microemboli having traveled to the brain (5). Carbon dioxide flooding involves passing large volumes of carbon dioxide gas, typically 1–10 Lmin, over the surgical field either throughout the duration of the operation or just at the end, before closure of the heart. Various means are used to disperse the gas flow, avoiding a direct stream (6, 7). The idea is that, if some gas bubbles inevitably remain after closure, they will be rapidly absorbed when they contain soluble carbon dioxide rather than comparatively insoluble air. This has been effectively demonstrated in surgical patients by TEE ultrasonic imaging (7). A small risk arising from this process is excessive lowering of blood pH by the dissolved carbon dioxide and the resultant
carbonic acid formed (8). Fortunately, the lungs are very effective at removing carbon dioxide, and no evidence of injury from this source has been presented. Techniques for minimizing the effects of carbon dioxide have been proffered (8, 9). Chemical Principles Gas Density Most students are able to manipulate the ideal gas law to show that the gas density, ρ, at a given pressure and temperature, is directly proportional to the molar mass, W, of the gas: ρ = mV = W(PRT ) This also follows directly from Avogadro’s principle. Accordingly, carbon dioxide gas is expected to be 1.5 (4429) times as dense as air. In the absence of turbulence, carbon dioxide should fill a cavity open at the top, displacing air under the influence of gravity. Measurements made during open-heart surgery have confirmed high partial pressures of CO2 (0.85–0.97 atm; ref 6 and 0.95 atm; ref 7) and low partial pressures of O2 (0.01–0.02 atm; ref 1a and less than 0.01 atm; ref 7 ) in the chest cavity under conditions of carbon dioxide flooding.
Gas Solubility Empirically, carbon dioxide gas is more soluble in water than oxygen and nitrogen are (1.17 g kg᎑1 at 1 atm and 35 ⬚C versus 0.035 g kg᎑1 and 0.016 g kg᎑1, respectively; ref 10). This may reasonably be attributed to formation of hydrogen bonds to the partially negative oxygen ends of the polarized C⫽O bonds (11) and to chemical reaction with the water. Formation of molecular carbonic acid, H2CO3, is too slight to account for the enhanced solubility and will be neglected for purposes of this discussion. However some of the dissolved CO2 reacts with water to form bicarbonate ion and hydrogen ion (12), causing the pH to drop. A water solution at 35 ⬚C in equilibrium with one atmosphere of CO2 gas would have a pH of about 4, based on a concentration of 0.0266 M for dissolved CO2 and an empirical Ka1 of 4.9 × 10᎑7 for carbon dioxide–carbonic acid. Such a pH change in the blood could not be tolerated and is prevented by the buffering action of the solutes in the blood. Blood serum (whole blood from which the cellular components have been removed by centrifugation) is a complex aqueous solution of electrolytes, proteins, and organic components. Sodium is the principal cation, balanced by chloride, bicarbonate, phosphate, and carboxylate anions (13). The effect of the electrolytes is to “salt out” CO2, whose solubility in 0.16 M sodium chloride solution at 37.5 ⬚C and 1 atm is reduced by 5–6% relative to pure water (14). Acid–Base Buffer Effect Blood is buffered at a pH of about 7.36. Several components contribute to the buffering action, including phos-
JChemEd.chem.wisc.edu • Vol. 80 No. 12 December 2003 • Journal of Chemical Education
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In the Classroom
phates and imidazole side chains on proteins, but the principal buffer, because of its relatively high concentration, ca. 26 mM, compared to 1 mM for phosphate, is bicarbonate (13). The pH requires a bicarbonate–carbon dioxide ratio of 11, so this system is principally effective at resisting acidification. This is achieved dynamically, through loss of CO2 from the lungs in response to falling pH, which induces hyperventilation. Indeed, if the concentration of dissolved CO2 in a static system rose to the value of 0.026 M, the value expected if blood is in equilibrium with one atmosphere of gaseous CO2 with no change in bicarbonate concentration, the pH would fall to a toxic 6.35. One reported case of “severe hypercarbia” owing to a malfunctioning atrial aspirator during surgical CO2 flooding resulted in a blood carbon dioxide concentration of 0.0035 M, based on a measured pressure of 100 mm and a pH of 7.06 (8). The incidence was transient and led to no harm to the patient. Published measurements indicate that the solubility of carbon dioxide in blood serum is essentially identical to that in the isotonic sodium chloride solution (15). The bicarbonate that should have been present in the serum seems neither to have decreased the solubility by the common ion effect nor to have increased it by virtue of the higher pH. The lack of apparent effect is resolved upon realizing that the workers making these measurements were interested only in the concentration of dissolved CO2, which they determined from measurements of the equilibrium pressure of gaseous CO2, applying Henry’s law. Accordingly, they had suppressed bicarbonate by acidifying the serum with lactic acid, to a pH below 5, before making measurements. In contrast, what is of interest in understanding the efficacy of carbon dioxide flooding is ability of the blood to absorb the CO2, measured by the total concentration of dissolved carbonic species in all forms. In the absence of direct measurements on unacidified serum, the estimate of 0.03 M total carbonic species in the hypercarbic patient indicates that serum could absorb as much as 0.7 liter of CO2 per liter at 37 ⬚C and 1 atmosphere. This is sufficient to ensure dissolution of a few milliliters of gas bubbles after surgical carbon dioxide flooding. Use in the Classroom A good example can be used in many ways. I use this scenario as an introductory exercise in a freshman honors chemistry course taken principally by university students with two years of high school chemistry, the second often an AP course. I describe the surgical practice of carbon dioxide flooding, then ask the students to interpret why it is done and why it works. Classroom discussion usually turns first to the gas laws (everybody remembers PV = nRT ) and their relevance. Further discussion leads to considerations of solubility and effects on pH as adumbrated above. This exercise provides a broad review and reminder of many of the basic principles the students have previously learned; more important, it provides an example of chemistry in an unexpected setting that grabs their interest. Many students cite this discussion in end-of-semester course evaluations, indicating that it does remain in their minds even after several months. This exercise could equally well be used as a wrap-up discussion at the end of a full-year high school or college chemistry course, encouraging the students to in1438
tegrate concepts from throughout the course and to apply them to a novel but real situation. Acknowledgment This practice was called to my attention by cardiothoracic surgeon Frank C. Seifert, for whose professional services I am most grateful. Literature Cited 1. (a) Mitchell, B. A. Carbon Dioxide Flooding the Operative Field to Minimize or Prevent Air Embolism During Open Heart Operations. In The Virtual Textbook of Extracorporeal Technology; available at http://perfline.com/textbook/local/flooding0599.html (accessed Sep 2003). (b) Frados, A. J. Extra-Corpor. Technol. 2001, 33, 91–93. 2. Nichols, H. T.; Morse, D. P.; Hirose, T. Surgery 1958, 43, 236– 244. 3. Burke, R. P. Surg. Clin. North Am. 2000, 80, 1593–1605. 4. (a) Furuya, H.; Suzuki, T.; Okumura, F.; Kishi, Y.; Uefuji, T. Anesthesiology 1983, 58, 124–129. (b) Oka, Y.; Inoue, T.; Hong, Y.; Sisto, D. A.; Strom, J. A.; Frater, J. M. W. J. Thorac. Cardiovasc. Surg. 1986, 91, 329–338. 5. (a) Pugsley W.; Treasure T.; Klinger L.; Newman M. F.; Pascalis C.; Harrison M. Vasc. Surg. 1990, 24, 34–43. (b) Mahanna, E. P.; Blumenthal, J. A.; White, W. D.; Croughwell, N. D.; Clancy, C. P.; Smith, L. R. Ann. Thorac. Surg. 1996, 61, 1342–1347. (c) McKhann, G. M.; Goldsborough, M. A.; Borowicz, L. M., Jr.; Selnes, O. A.; Mellits, E. D.; Enger, C.; Quaskey, S. A.; Baumgartner, W. A.; Cameron, D. E.; Stuart, R. S.; Gardner, T. J. Ann. Thorac. Surg. 1997, 63, 510–515. (d) Goldstein, L. J. J. Thorac. Cardiovasc. Surg. 2001, 122, 935–945. 6. Selman, M. W.; McAlpine, W. A.; Albregt, H.; Ratan, R. J. Thorac. Cardiovasc. Surg. 1967, 53, 618–622. 7. (a) Webb, W. R.; Harrison, L. H., Jr.; Helmcke, F. R.; CaminoLopez, A.; Munfakh, N. A.; Heck, H. A., Jr.; Moulder, P. V. Ann. Thoracic Surg. 1997, 64, 1489–1491. (b) Olinger, G. N. J. Cardiovasc. Surg. 1995, 109, 187–190. 8. (a) Burbank, A.; Ferguson, T. B.; Burford, T. H. J. Thorac. Cardiovasc. Surg. 1965, 50, 691–698. (b) O’Connor, B. R.; Kussman, B. D.; Park, K. W. Anesth. Analg. 1998, 86, 264–266. 9. Nadolny, E. M. Perfusion 2000, 15, 151–153. 10. Gevantman, L. H. Solubility of Selected Gases in Water. In CRC Handbook of Chemistry and Physics, 82nd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2001–2002; pp 8-86–8-89. 11. Sato, H.; Matubayashi, N.; Nakahara, M.; Hirata, F. Chem. Phys. Lett. 2000, 323, 257–262. 12. For general discussions of CO2 and its chemical properties, see (a) Bent, H. A. J. Chem. Educ. 1987, 64, 167–171. (b) Chemical of the Week—Carbon Dioxide. http://scifun.chem.wisc.edu/ chemweek/CO2/CO2.html (accessed Sep 2003). (c) Kern, D. M. J. Chem. Educ. 1960, 37, 14–23. 13. Harrow, B.; Mazur, A. Textbook of Biochemistry; W. B. Saunders: Philadelphia, PA, 1966; pp 428–429, 461–465. 14. (a) Harned, H. S.; Davis, R., Jr. J. Amer. Chem. Soc. 1943, 65, 2030–2037. (b) Siesjo, B. K. Acta Physiol. Scand. 1962, 55, 325–341. (c) Yeh, S-Y.; Peterson, R. E. J. Pharm. Sci. 1964, 53, 822–p4. 15. (a) Van Slyke, D. D.; Sendroy, J., Jr.; Hastings, A. B.; Neill, J. M. J. Biol. Chem. 1928, 78, 765–799; (b) Austin, W. H.; Lacombe, E.; Rand, P. W.; Chatterjee, M. J. Appl. Physiol. 1963, 18, 301–304.
Journal of Chemical Education • Vol. 80 No. 12 December 2003 • JChemEd.chem.wisc.edu
