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Understanding the meaning of the shunt fraction calculation

Abstract

The pulmonary shunt fraction (\({{\dot Qs} \mathord{\left/ {\vphantom {{\dot Qs} {\dot Qt}}} \right. \kern-\nulldelimiterspace} {\dot Qt}}\)) is frequently calculated in critically ill patients to monitor the effectiveness of pulmonary oxygenation. The breathing of pure oxygen often results in higher calculated\({{\dot Qs} \mathord{\left/ {\vphantom {{\dot Qs} {\dot Qt}}} \right. \kern-\nulldelimiterspace} {\dot Qt}}\) values that have been attributed to the development of atelectasis, ventilation-perfusion imbalance, or both. To interpret properly the changes in calculated\({{\dot Qs} \mathord{\left/ {\vphantom {{\dot Qs} {\dot Qt}}} \right. \kern-\nulldelimiterspace} {\dot Qt}}\) that occur when the inspired oxygen fraction is altered, the changes produced in all the variables affecting\({{\dot Qs} \mathord{\left/ {\vphantom {{\dot Qs} {\dot Qt}}} \right. \kern-\nulldelimiterspace} {\dot Qt}}\) must be known. This tutorial presents an in-depth analysis of the four variables affecting the calculation of\({{\dot Qs} \mathord{\left/ {\vphantom {{\dot Qs} {\dot Qt}}} \right. \kern-\nulldelimiterspace} {\dot Qt}}\) \(\dot Vo_2 \) (oxygen uptake),\(\dot Qt\) (cardiac output), Cc'O2(oxygen content in pulmonary end capillaries), and\(C\bar vO_2 \) (oxygen content in mixed venous blood). These variables are related according to the following equation, which derived by combining the Fick and the classic shunt equations:\({{\dot Qs} \mathord{\left/ {\vphantom {{\dot Qs} {\dot Qt}}} \right. \kern-\nulldelimiterspace} {\dot Qt}} = 1 - [({{{{\dot Vo_2 } \mathord{\left/ {\vphantom {{\dot Vo_2 } {\dot Qt}}} \right. \kern-\nulldelimiterspace} {\dot Qt}})} \mathord{\left/ {\vphantom {{{{\dot Vo_2 } \mathord{\left/ {\vphantom {{\dot Vo_2 } {\dot Qt}}} \right. \kern-\nulldelimiterspace} {\dot Qt}})} {(Cc'O_2 }}} \right. \kern-\nulldelimiterspace} {(Cc'O_2 }} - C\bar vO_2 )]\). Three-dimensional surface representations relating these variables are also presented to help the reader understand the effects of these variables on the calculated\({{\dot Qs} \mathord{\left/ {\vphantom {{\dot Qs} {\dot Qt}}} \right. \kern-\nulldelimiterspace} {\dot Qt}}\).

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References

  1. Bartels H, Dejours P, Kellogg RH, Mead J. Glossary on respiration and gas exchange. J Appl Physiol 1973;34:549–558

    Google Scholar 

  2. Comroe JH Jr, Forster RE II, Dubois AB, et al. The lung, 2nd ed. Chicago: Year Book, 1962;343–345

    Google Scholar 

  3. Douglas ME, Downs JB, Dannemiller FJ, et al. Change in pulmonary venous admixture with varying inspired oxygen. Anesth Analg 1976;55:688–695

    PubMed  Article  CAS  Google Scholar 

  4. McAslan TC, Matjasko-Chiu J, Turney SZ, Cowley RA. Influence of inhalation of 100% oxygen on intrapulmonary shunt in severely traumatized patients. J Trauma 1973;13:811–821

    PubMed  CAS  Article  Google Scholar 

  5. Kerr JH. Pulmonary oxygen transfer during IPPV in man. Br J Anaesth 1975;47:695–705

    PubMed  Article  CAS  Google Scholar 

  6. Suter PM, Fairley HB, Schlobohm RM. Shunt, lung volume and perfusion during short periods of ventilation with oxygen. Anesthesiology 1975;43:617–627

    PubMed  Article  CAS  Google Scholar 

  7. Quan SF, Kronberg GM, Schlobohm RM, et al. Changes in venous admixture with alterations of inspired oxygen concentration. Anesthesiology 1980;52:477–482

    PubMed  Article  CAS  Google Scholar 

  8. Oliven A, Abinader E, Bursztein S. Influence of varying inspired oxygen tensions on the pulmonary venous admixture (shunt) of mechanically ventilated patients. Crit Care Med 1980;8:99–101

    PubMed  Article  CAS  Google Scholar 

  9. Gallagher TJ, Civetta JM. Goal-directed therapy of acute respiratory failure. Anesth Analg 1980;59:831–834

    PubMed  Article  CAS  Google Scholar 

  10. Rahn H, Fenn W. A graphical analysis of the respiratory gas exchange. The O2-CO2 diagram. Washington, DC: American Physiological Society, 1955:1–41

    Google Scholar 

  11. Farhi LE. Recent advances in respiratory physiology. Ventilation-perfusion relationship and its role in alveolar gas exchange. London: WH Arnold, 1965:148–197

    Google Scholar 

  12. Philbin DM, Sullivan SF, Bowman FO, et al. Postoperative hypoxemia: contribution of cardiac output. Anesthesiology 1970;32:136–142

    PubMed  Article  CAS  Google Scholar 

  13. Berggren SM. The oxygen deficit of arterial blood caused by non-ventilating parts of the lung. Acta Physiol Scand 1942; 4:Suppl 11:1–92

    Google Scholar 

  14. Johnson SF, Cruz JC, McDonald JS. Spatial representation of the Fick and shunt models. Fed Proc 1982;41:1129

    Google Scholar 

  15. Cruz JC, Reilley TE. A new approach to analyze the shunt in the critically ill patient. Crit Care Med 1982;10:236

    Article  Google Scholar 

  16. Cruz JC, Reilley TE. Mechanisms of changes in venous admixture by augmenting inspired oxygen concentration. Fed Proc 1982;41:1129

    Google Scholar 

  17. Kelman GR, Nunn JF, Prys-Roberts C, Greenbaum R. The influence of cardiac output on arterial oxygenation: a theoretical study. Br J Anaesth 1967;39:450–457

    PubMed  Article  CAS  Google Scholar 

  18. Smith G, Cheney FW Jr, Winter PM. The effect of change in cardiac output on intrapulmonary shunting. Br J Anaesth 1974;46:337–342

    PubMed  Article  CAS  Google Scholar 

  19. Bishop MJ, Cheney FW. Effects of pulmonary blood flow and mixed venous O2 tension on gas exchange in dogs. Anesthesiology 1983;58:130–135

    PubMed  CAS  Article  Google Scholar 

  20. Cruz JC. The open book, model for the oxygen uptake, cardiac output and shunt fraction relationships. Proc Int Union Physiol Sci 1986;16:176

    Google Scholar 

  21. Cruz JC, Beaver BL, Reilley TE, McDonald JS. Changes in oxygen uptake, cardiac output and/or mixed venous O2 difference produced by augmenting inspired O2. Anesthesiology 1982;57:A122

    Article  Google Scholar 

  22. Schaefer KE. Circulatory adaptation to the requirements of life under more than one atmosphere of pressure. In: Hamilton WF, Dow P, eds. Handbook of physiology. Vol 3. Washington, DC: American Physiological Society, 1965:1843–1873

    Google Scholar 

  23. West JB. Regional differences in gas exchange in the lung of erect man. J Appl Physiol 1962;17:893–898

    PubMed  CAS  Google Scholar 

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Cruz, J.C., Metting, P.J. Understanding the meaning of the shunt fraction calculation. J Clin Monitor Comput 3, 124–134 (1987). https://doi.org/10.1007/BF00858361

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  • DOI: https://doi.org/10.1007/BF00858361

Key words

  • Oxygen content inspired oxygen fraction
  • Heart: cardiac output
  • Lungs: pulmonary oxygenation oxygen uptake Fick equation shunt equation