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BJA Advance Access published online on August 26, 2005
British Journal of Anaesthesia, doi:10.1093/bja/aei212
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© The Board of Management and Trustees of the British Journal of Anaesthesia 2005. All rights reserved. For Permissions, please e-mail: journal.permissions@oupjournals.org

Laboratory Investigation

Effects of alveolar dead-space, shunt and {Vdot}/{Qdot} distribution on respiratory dead-space measurements

Y. Tang 1, M. J. Turner 1, and A. B. Baker 1*

1 Department of Anaesthetics, University of Sydney, Royal Prince Alfred Hospital, Sydney, NSW 2050, Australia

* To whom correspondence should be addressed.
A. B. Baker, E-mail: bbaker{at}usyd.edu.au


  Abstract

Background. Respiratory dead-space is often increased in lung disease. This study evaluates the effects of increased alveolar dead-space (VDalv), pulmonary shunt, and abnormal ventilation perfusion ratio ({Vdot}/{Qdot}) distributions on dead-space and alveolar partial pressure of carbon dioxide (PACO2) calculated by various methods, assesses a recently published non-invasive method (Koulouris method) for the measurement of Bohr dead-space, and evaluates an equation for calculating physiological dead-space (VDphys) in the presence of pulmonary shunt.

Methods. Pulmonary shunt, {Vdot}/{Qdot} distribution and VDalv were varied in a tidally breathing cardiorespiratory model. Respiratory data generated by the model were analysed to calculate dead-spaces by the Fowler, Bohr, Bohr-Enghoff and Koulouris methods. PACO2 was calculated by the method of Koulouris.

Results. When VDalv is increased, VDphys can be recovered by the Bohr and Bohr-Enghoff equations, but not by the Koulouris method. Shunt increases the calculated Bohr-Enghoff dead-space, but does not affect Fowler, Bohr or Koulouris dead-spaces, or VDphys estimated by the shunt-corrected equation if pulmonary artery catheterization is available. Bohr-Enghoff but not Koulouris or Fowler dead-space increases with increasing severity of {Vdot}/{Qdot} maldistribution. When alveolar PCO2 is increased by any mechanism, PACO2 calculated by Koulouris' method does not agree well with average alveolar PCO2.

Conclusions. Our studies show that increased pulmonary shunt causes an apparent increase in VDphys, and that abnormal {Vdot}/{Qdot} distributions affect the calculated VDphys and VDalv, but not Fowler dead-space. Dead-space and PACO2 calculated by the Koulouris method do not represent true Bohr dead-space and PACO2 respectively, but the shunt-corrected equation performs well.

Keywords: computers; lung, shunting; ventilation, deadspace; ventilation, ventilation-perfusion.
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