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BJA Advance Access originally published online on August 26, 2005
British Journal of Anaesthesia 2005 95(4):538-548; 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

RESPIRATION AND THE AIRWAY

Effects of alveolar dead-space, shunt and V/Q distribution on respiratory dead-space measurements

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

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

* Corresponding author: Department of Anaesthetics, Royal Prince Alfred Hospital, Building 89 Level 4, Missenden Rd, Camperdown, NSW 2050, Australia. E-mail: bbaker{at}usyd.edu.au

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 (V/Q) distributions on dead-space and alveolar partial pressure of carbon dioxide () 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, V/Q 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. 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 V/Q maldistribution. When alveolar is increased by any mechanism, 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 V/Q distributions affect the calculated VDphys and VDalv, but not Fowler dead-space. Dead-space and calculated by the Koulouris method do not represent true Bohr dead-space and respectively, but the shunt-corrected equation performs well.


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