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

Respiratory dead-space is often increased in lung disease. This study evaluates the effects of increased alveolar dead-space (Vd(alv)), pulmonary shunt, and abnormal ventilation perfusion ratio (/) distributions on dead-space and alveolar partial pressure of carbon dioxide (Pa(co(2))) 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 (Vd(phys)) in the presence of pulmonary shunt.

Pulmonary shunt, / distribution and Vd(alv) 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. Pa(co(2)) was calculated by the method of Koulouris.

When Vd(alv) is increased, Vd(phys) 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 Vd(phys) 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 / maldistribution. When alveolar Pco(2) is increased by any mechanism, Pa(co(2)) calculated by Koulouris' method does not agree well with average alveolar Pco(2).

Our studies show that increased pulmonary shunt causes an apparent increase in Vd(phys), and that abnormal / distributions affect the calculated Vd(phys) and Vd(alv), but not Fowler dead-space. Dead-space and Pa(co(2)) calculated by the Koulouris method do not represent true Bohr dead-space and Pa(co(2)) respectively, but the shunt-corrected equation performs well.