The definition of pure dead space is ventilation without perfusion, whereby alveolar gases do not contact blood flowing through the pulmonary capillaries. 2 Physiologic dead-space fraction (dead space divided by tidal volume ), as defined by Bohr and Enghoff, is the sum of anatomic or airway dead space (V D-anat) and alveolar dead space (V D-alv) divided by the V T. In physiology the respiratory rates predicted by Otis' formulae is in keeping with the body's choice for a given minute volume.Dead-space ventilation, the portion of a tidal volume that does not contribute to gas exchange, was first described and calculated by the Bohr equation in 1891, 1 and later by the Enghoff modification of the Bohr equation in 1938. The optimal frequency will increase with increasing alveolar ventilation The optimal frequency will decrease with increasing complianceĤ. The optimal frequency will decrease with increasing dead spaceĢ. The optimal frequency will decrease with increasing non-elastic resistanceģ. Taking this into account, there must be an optimal rate at which the sum of the two effects lead to minimal WOB.Īdding non-elastic WOB into the calculations will yield a lower optimal rate than with just elastic WOB.Īpplying the above equation leads to the following conclusions for a given alveolar ventilation:ġ. A higher rate decreases elastic WOB because the tidal volumes are smaller A higher rate increases the elastic work because there is more dead space ventilationĢ. If only non-elastic work was involved, the lowest breathing rate would yield the lowest WOB.Ĭhanging in frequency influence the elastic part of WOB in two ways:ġ.

As per the above formula the non-elastic work increases with the respiratory rate simply because there is more total ventilation.

Acceptable dead space in lungs