Components of the Respiratory Dead Space and Their Variation With Pressure Breathing and With Bronchoactive Drugs

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Abstract
The concept of series and parallel components of the respiratory dead space is presented. The "series" dead space is associated with the volume of the respiratory passages leading to the lung alveoli. The "parallel" dead space is defined in terms of lung alveoli which are ventilated but poorly perfused. In anesthetized cats breathing at normal transpulmonary pressures (3-5 cm H2O), approximately 30% of the series dead space is located in the bronchial tree below the bifurcation of the trachea. The series dead space increases with increasing transpulmonary pressure, reaching 150$ of its normal value when the transpulmonary pressure is 15-20 cm H2O. Most of this increase can be accounted for by the distensible bronchial tree within the lungs. Histamine, epinephrine and other broncho-active drugs have little effect on the series dead space provided that the transpulmonary pressure is maintained constant. This implies that the chief resistance to gas flow is offered by the small bronchioles which contribute little to the intrapulmonary series dead space. The Bohr equation for respiratory dead space is extended to include both series and parallel components. This equation accurately characterizes the parallel dead space created by ligation of one pulmonary artery in anesthetized cats. In 2 normal human subjects the series dead space was increased by 50% during pressure breathing against 20 cm H2O. In addition, a parallel dead space was created which was equivalent to complete obstruction of the circulation to 10% of the ventilated alveoli.

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