The response of the respiratory mechanism to rapid changes in the reaction of the blood

Abstract

For the past year we have been studying the reaction of the blood by two parallel methods: the Fridericia method for alveolar CO₂ and the Van Slyke determination of the carbon dioxide combining power of the blood. We believe that this combination offers a simple method for the study of the state and the reaction of the respiratory mechanism.

In no normal person have we found a disagreement of over 10 per cent. in the ratio of alveolar/plasma CO₂. Discrepancies observed seem to fall into two large classes: (1) Those due to a mechanical interference with the gaseous exchange, and (2) those due to changes in the control of the respiratory mechanism.

The first class is best illustrated by patients with cardiac dyspnea or very great dimunition of the pulmonary capacity. In these the alveolar is invariably lower than the plasma reading. If compensation is established the two again come into agreement. This, we believe, is due to an impairment of the lungs that renders the excretion of CO₂ more difficult. The body overcomes this, in response to stimulation of the respiratory center by the retained CO₂, with a greater pulmonary ventilation. In this way a pressure difference is established between the carbon dioxide tension in the blood and in the alveoli, sufficient to compensate for the impairment in the lungs. The increased minute volume, the intolerance to carbon dioxide in the inspired air, the dyspnea and the disproportionate response to exercise are all expressions of the same thing.

The second class is best illustrated by the discrepancies observed after rapid changes in the reaction of the blood. The alveolar air tends to lag behind the plasma in its response. This is susceptible of an easy explanation during a sudden increase of the H-ion concentration of the blood, when the demand on the respiratory mechanism is overwhelming and the lungs are flooded with CO₂.