CO2‐breathing and Output of the Heart

Summary. On two subjects it is shown, that hyperventilation, caused by CO2‐breathing, has an increasing effect on the cardiac output if the subject lies horizontally, none, however, if the subject is tilted to an upright (60°) position. It is concluded, that the effect of CO2‐breathing on the cardiac output must be due to the mechanical forces of hyperventilation and not to a chemical action of the CO2 on the circulation, as otherwise the difference of the effect in the two positions is unexplained. By increasing the inspiratory or exspiratory resistance artificially the negativity of the intra‐pleural pressure could be increased, respectively decreased. It was found that this had very slight and irregular effects on the cardiac output, and it is concluded, that the augmented negativity of the intra‐pleural pressure during hyperventilation cannot be the cause of the increased cardiac output. It is discussed how the deep respirations of the hyperventilation might influence the cardiac output, and the conclusion is put forward tentatively, that this is possible if one assumes that the enlarged respiratory movements cause a pressing‐out of blood from the intra‐pulmonary (or, and from the abdominal) vessels, when these are “filled” with blood (as in the horizontal position), not, however, when they are “empty” (as in the tilted position). The conclusion is supported by experiments, in which part of the blood was shut off in the legs.