The mechanical factors of excessive artificial respiration and a consideration of their relation to the acapnial theory of shock

It has been claimed that the most important factor in the causation of shock is diminution of CO₂ within the blood, and that this diminution is a regular consequence of all influences resulting in shock. That CO₂ possesses important physiological functions cannot be denied. An investigation therefore of the true significance of a diminution of its normal amount within the blood is important and bears a special relation to various methods of artificial respiration utilized in thoracic surgery. The present experiments were undertaken for the purpose of investigating the effects of acapnia and the relation of some factors concerned in its production to shock. In all of them dogs were used. The first series was performed for the purpose of studying the effect of variations in intrapulmonic air pressure upon the blood pressure. The thorax was opened laterally, a T-tube connected with a water manometer was tied in a small bronchus, and the heart enclosed in a Henderson cardiometer in series with a recording tambour. The blood pressure was recorded from the carotid artery. The thorax was closed and the animal was subjected to intratracheal insufflation from an apparatus provided with an exhaust valve which reduced the pressure to approximately zero about four (4) to twelve (12) times per minute. The blood pressure averaged 150 mm. Hg. when the intrapulmonic air pressure was not allowed to exceed 6 mm. Hg.

In one experiment, with an increase of intrabronchial pressure from 8 mm. to 30 mm. Hg, blood pressure fell from 122 mm. to 55 mm. Hg and the volumetric tracing indicated that the output from the heart had diminished about 44 per cent. These variations in blood pressure were completed within a few seconds after the change in intrabronchial pressure, and could be duplicated at will.