[Transient variations of alveolar gas composition during haemodynamic disturbances (author's transl)].

Passive changes in the posture induce haemodynamic disturbances, and especially a decrease of the cardiac output. This in the steady state does not involve any modification of the alveolar gas. There are however transient variations of PAO2 and PACO2, studied in this work at constant states of ventilation and metabolism. The haemodynamic disturbances are produced by a tip-up seat, actuated by a sinusoidal motion to different velocities and a constant amplitude (90 degrees). The frequency response is analyzed according to servosystems theories. The results show that there are causes of non-linearity in PAO2 and PACO2 variations, more complex for PAO2, and that the amplitude of the variations is two to three times greater for PAO2 than for PACO2. The observed transient phenomena are essentially interpreted as resulting from the differences in dynamic behavior between cardiac output on the one hand, variations in the arterio-venous gradient on the other; this effect is more pronounced for PAO2 variations. From a practical point of view, the great amplitude of transient PAO2 variations (as much as 11 mmHg) could, under certain conditions, allow, by the single continuous recording of PO2 to the mouth, to demonstrate and define the time of the arrival of an haemodynamic disturbance in patients with assisted ventilation.