The efficacy of four respiratory analeptics on healthy young men in a CO2 rebreathing experiment.

Four respiratory analeptics were examined in a CO2 rebreathing experiment with increase from 0--6% in the inspiratory CO2 concentration during 30 min on 24 healthy young men. The respiratory response curves (VE/pCO2) showed a parabolic shape. They were examined on parallel shift and change of slope. The "excitability ratio" is the first derivative of the response parabola and as such becomes a linear function of the actual arterial CO2 partial pressure. The following effects have been shown: 1. 150 mg amiphenazole (2.07 mg/kg) cause a small rise in the ventilation by accelerating the frequency at increased CO2 partial pressures above 45 mmHg. 2. 240 mg theophylline ethylenediamine (3.32 mg/kg) produce a nearly parallel upward-shift of the respiratory response curve, i.e., the respiratory minute volume increases independently by a deepening of the respiration. 3. 450 mg prethcamide (6.22 mg/kg) cause a slight increase of the "excitability ratio" at CO2 partial pressures above 45 mmHg. The CO2-dependent respiratory minute volume is not changed significantly but the pattern of breathing changes by accelerating the frequency and decreasing the tidal volume. 4. 40 mg fominobene (0.55 mg/kg) raise the "excitability ratio" convincingly at CO2 partial pressures above 40 mmHg. However, the total volume ventilated during the experiment does not increase because of a diminished ventilation at rest.