Fig. 6. Effect of 0.15 mg/kg morphine and 0.8 mg/kg morphine-6-glucuronide (M6G) on the ventilatory response to carbon dioxide. The control response is also given. During anesthesia, resting ventilation occurs at the intersection of the ventilatory carbon dioxide response curve and the metabolic hyperbola (M). While resting ventilation did not differ between the two drugs, the end-tidal partial pressure of carbon dioxide (Petco²) to reach a ventilation level of 2 l/min was 7 mmHg (approximately 1 vol%) greater for morphine than for M6G (50  vs. 57 mmHg). This is an indication that M6G produces less respiratory depression than morphine at the drug doses used.