Evidence for the emergence of an opioid-resistant respiratory rhythm following fentanyl overdose.

Breathing resumes within one to two minutes following fentanyl overdose induced apnea in spontaneously breathing rats. As this regular rhythm is produced at a time wherein fentanyl concentrations and receptor occupancy are likely to be extremely high, the mechanisms initiating and sustaining such a respiratory activity remain unclear. Forty-four un-anesthetized adult rats were studied in an open-flow plethysmograph. Regardless of the dose of fentanyl that was used, i.e. 50 μg.kg (n = 8), 100 μg.kg (n = 8) or 300 μg.kg (n = 7), all rats developed an immediate central apnea followed by a depressed regular rhythm that was produced 118, 97 and 81 s (median) later, respectively. Only one rat did not recover. This inspiratory and regular activity consisted of a low frequency and tidal volume pattern with a significant reduction in V̇E/V̇CO2 ratio, which persisted for at least 30 min and that was not different between 100 or 300 μg.kg. The time at which this respiratory rhythm emerged, following the highest dose of fentanyl, was not affected by 100 % O or 8% CO/15 % O. The absolute level of ventilation was however higher in hypercapnic and moderately hypoxic conditions than in hyperoxia. When a second injection of the highest dose of fentanyl (300 μg.kg) was performed at 10 min, ventilation was not significantly affected and no apnea was produced in major contrast to the first injection. When a similar injection was performed 30 min after the first injection, in a separate group of rats, an apnea and breathing depression was produced in 30 % of the animals, while in the other rats, ventilation was unaffected. We conclude that the depressed regular respiratory activity emerging during and following fentanyl overdose is uniquely resistant to fentanyl.