Ventilatory effects of nitrous oxide during continuous infusion of fentanyl or alfentanil

Two groups of eight patients received infusions of either fentanyl at 3 micrograms kg-1 h-1 or alfentanil at 20 micrograms kg-1 h-1 as supplements to 66% N2O in oxygen anaesthesia, during and after body surface surgery. At the end of surgery, the N2O was reduced to 50% and after measurement of ventilatory frequency, minute ventilation, and the ventilatory response to carbon dioxide, N2O was discontinued. The opioid infusions were continued for a further hour and the ventilatory measurements repeated. Both sets of measurements were compared with preoperative values. Minute ventilation (P less than 0.01), frequency (P less than 0.01) and the response to carbon dioxide (P less than 0.01) were reduced during the infusion of fentanyl with N2O; with fentanyl alone, minute ventilation (P less than 0.05) and the response to carbon dioxide (P less than 0.01) were reduced but to a lesser degree. The elimination of nitrous oxide from the inspired gas mixture produced an increase in frequency (P less than 0.05) and increases in the slope (P less than 0.01) and ventilation at 7.3 kPa (P less than 0.025) of the carbon dioxide response curve. Minute ventilation (P less than 0.01) frequency (P less than 0.05) and response to carbon dioxide (P less than 0.01) were all reduced during the infusion of alfentanil with nitrous oxide; with alfentanil alone, minute ventilation (P less than 0.01), tidal volume (P less than 0.05), the slope (P less than 0.025) and the ventilation at 7.3 kPa (P less than 0.01) of the carbon-dioxide response curve were still reduced.(ABSTRACT TRUNCATED AT 250 WORDS)