Post-surgical pain relief with zero-order intravenous infusions of meptazinol and morphine: a double-blind placebo-controlled evaluation of their effects on ventilation
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A double-blind, placebo-controlled study has been made of the analgesic and respiratory effects of constant rate infusions of meptazinol and morphine in 30 patients after abdominal surgery.
Group I received meptazinol, loading dose 50 mg followed by i.v. infusion 0.5 mg · kg−1 · h−1, Group II received morphine, loading dose 5 mg and then an infusion of 0.05 mg · kg−1 · h−1, and Group III received saline.
After recovery from inhalation anaesthesia (without opiates or a local anaesthetic) pain relief and chemoreceptor carbon dioxide tolerance were assessed before and at various times after starting the analgesic infusion. A similar degree of pain relief was found after 10 min in Groups I and II, which lasted until the end of observation period (20 h). Heart rate and systolic and diastolic blood pressures were lower in Group II than in Groups I and III, and respiratory rate fell in Groups I and II.
After 6 h arterial carbon dioxide tensions (PaCO2) became significantly higher in Group II than Group III. The maximum percentage fall in mean tidal volume (VT) and expired minute volume (0VE) from the preinjection values was significant in Groups I and II. End-tidal carbon-dioxide (PETCO2) and PaCO2 were significantly higher after 20 h of infusion in Group II compared to Group I. The slope of 0VE/PETCO2 (<S>) was increased in Group I and it was significantly reduced in Group II. Analysis of derived variables, such as the CO2 intercept (CO2I) and minute ventilation at 7 kPa (0VE7), indicated a shift to the right of the slopes in Groups I and II, initially more so in Group I.
It is concluded that constant rate infusions of meptazinol and morphine were effective in providing postoperative pain relief. However, their effects on the central regulation of respiration were different, as meptazinol did not impair CO2 sensitivity whereas morphine did.
Key wordsanalgesia morphine meptazinol ventilatory response
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