Comparison of morphine alone with morphine plus clonidine for postoperative patient-controlled analgesia

C. Carey1, S. Jagger1, S. A. Jeffs2, J. E. Hall2 and S. Morris2

1 London, UK 2 Cardiff, UK

Editor—We were interested to read the article by Jeffs and colleagues1 on the addition of clonidine to morphine PCA devices, and would like to make some comments. First, the median pain score in the morphine group for the first 12 h was given as 3 with a range of 1–4, although it was stated that the pain scale used was 0–3. If this should be read as a median of 2 with a range of 1–3, it suggests that these patients experienced significant pain and we wondered if any rescue analgesia was given after the initial boluses of morphine in recovery.

Second, in a recent study by Marinangeli and colleagues2 the optimum analgesic dose of clonidine in the postoperative period was found to be a 3 µg kg–1 bolus followed by an infusion of 0.3 µg kg–1 h–1. This was shown to significantly decrease morphine PCA requirements, although the study only lasted for 12 h. The rate of infusion used equated to 21 µg h–1 in a 70 kg adult, compared with an average clonidine dose equivalent to 40 µg h–1 over the entire duration of this study (together with morphine 72 mg over 36 h in the clonidine group). Thus, the patients in this study received approximately twice the average hourly dose of clonidine compared to Marinangeli’s study, and there was no reduction in morphine requirements in the first 12 h. The current study therefore does not support Marinangeli’s data and, as the authors stated, checking plasma concentrations may add useful information in trying to optimize the dose delivered in each PCA bolus.

We also note that patients in this study were given droperidol 1 mg at induction of anaesthesia. Droperidol is a butyrophenone agent with a variety of actions and, of particular relevance to this study, is known to have alpha-antagonist effects.3 It is therefore possible that droperidol may have a significant interaction with the effects of clonidine at alpha-receptors that would affect the results of this study.

Finally, the scoring systems used by the investigators seem to have been devised de novo for this study. Scoring systems, especially for subjective measurements such as pain and sedation, should be validated before use in a trial such as this one and we wondered whether this had occurred.

C. Carey

S. Jaggar

London, UK

Editor—Thank you for allowing us to reply to the letter by Drs Carey and Jaggar. We would first like to apologize for an error in Table 2. As they rightly say, the median pain score in the morphine group for the first 12 h should read 2 with a range of 1–3. Rescue analgesia was not given, but the patients were encouraged to use their PCA.

With regard to our use of droperidol, it was given to both groups at induction only. We would therefore expect any difference in our findings to be because of the clonidine and not the droperidol.

We agree that our scoring system had not been validated and, whilst we agree this would have been optimal, this scoring system was chosen as it is the one used on our PCA/pain charts. It was felt that less confusion would occur if a scoring system familiar to the ward staff was used.

S. A. Jeffs

J. E. Hall

S. Morris

Cardiff, UK

References

1 Jeffs SA, Hall JE, Morris S. Comparison of morphine alone with morphine plus clonidine for postoperative patient-controlled analgesia. Br J Anaesth 2002; 89: 424–7[Abstract/Free Full Text]

2 Marinangeli F, Ciccozzi A, Donatelli F, et al. Clonidine for treatment of postoperative pain: a dose-finding study. Eur J Pain 2002; 6: 35–42

3 Castillo C, Castillo EF, Valencia I, Ibarra M, Bobadilla RA. Droperidol interacts with vascular serotonin receptors and alpha-adrenoceptors. Arch Int Pharmacodyn Ther 1995; 330: 53–65[ISI][Medline]





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