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LETTERS TO THE EDITOR

One Drop of Morphine Added to Local Anesthetics by Means of a 23-Gauge Injection Needle Can Relieve Postoperative Pain Under Spinal Anesthesia

Department of Anesthesia Osaka Kosei-Nenkin Hospital Osaka 553-0003, Japan

We read with great interest the article by Slappendel et al. (1). We agree with that the optimum dose of intrathecal morphine in total hip surgery is 0.1 mg. Another report (2) has shown excellent postoperative pain relief under spinal anesthesia obtained with a small dose of subarachnoid morphine. However, it was not clear how exactly a small dose of morphine was added to the local anesthetic. We assume that the morphine after repeated dilutions has to be added to the local anesthetic and that this troublesome procedure may cause contamination of the drugs. Moreover, the increase in volume as a result of the addition of morphine could have an undesirable influence on the extent of the spinal blockade.

We have routinely used a 23-gauge injection needle (MN-2325R, TERUMO, Tokyo, Japan) for adding 0.15 mg of morphine to local anesthetics when we perform spinal anesthesia. Our technique of using this needle means that one drop equals 0.015 mL of morphine (10 mg/mL), as shown in Figure 1. Our method offers two major advantages. First, the procedure is clean and simple, and second, one drop of morphine doesn’t result in a change in the total volume of local anesthetics.

View larger version (102K): [in this window] [in a new window]   Figure 1. A clean and simple method for adding a small dose of morphine to local anesthetic. When the bevel of a 23-gauge injection needle is turned upward and kept horizontal, one drop of morphine equals 0.015 ± 0.0009 mL (mean ± SD for 100 trials). With only one drop of morphine added to a local anesthetic, it is easy to perform spinal anesthesia with 0.15 mg of subarachnoid morphine.

1. Slappendel R, Weber ERG, Dirksen R, et al. Optimization of the dose of intrathecal morphine in total hip surgery: a dose-finding study. Anesth Analg 1999;88:822–6.[Abstract/Free Full Text]
2. Yamaguchi H, Watanabe S, Motokawa K, et al. Intrathecal morphine dose-response data for pain relief after cholecystectomy. Analg 1990;70:168–71.[Abstract/Free Full Text]

Response

Sint Maartenskliniek6500 GM Nijmegen, The Netherlands

We thank our Dr. Toyoyama et al. for their response dealing with the preparation of drugs in our article. However, we do not support their proposal that the anesthesiologists themselves dilute or add the morphine to the bupivacaine. To guarantee the best quality of all mixtures, which are not commercially available, we have these prepared by our hospital pharmacy department. For all routes of administration of drugs in our anesthesiology practice, we must be certain that the exact concentration of the drug is present and the preparations are made to Good Manufacturing Practice standards.

The actual method for preparation of the drug mixtures in our study (1), bupivacaïne 0.5% solution plus morphine 0.1 mg per 4 mL is given below.

A dry 4-L bottle is filled with 1500 mL sterile water for injection. Ten grams of bupivacaïne hydrochloride 1.00 H₂O Pharmacopea European (PhEur) and 15 grams of sodium chloride H₂O PhEur are weighed and added to the water and mixed by a magnetic stirrer. The amount of 50 mg morphine is weighed and added to the solution. Diluted hydrochloric acid is added to a pH of 4.0. Sterile water is added up to 2 kg. When the sterile water has been added, the magnetic stirrer mixes the fluid during another 10 min until a homogeneous fluid exists while nitrogen is led through the solution for 15 min. The bottle is closed with a paraffin film.

In an ampulla’s filling machine, the ampulla’s are filled up to 5.3 mL while nitrogen is led through and over the solution to remove all oxygen. The ampulla’s are sterilized in a steam autoclave for 16 min at 121°C. The ampulla’s are labeled and numbered. A list of the labels is kept in the pharmacy.

For each batch of 70 ampulla’s, 60 ampulla’s are used in clinical practice. The remaining 10 ampulla’s are used for quality checks. The concentrations of morphine and bupivacaïne are measured by using high-pressure liquid chromatography. A glass electrode measures the pH. The osmolality is measured (normal, 271–301 mmol/L). Finally the germ number is determined before filtration and must be below 10 germs per mL.

In all, we strongly recommend to refrain from less accurate methods. The safety range of intrathecal morphine is just one drop.

References

1. Slappendel R, Weber EWG, Dirksen R, et al. Optimization of the dose of intrathecal morphine in total hip surgery: a dose finding study. Anesth Analg 1999;88:822–6.

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