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University of Tasmania, Hobert, Australia Department of Paediatrics, University of Tasmania, Hobart, Australia
To the Editor:
The recent article by Paut Effectiveness of Blinding When Comparing Nitrous Oxide with Topical Anesthetic Cream (1) reported a double-blind methodology and emphasized the lack of blinding in previous studies.
We recently conducted a pilot study that casts considerable doubt upon the ability to effectively blind the use of nitrous oxide and local anesthetic creams. Eleven children aged between 3 and 17 yr undergoing either cannulation or venepuncture were randomized to either 50% nitrous oxide in oxygen and placebo cream or 50% nitrogen in oxygen with Amethocaine 4% cream. The proceduralist and an investigator independently indicated whether they believed the child had received active gas or active cream. The proceduralist indicated correctly in 9/11 cases (P = 0.03), and the investigator indicated correctly in 11/11 cases (P < 0.001).
Blinding was almost impossible owing to the striking central nervous system effects of nitrous oxide, and the erythema associated with Amethocaine. Although EMLA does not produce erythema, it does cause blanching, which is also readily apparent.
Although the idea of double blinding is commendable, we and other authors believe that it is not possible to effectively blind such a study (2). In studies where blinding may be difficult, an assessment of the success of blinding methods is essential.
References
La Timone Children Hospital, Department Of Pediatric Anesthesia and Critical Care, Marseille, France La Timone Children Hospital, Marseille, France
In Response:
We appreciate the opportunity to respond to the pertinent comments made by Drs. Flanagan and Sherrington about our article (1).
They raise an important concern on the double-blinded methodology of our study. On the basis of their pilot study, these authors suggested that the investigators can easily identify the treatment group each child received randomly and concluded that it is not possible to perform such study in a double-blinded manner. Based on this observation, we would like to give more information about our study design.
A review of the research process was performed at the end of the study; blindness was cleared at this time. The following were noted during these discussions among the investigators:
In conclusion, we agree with Drs. Flanagan and Sherrington that EMLA and nitrous oxide are two effective medications and that their respective effects on the skin and the central nervous system are sometimes difficult to blind from the investigators. However in view of the false positive signs observed in some patients (i.e., side effects expected to be those of the treatment the patient did not receive), the investigators must be careful not to speculate on the patient’s assignment group. We can thereby confirm that our study was effectively randomized and that the double-blinded design was respected.
References
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