At many institutions, it is routine for anesthesiologists to use intradermal anesthesia to provide analgesia with lidocaine, with or without bicarbonate, before IV catheter placement. Local anesthetics can produce pain during skin infiltration. A 25-gauge needle is routinely used for local anesthetic infiltration. At other institutions, 27- or 30-gauge needles are used for skin infiltration. Sodium bicarbonate has previously been shown to attenuate pain on skin infiltration with lidocaine [1-5]; however, in one large controlled study (n = 100), bicarbonate did not diminish pain on injection . No study has examined whether the needle size (25- vs 30-gauge) influences the pain of intradermal injection or whether the pain of intradermal injection is affected differently by bicarbonate using different size needles. We tested the hypothesis that there is a difference between the pain induced by 30-gauge versus 25-gauge needles when used for local infiltration with lidocaine. We also determined whether the addition of sodium bicarbonate attenuates the pain of lidocaine local infiltration using either the 25- or 30-gauge needle.
After approval by our institution's human studies review board, 40 healthy adult volunteers gave their consent to participate in this study. Volunteers were recruited from within our department. All of the volunteers received four intradermal injections and were blinded as to the contents of the injectate and to which needle size was used for each injection. To achieve this, participants were asked to close their eyes or to look away as each injection was administered. Each volunteer randomly received a 0.25-mL intradermal injection of the following four solutions: 1) lidocaine 2% administered through a 25-gauge needle (lido-25); 2) lidocaine 2% mixed with sodium bicarbonate administered through a 25-gauge needle (lido-bicarb-25); 3) lidocaine 2% administered through a 30-gauge needle (lido-30); and 4) lidocaine 2% mixed with sodium bicarbonate administered through a 30-gauge needle (lido-bicarb-30). The lidocaine/bicarbonate mixture was made by adding 1 mL of sodium bicarbonate (8.4%) to 4 mL of commercially prepared lidocaine hydrochloride (Abbott Laboratories, North Chicago, IL) with a resultant pH of 7.26. The pH of the solution was analyzed by using a Radiometer ABL electrode system (Copenhagen, Denmark). The injection sequence was randomized in each patient. Because it is generally difficult to draw up fluid with a smaller-gauge needle through a rubber stopper, the investigator used a 19-gauge needle to draw up all of the solutions. Therefore, the needle was not dulled while drawing up the solutions. All injections were performed over a 5-s period to the volar surface of the forearm by the investigator after the skin was cleansed with isopropyl alcohol and allowed to dry. The skin wheal was also tested for appropriate anesthesia using a 19-gauge needle on the skin wheal. A visual analog pain scale score was recorded after each intradermal injection. Markings on the pain scale were measured to the nearest millimeter. The volunteers were instructed to score a pain-free injection as 0 and the worst pain imaginable as 10.
All data are presented as mean +/- SEM. Data were analyzed by using two-way analysis of variance with a post hoc Newman-Keuls test to distinguish between the effects of the needle gauges with and without bicarbonate. Student's t-test was used to compare gender with pain scores. Pearson correlations were used to correlate age with pain scores. Statistical significance level was set at 0.05 in all tests.
The volunteer population consisted of anesthesia personnel (residents, faculty, secretaries, a nurse anesthetist, and technicians). Most were anesthesia residents (n = 23). There were 22 male and 18 female volunteers in the study. Their ages ranged from 28 to 61 yr. The pain scores were similar between genders, and there was no correlation between age and pain score. The only complication noted was mild bruising at the injection site on some of the volunteers. No infections or other complications were noted. None of the volunteers had a history of adverse reaction to lidocaine, and none were known to be pregnant.
There was a strong interaction between needle size and pH (P = 0.002). The lido-bicarb-30 injection was the least painful, and the lido-25 injection was the most painful. The pain scores were significantly higher with the lido-25 (3.2 +/- 0.2) group than with the lido-30 (2.5 +/- 0.3), lido-bicarb-25 (1.9 +/- 0.2), and lido-bicarb-30 (1.3 +/- 0.2) groups. The lido-bicarb-30 injection was also less painful than the lido-30 injection. No differences were found between the lido-bicarb-25 and the lido-bicarb-30 injections (Figure 1). The lido-bicarb-25 injection was not any less painful than lido-30 injection. A bicarbonate-induced reduction of injection pain from intradermal lidocaine was not different for the two needle sizes tested in this study. Complete analgesia for the 19-gauge needle pain stimulus was achieved in all patients.
Overall, the intensity of the pain of an intradermal injection of 2% lidocaine was low. Although some studies do not support our findings [6,7], our study demonstrates that pain resulting from the intradermal infiltration of a lidocaine local anesthetic can be diminished by the addition of sodium bicarbonate, regardless of the needle size. There are several other studies in the anesthesia, radiological, and dermatological literature that advocate the use of sodium bicarbonate [1-5,8]. Many institutions use needle sizes ranging from 25- to 30-gauge when administering lidocaine, whether buffered or unbuffered. We hypothesized that the smaller needle size would be correlated with less pain during intradermal local anesthetic infiltration. No study has previously examined whether needle size is important in decreasing the pain of injection. We discovered that although the smaller needle gauge can decrease the pain of injection when sodium bicarbonate is not used, the buffering of the lidocaine is more important than needle size in decreasing the pain of intradermal injection.
The volunteers were recruited through memorandum announcements and the departmental electronic mail system. There is a potential for bias in that those with a low pain tolerance may have declined to participate. However, according to our results, there were clearly volunteers who had a low pain tolerance. Therefore, we believe that the volunteer population is a good representation of our patient population. The pain score was determined only immediately after the wheal of local anesthetic was placed. We did not determine whether the pain related to needle puncture alone was affected by needle size because this is an insignificant component of the pain related to subdermal lidocaine infiltration. For example, many of the volunteers could initially tell which needle was placed, but they were more concerned with the injection, which often stings if the lidocaine is not buffered. The efficacy of the skin wheal of all of the injections was determined with a 19-gauge needle to assure that proper analgesia was attained. We also injected the solutions slowly over a five-second period, because rapid injection techniques cause more pain [7-12]. The five-second injection time for 0.25 mL may be slower than standard clinical practice, and, therefore, the results may not apply if faster injection times are used.
It is unknown why infiltration with local anesthetics produce pain. It is hypothesized to be a reaction to the nonphysiologic pH of the anesthetic solution. The pH of our 2% lidocaine was approximately 6.2, and with the addition of 1 mL of bicarbonate for every 4 mL of lidocaine, the pH was approximately 7.26. Other studies suggest mixing 1 mL of 8.4% bicarbonate with 10 mL of local anesthetic to produce an adequate sting-free local infiltration . To ensure that the differences observed did not result from differences in lidocaine concentration, rather than from effects of NaHCO3, we compared the pH of a solution of 1 mL of NaHCO3 plus 4 mL of 2% lidocaine with a solution of 1 mL of saline plus 4 mL of 2% lidocaine. The pH of the solution of 2% lidocaine with 1 mL of saline was 6.22. Although there was some dilutional effect with the addition of saline, the effect was not sufficient to account for the larger change in pH with the addition of NaHCO3. Increasing pH may not be the mechanism for decreasing pain in buffered lidocaine solutions. In a similar study, chloroprocaine (pH 3.4) was among the least painful on injection when compared with etidocaine (pH 4.7), which was the most painful. In the previous study , lipid solubility seemed to be related to the painful effects of the various local anesthetics. Etidocaine was the most painful anesthetic and was also the most lipid-soluble. Chloroprocaine was the least painful and the least lipid-soluble . Because it is possible that drugs related to lidocaine (including etidocaine) are more painful than other, chemically unrelated agents (e.g., 2-chloroprocaine), our results may not be generalizable to other chemically unrelated local anesthetics.
We have demonstrated that the addition of sodium bicarbonate to commercially prepared lidocaine is more important than needle size in decreasing the pain associated with an intradermal skin wheal. In most cases, EMLA cream (Astra; Westborough, MA) would be beneficial as a local anesthetic in needle-phobic patients. However, in many cases, patients reveal their needle phobia immediately before surgery and, therefore, there may not be enough time available for the full effect of the EMLA cream to be achieved. Offering a smaller-gauge needle with sodium bicarbonate for intradermal injections to needle-phobic patients may be a good option in a stressful situation.
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