RESULTS
Twenty-eight out of 31 patients reported that the unbuffered LA
was more comfortable and less painful upon injection than the
buffered LA. The distribution of pain fibers was similar because
the injection sites were on the face where two biopsies were
needed to rule out two separate potential skin cancers. Pain reported
upon biopsy was negligible (the average reported pain
was 0 on the VAS). The sample size was determined before data
collection and based on a potential size that would be of statistical
significance. The sample size calculation of 31 was based
on a power of 80% and a 95% confidence interval
Since injections were given in separate, discrete locations,
we may assume the independence of samples and, therefore,
rule out sequence or blocking effects. The observations in each
group were independent. Since each patient was acting as his
own control, there were no differences in baseline characteristics
between treatment groups.
Using a VAS of 0 to 10 for perceived pain, when comparing
pain upon injection in the buffered LAs vs. unbuffered LAs,
nearly every patient reported less pain from the injection with
the unbuffered LA compared with the buffered LA (28 out of
31) with one patient reporting no difference and one patient
reporting a 0.5 difference in pain favoring the unbuffered LA.
The average difference in pain reported was 2.7 (see Table 1 for
results). Traditionally, a difference in VAS of greater than 1.520
is considered clinically relevant.2 Statistical analysis of this difference
between pain reported upon injection with the buffered
LA versus the unbuffered LA using a two-tailed Student paired
t test revealed this to be a significant difference (P=2.3x10-9 =
.0000000023). Therefore, the authors conclude that there is a
significant difference in reported pain upon injection with the
buffered LA vs the unbuffered LA, with less pain associated
with the latter.
DISCUSSION
In search for painless anesthesia, physicians continue to use a myriad of techniques that will decrease the pain (analgesia) or
eliminate the pain (anesthesia) associated with LAs. Hand-holding
and talking (talkesthesia) are viewed as helpful.3,4 Vibration
devices have also been used to reduce pain.5 Cooling the skin
with cryogel packs before local anesthetic injection has been shown to decrease patient discomfort and improve the overall anesthetic experience.6 Iontophoresis has been reported to be
useful, but it requires training and instrumentation.7 Warming
the lidocaine to body temperature has also been shown to be
somewhat effective in increasing analgesia, and buffering and
warming the lidocaine solution before infiltration is significantly
superior to buffering alone.8
A variety of topical anesthetics with or without occlusion have also been helpful during the needle insertion, but they
often do not provide sufficient pain relief during infiltration
of the local anesthetic.9
Of all the commonly employed techniques to induce analgesia, buffering lidocaine is the preferred technique among many
dermatologists.10 A minor setback is that buffering lidocaine
shortens its shelf life.11 As with buffered lidocaine, we found
that the shelf life of our mixture is also shortened to approximately
one month.
To date, the mechanism through which the infiltration of lidocaine causes pain is not completely understood. Many
hypotheses exist that try to determine what influences pain
perception. Liposolubility, changes in protein kinase A (PKA),
and protein-binding properties are just a few of the theories
currently found in the medical literature.12
The reason why adding sodium bicarbonate leads to pain reduction is also not well understood. Some authors believe
that the increase in pH reduces the concentration of hydrogen
ions, while others support the hypothesis that an increase in
pH leads to an increase in the quantity of nonionized anesthetic.13 It is believed that an increase in nonionized anesthetic will
increase the amount of tissue diffusion, the concentration in
nervous fibers, and the onset of the block, thereby masking the
perception of pain.
The aim of our study was to determine if diluting lidocaine with normal saline would decrease the pain associated with subcutaneous
infiltration and how this mixture would compare with
traditionally buffered lidocaine. Lidocaine diluted with normal
saline proved to be superior to traditionally buffered lidocaine
for pain attenuation. Reduced pain secondary to tumescent anesthesia
did not contribute to the decreased pain perceived by
subjects, as both types of injection were of equal volume. In
addition, tumescent anesthetics have a longer onset of action
as compared with the total time of the injection and subsequent
biopsy. The perceived pain of the injection in the normal saline
group may be accounted for by the presence of benzyl alcohol
in the normal saline.
Limitations of this study included the patient population, which consisted of Caucasian and Hispanic patients in a private,
