Biomedical Engineering Reference
In-Depth Information
The effi cacy of topical anesthetic creams may be affected by
a variety of conditions. A decrease in penetration of the cream
can occur if the skin contains makeup moisturizing creams
and/or sunscreens. In addition, some topical compounds, such
as benzoyl peroxide, may affect the anesthetic effi cacy. One
study demonstrated a 75% increased perception of pain when
5% benzoyl peroxide was applied prior to application of 6%
benzocaine cream (36). Benzoyl peroxide reacts with tetra-
caine, procaine, pramoxine, prilocaine, and lidocaine. Thus,
since benzoyl peroxide is one of the most commonly used
topical antiacne creams, one should make sure that the skin
where topical anesthetics are applied is totally devoid of any
previous treatment with benzoyl peroxide.
Iontophoresis of Lidocaine
Iontophoresis describes the process of transporting charged
ions across a membrane by means of an electric current. This
technique can be used to deliver a large amount of medication
into the skin, including local anesthetics (37). Anesthesia
induced by iontophoretic delivery of lidocaine is most effec-
tive when treating lesions within the epidermis and upper
dermis (4). The procedure entails adding 1.5 mL lidocaine
hydrochloride 4% with epinephrine 1:50,000 to the polymer
gel medication delivery electrode. Anesthesia is achieved with a
minimum current delivered over 20 minutes. The duration
of anesthetic effect with this mixture lasts between 60 and
90 minutes (38,39). Two studies evaluating three (40) and
eleven (41) patients, respectively, with PWS were conducted to
test the efficacy of iontophoresis. Patients demonstrated
significant decreases in discomfort of pulsed dye laser impulses
with this iontophoretic mixture. In addition, no detrimental
effect on flashlamp-pumped pulsed dye laser (FLPDL) ablation
of PWS occurred despite significant decreases in perfusion, as
measured by laser-Doppler velocimetry of the PWS receiving
iontophoresis with the lidocaine/epinephrine mixture (41).
The average time of iontophoresis is approximately 12 min-
utes. This usually yields a 30-minute anesthetic effect. The
most common side effects to this treatment are prolonged ery-
thema under the dispersive electrode and a burning sensation
to underlying skin. A metallic taste has been noted by patients
during some procedures when the electrode was placed on the
face (40,41). In a cooperative patient with a limited lesion, this
modality may be useful (Fig. 16.4).
Figure 16.3 Appearance of EMLA under Tegaderm dressing. The level of anes-
thesia has not been reported to be proportional to the thickness of anesthetic
cream applied.
Unfortunately, there has been some indication that experi-
ence with EMLA cream may be variable. Anecdotal evidence
indicated that some patients will have excellent anesthetic
results with an almost imperceptible reaction to laser pulses,
whereas on another day the same patient may have absolutely
no response to the cream. Therefore, it has been recommended
by some to use EMLA for small lesions, but that it may not be
useful for total, predictable anesthesia.
Lidocaine/Tetracaine (LaserCaine)
Another topical anesthetic agent available for physician use
within the office is LaserCaine (Unit Dose Pharmacy and
Packaging, Phoenix, Arizona, USA). LaserCaine Compound
contains 1% lidocaine and 3% or 4% tetracaine in a proprie-
tary formulation. It is available in 2- and 3-oz sizes in two
strengths, Classic, 3% tetracaine, and Forte, 4% tetracaine. It is
intended for use on intact skin.
Preliminary observations indicate that LaserCaine has a
30-50% faster onset than EMLA, produces little to minimal
skin changes, and does not require the use of an occlusive
dressing. It appears to have minimal toxicity. LaserCaine is
rapidly metabolized in tissue through hydrolysis by serum
cholinesterase. The half-life in serum is 1½ to 2½ minutes with
a toxic blood level of 8 mg/mL, which is higher than the toxic
level of lidocaine, 5 mg/mL. In addition, only 5% of tetracaine
is present in the nonionized or active form at physiologic pH.
However, patients with atypical plasma cholinesterase enzyme
may be at increased risk for developing excessive plasma con-
centrations of local ester anesthetics because of absent or min-
imal hydrolysis. There have been reports of anaphylaxis with
the use of tetracaine-containing topical anesthetics.
General Anesthesia
Use of general anesthesia is highly regulated by the Federal and
State governments and can only be administered in approved
surgical centers that are meticulously monitored. Each surgical
center must have specified updated equipment, including
anesthesia machines with appropriate vaporizers, intraopera-
tive and postoperative monitoring systems, and resuscitation
equipment (defibrillator, oxygen delivery, intubation equip-
ment, various intravenous (IV) drugs, etc.). A strict record-
keeping system of the monitoring environment and procedures,
as prescribed by the US Occupational Safety and Health
Administration (OSHA), is also mandatory. Finally, specifically
trained personnel are required. The physician is referred to the
American Academy of Pediatrics (AAP) guidelines for the use
of conscious sedation, deep sedation, and general anesthesia in
pediatric patients for further information on this subject (41).
 
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