Biomedical Engineering Reference
In-Depth Information
airway (LMA) allows the procedure to be accomplished without
ET intubation or an anesthesia machine.
Propofol is the primary agent used and provides rapidly
induced, easily maintained, and quickly terminated loss of
consciousness. IV midazolam is used to provide initial seda-
tion and amnesia, while analgesia is provided by fentanyl
(Sublimaze®) supplemented intermittently with ketamine
(Ketalar®). To counteract potentially troublesome side effects
of ketamine, all patients receive glycopyrrolate (Robinul®) as a
drying agent and midazolam and propofol before receiving
ketamine. By using this regimen and limiting the total ket-
amine dose to about 1 mg/kg, patients have had no problems
with “bad dreams” or adverse psychic experiences (155).
The requisite equipment for this technique consists of an
electrocardiogram (ECG), blood pressure monitor, pulse
oximeter, supplemental oxygen source, LMA, and an intrave-
nous infusion pump, as well as an anesthesiologist to adminis-
ter the anesthesia. Capnohepatography, while optional, is
desirable to monitor adequacy of ventilation. Prudence dic-
tates that any facility administering narcotics or sedatives has
appropriately trained medical personnel, a fully stocked resus-
citation cart, intubation equipment (laryngoscope, blades, ET
tubes), a method for delivering positive-pressure ventilation
(Ambubag or Jackson-Rees circuit), and a charged and func-
tional defi brillator. A recovery area staffed with qualifi ed nurs-
ing personnel should have the capacity to monitor ECG, blood
pressure, and pulse oximetry.
Concern over the safety of laser energy in proximity to oxygen
and the patient's face is understandable. All modern inhaled
anesthetics are nonfl ammable, and although oxygen and nitrous
oxide support combustion, neither is by itself fl ammable. It is
imperative that fl ammable liquids not be used in the facial prep-
aration. The patient must be instructed not to use hairspray or
other topical skin and hair products before surgery to avoid the
introduction of unknown chemicals into the surgical fi eld.
The oxygen delivery system consists of the LMA and the
tubing from a green oxygen mask connecting the oxygen cyl-
inder and the LMA, which includes a small pilot tube used to
infl ate the pharyngeal cuff. The LMA is very resistant to dam-
age from the laser. An informal study performed in our facility
revealed it to be resistant to penetration or infl ammation
despite 60 consecutive laser impacts while oxygen was fl owing
to enrich the immediate open-air vicinity. The pilot tube is
also very resistant to laser damage, requiring 22 impacts before
it was cut through, but never ignited. The green oxygen tubing,
on the other hand, is easily damaged, and a few pulses will melt
it. However, the oxygen fl owing through the tube will not
ignite, although any fl ammable object present in the fi eld (e.g.,
dry gauze and dry hair) will ignite. ET tubes and LMA tubes
can be fully protected by wrapping them with saline-soaked
gauze or towels. With proper procedures and caution that no
combustible liquids or other products are used near surgical
areas, laser resurfacing in the presence of oxygen and an anes-
thetic delivery system such as an ET tube or LMA is safe.
for superfi cial resurfacing with the Er:YAG laser. For deeper
resurfacing with the short-pulsed Er:YAG laser, regional nerve
blocks (supraorbital, supratrochlear, infraorbital, and mental
nerve blocks can be easily performed) and/or local infi ltration
with 1-2% lidocaine with 1:100,000 epinephrine can be used.
However, for ablation into the dermis or full-face resurfacing
with the Er:YAG laser, a more potent topical cream or IV seda-
tion performed by an anesthesiologist as described earlier
under careful monitoring is generally required.
wound healing
Observations in animal models and clinical studies suggest
a general sequence of events that occur in wound healing,
largely controlled by fi broblasts (156-162). This has been
divided into three phases: infl ammatory, proliferation, and
maturation.
Inflammatory Phase
The infl ammatory phase lasts 3-10 days (159,162). After der-
mal trauma, blood vessels constrict, and leakage of plasma pro-
teins (fi brinogen, fi bronectin, and plasminogen) and platelets
occurs. Plasma and blood clot when exposed to tissue factors
(163) and form a gel-like fi brin-fi bronectin matrix. Infl amma-
tory cells, new capillaries, and fi broblasts derived from the
wound edges migrate into this matrix. Activated macrophages
are probably the most important cells in this phase of healing,
particularly for wound debridement (164-167).
The fi brin-fi bronectin matrix is degraded by infl ammatory
cells. Fibroblasts synthesize fi bronectin, interstitial collagens,
and GAGs to form a new fi brovascular connective tissue or
granulation tissue, within 2-4 days of injury (157,168,169).
Proliferation Phase
The prominent collagen formed during the infl ammatory
phase is type III collagen, which has a gel-like consistency
(170). Its synthesis is maximal between 5 and 7 days. The pro-
liferation phase occurs over the next 10-14 days and is domi-
nated by the proliferation of fi broblasts and synthesis of
collagen as well as regeneration of the epidermis and neoan-
giogenesis. Reepithelialization is heralded by mitoses at wound
edges and at appendages within 24 hours (160). Epidermal
proliferation is maximal at 24-72 hours (161). Direct contact
with fi bronectin and type I collagen guides and stimulates epi-
thelial cell migration in culture (162). Soluble substances such
as growth factors derived from platelets, macrophages, dermal
parenchymal cells, and keratinocytes stimulate reepithelializa-
tion in animal models (169). Continued collagen synthesis
proceeds, stimulated by macrophage-derived and platelet-
derived factors (171), followed by capillary resorption and dis-
appearance of fi broblasts (172).
Lymphokines, complement, native collagens of types I to V,
fi bronectin, and platelet-derived growth factor may be potent
mitogens and chemotactants for fi broblasts (156,173). In addi-
tion, macrophages may be activated by blood and may play a
key role in inducing a fi broproliferative response when there is
signifi cant blood within the healing wound (156,165,174).
In the biosynthesis of collagen, procollagen is formed intra-
cellularly by fi broblasts and secreted into the extracellular
space (172). It is then biochemically transformed into tropo-
collagen by proteases (175-177). The tropocollagen molecules
anesthesia for e
:yag laser resurfacing
While a topical-only anesthesia approach has been found
impractical for CO
2
laser skin resurfacing, topical anesthetics
such as LMX-5 (Ferndale Laboratories, Ferndale, Michigan,
USA) have been able to provide an effective level of anesthesia
r
