Alice I. Andreasen, B.S.
University of Washington
School of Medicine
Seattle, Washington
Keith D. Knopes, M.D.
Virginia Mason Medical Center
1100 Ninth Avenue, B2-AN
Seattle, Washington 98101
anekdk@vmmc.org
Thiopental is the most commonly used induction
agent in the United States. Barbiturates decrease the rate of
dissociation of GABA from its receptors, thus depressing the
reticular activating system. This unique property of barbiturates
is responsible for their ability to induce unconsciousness.
Administration of intravenous barbiturates typically produces
peripheral vasodilation with a moderate decrease in blood pressure.
This hypotension reflects the drugās depression of the medullary
vasomotor center and outflow from the sympathetic nervous system.
In a healthy young patient, this is compensated for by the baroreceptor
reflex, but in an elderly patient with a decreased baroreceptor
reflex and increased vascular wall rigidity, barbiturates may
cause a dangerous drop in blood pressure. Barbiturates also
depress medullary ventilatory centers, leading to a dampened
response to carbon dioxide. This apnea will be more pronounced
if a barbiturate is used in conjunction with an opioid.
In the elderly, thiopentalās elimination half-life is 13-25
hours as opposed to 6-12 hours in the young (due in part to
an increase in volume of distribution at steady state). The
thiopental dosage requirement decreases 25-75 percent as one
ages, but it takes longer to induce unconsciousness. Methohexital
is rapid acting and has a higher hepatic clearance rate and
shorter elimination half-life than thiopental. For these reasons,
it is favored over thiopental by some for use in outpatient
anesthesia. Elderly patients treated with methohexital show
a postoperative prolongation of central nervous system depression
(fatigue and motor skill impairment).
Ketamine is a sedative-hypnotic amnestic and a potent analgesic
that can be injected intravenously or intramuscularly. This
drug stimulates the cardiovascular system, which is beneficial
in hypovolemic patients and disadvantageous in patients with
ischemic heart disease because it may increase myocardial oxygen
demand. When used in combination with a benzodiazepine, the
cardiovascular stimulation will be attenuated. Ketamine increases
airway secretions, decreases airway resistance and increases
intracranial pressure.
Etomidate is a rapid, short-acting carboxylated imidazole derived
hypnotic. It is a good choice for inducing anesthesia in the
hemodynamically tenuous elderly because it possesses less cardiovascular
depression than the barbiturates. Rapid recovery is due to the
extensive hydrolysis of etomidate to inactive metabolites, but
clearance is hepatic blood flow dependent. Disadvantages of
using etomidate include a high incidence of postoperative nausea
and vomiting (decreased with prophylaxis with an antiemetic
drug) and a postoperative suppression of adrenocortical function
seen with infusion of this sedative-hypnotic.
Propofol is a rapid, short-acting alkylphenol with few side
effects. It has a high lipid solubility and is solubilized in
a lecithin containing emulsion. Induction using 1.2 to 1.7 mg/kg
in the elderly (versus 2.0 to 2.5 mg/kg in younger patients)
produces a rapid onset of anesthesia (less than one minute)
lasting five to 10 minutes. There is an age-related decrease
in propofol clearance, resulting in a decreased maintenance
anesthetic requirement with age. Propofol produces dose-dependent
cardiovascular and respiratory depression, leading to greater
decreases in systemic blood pressure than thiopental when used
for induction in elderly patients. These effects can be minimized
if propofol is injected slowly with sufficient time allowed
to achieve the full effect of the dose, thereby decreasing the
total dose. Nevertheless, propofol is a good choice for many
elderly patients because it offers quick recovery with few side
effects. For example, patients older than 80 exhibit less post-anesthetic
mental impairment with propofol compared to other agents. Thus,
although propofol has theoretical advantages in the elderly,
its hypotensive side effects will require slow administration
of a reduced dose, titrated to effect rather than administering
a preselected standardized dose which might be more applicable
to younger patients.
Bibliography:
- White PF. Clinical pharmacology of intravenous induction
drugs. Int Anesthesiol Clin. 1988; 26(2):98-104.
This is a very readable review of the major induction agents.
The dosing recommendations are useful and appropriate for
routine clinical care.
- White PF. Anesthetic techniques for the elderly outpatient.
Int Anesthesiol Clin. 1988; 26(2):105-111.
This practical article places the induction agents in the
context of complete anesthesia care. Also, the use of induction
agents as premedication is reviewed.
- Schnider TW, Minto CF, Shafer SL, et al. The influence of
age on propofol pharmacodynamics. Anesthesiology. 1999;
90:1502-1516.
This investigation uses EEG as an endpoint in investigating
the hypnotic effects of propofol in the elderly. The increased
sensitivity of the elderly to propofol is quantified.
- Dundee JW, Robinson FP, McCollum JSC, Patterson CC. Sensitivity
to propofol in the elderly. Anaesthesia. 1986; 41:482-485.
- Mirakhur RK, Shepherd WF. Intraocular pressure changes
with propofol (Īdiprivanā): comparison with thiopentone. Postgrad
Med J. 1985; 61:41-44.
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