Download A Comparison of Target-Controlled Infusion Versus Volatile Inhalant

REPORTS
A Comparison of Target-Controlled Infusion Versus
Volatile Inhalant Anesthesia for Heart Rate,
Respiratory Rate, and Recovery Time
in a Rat Model
SANDER O. HACKER,1,* CHARLES E. WHITE,2 AND IAN H. BLACK3
We conducted this study to determine whether heart rate, respiratory rate, and recovery time differed significantly between rats
receiving target-controlled infusion (TCI) and those under volatile inhalant anesthesia. TCI rats received intravenous propofol at an
average effect site concentration of 11.3 μg/ml or propofol plus ketamine (5 mg/ml of propofol) at an average effect site concentration of 8.7 μg/ml. Inhalant anesthesia rats received isoflurane (average, 1.8%) delivered in medical-grade air. We used a tail-clamp
response test to determine when a surgical plane of anesthesia was attained. Anesthesia was continued for 1 h from the first negative
tail-clamp test. During this time the test was repeated every 10 min to confirm that a surgical plane of anesthesia was being maintained. Anesthesia then was discontinued, and the animals were monitored continuously until they recovered. Average heart rate
was higher for rats during anesthesia with isoflurane compared with TCI propofol–ketamine (P = 0.0053). Average respiratory rate
was higher for TCI regimens compared with isoflurane anesthesia, with male rats having consistently faster respiratory rates than
females (P < 0.001). Recovery time was longer for both TCI regimens compared with isoflurane (P < 0.001). Once venous access was
accomplished, TCI anesthesia with propofol or propofol combined with a low dose of ketamine was comparable to an isoflurane
inhalant regimen in ease of administration and control of the anesthetic event when used in rats for procedures of 1-h duration.
Respiratory rate was increased and recovery time was longer for rats receiving the TCI regimens.
Target-controlled infusion (TCI) is a relatively new method of total
intravenous anesthesia (TIVA) administration in veterinary medicine.
This method uses a computer-controlled infusion pump to deliver
a drug based on a mathematically predicted plasma or “effect site”
concentration. The effect site for general anesthetics is the brain. The
effect site concentration is the theoretical drug concentration at the
receptors in the brain that produces the desired effect, i.e. anesthesia.
This concentration cannot be measured directly, but the time for
equalization between plasma concentration and drug effects can be
characterized and then quantified as a mathematical constant (6). Effect site concentration accounts for the slight time lag between peak
plasma concentration and peak effect of a drug. Plasma and effect
site drug concentrations are derived from known pharmacokinetic
and pharmacodynamic information about a particular animal model.
TCI provides a high degree of control of intravenous anesthesia in
the patient because the computer correlates drug effect more closely
to blood or effect site concentration than to infusion rate (19).
Patients can be maintained at an appropriate plane of anesthesia
with minimal accumulation of drug in body tissues. The result is a
smoother anesthesia with more stable respiratory and cardiovascular
responses and a shorter recovery time. TCI in the rat has shown that
actual plasma levels of an anesthetic correlate to the calculated target
blood level or desired effect site concentration (2, 7, 8). The system
enables the user to precisely control the patient’s blood or effect site
concentration of chemical anesthetic, making it comparable to using inhalant regimens. This control provides the anesthetist with an
objective means of assessment of the pharmacodynamic effects of
drugs. With manually controlled infusion methods, drug levels change
Walter Reed Army Institute of Research, Division of Veterinary Medicine,1 and Division
of Biometrics,2 Silver Spring, Maryland; U.S. Army Institute of Surgical Research, Fort
Sam Houston, Texas3.
*Corresponding author.
Volume 44, No. 5 / September 2005
over time, secondary to accumulation, and it is difficult to discern
whether the effect that one observes or measures is dose-related or
pharmacodynamic (6, 19). TCI has the potential to minimize any
effects that changing drug concentrations may have on physical and
physiologic parameters during the anesthetic event.
Bolus intramuscular or intraperitoneal injection of a chemical
cocktail is a common means of administering rodent anesthesia. This
method is effective, easy to perform, and relatively inexpensive; however,
it provides little control of anesthesia once the drug dosage has been
administered. The experience of the surgeon and the technical difficulty
of the procedure dictate how long a surgery will take. Consequently, additional doses of anesthetic may be needed to permit completion of the
task. When compared with bolus administration, TIVA via manually
controlled infusion (MCI) offers better anesthesia management because
the technique permits a more uniform delivery of drug. The relatively
stable plasma concentration attained using MCI avoids the peak and
trough blood and effect site concentrations associated with intermittent
bolus administration and the concurrent potential for inadequate or
excessive anesthesia. However, with MCI of an anesthetic delivered at
a constant rate, the plasma concentration gradually changes over time.
This change can be managed by the anesthetist through rate adjustments
in light of patient clinical parameters observed during the anesthesia
period. TCI offers a finer degree of control for TIVA compared with
MCI. The technique permits rapid titration of infused anesthetic
to a programmed setting of plasma or effect site concentration. The
computer then automatically adjusts the infusion rate to maintain the
concentration that is set by the anesthetist. If the setting is increased,
the computer responds by initiating a bolus of drug to rapidly increase
blood or effect site concentration to the desired level. Decreasing the
setting causes the computer to stop the infusion pump until the desired
concentration is attained. The pump then is reactivated and run to
maintain the lower concentration.
CONTEMPORARY TOPICS © 2005 by the American Association for Laboratory Animal Science
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The overall goal of this study was to provide investigators with an
intravenous anesthesia technique for rodent surgery that could be
performed and controlled comparable to an inhalation agent. This
method should avoid the pitfalls of bolus injections while providing
anesthesia of appropriate duration and effect for the procedure being
performed. MCI and TCI of propofol both have the potential to
meet this goal. Comparisons of MCI and TCI propofol anesthesia in
human subjects have been published and report that the methods are
similar in terms of safety and efficacy when performed by a dedicated
anesthetist (10, 17). The bispectral index, an objective measure of
anesthetic depth, has been used to show that both techniques can
result in a similar depth of anesthesia when plasma concentrations are
titrated using standard clinical parameters (5). Russell and colleagues
found that both techniques were easy to learn and perform, yet a
majority of anesthetists preferred TCI over MCI (16). This preference may be due to the similarity of adjusting target concentrations
during TCI with varying inspired concentrations on a gas anesthesia
machine. Ease of operation and similarity in function to a volatile
inhalant system were primary factors in our selection of TCI over
MCI for this study.
A secondary goal was to investigate whether ketamine could be
used as an adjunct to propofol during TCI in order to provide a balanced general anesthesia for surgery with low residual effects so that
the patients would recover as quickly as possible. The ability of the
animal to perform purposeful movements about the recovery cage
postoperatively is a criterion often used to determine the point of
anesthetic recovery. This time frame can be extremely variable depending on the drug combination and the dosing regimen used by
the investigator. Because recovery from inhalation anesthesia is rapid,
with a correspondingly short recovery monitoring period, we used it
as the “gold standard” for comparison with the TCI regimens.
Material and Methods
We acquired 42 Crl:CD(SD)IGSBR (21 male and 21 female) rats,
each weighing 250 to 350 g, from Charles River Laboratories, Inc.
(Wilmington, Mass.). Rats were verified as being specific pathogen
free (SPF) through quarterly testing at Charles River Laboratories
for the following infectious agents: Sendai virus, pneumonia virus of
mice, sialodacryoadenitis virus/rat coronavirus, Killam rat virus, H-1
virus, reovirus, rat enterovirus, Mycoplasma pulmonis, lymphocytic
choriomeningitis virus, Hantaan virus, mouse adenovirus (FL/K87),
Encephalitozoan cuniculi, carbacillus, rat parvovirus, and rat minute
virus. Animals were housed individually in polycarbonate shoebox
caging under controlled conditions of temperature (64 to 79°F [ca.
18 to 21°C] ), humidity (50 to 70%), and light (12:12-h light:dark
cycle). They were provided ad libitum access to a pelleted rodent
diet (Lab Diet ProLab RMH3000, PMI Feeds, Inc., St. Louis, Mo.)
and reverse osmosis-treated water via automatic watering system. All
animals were acclimated to the facility for 7 days prior to being used in
the study. All procedures associated with this study were reviewed and
approved by the institutional animal care and use committee and were
performed in a facility accredited by the Association for the Assessment
and Accreditation of Laboratory Animal Care, International.
Animals were assigned randomly to one of three anesthesia treatment groups, ensuring that there were an equal number of male and
female rats per group. The order of treatments was then randomized
with respect to treatment and sex.
On the day of anesthesia, a rat was weighed and placed in a flexible
polyethylene cone (DecapiCone, Braintree Scientific, Inc., Braintree,
Mass.). Bandage tape was placed on the plastic of the restrainer
around the base of the tail to secure the animal inside. A 22-gauge,
over-the-needle catheter was placed in the tail vein and secured with
bandage tape. The catheter then either was connected to a syringe
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CONTEMPORARY TOPICS © 2005 by the American Association for Laboratory Animal Science
on a syringe pump for administration of TCI or was fitted with an
injection cap, and the animal was moved to an induction chamber
for administration of isoflurane.
Induction using TCI. A syringe pump (Harvard Pump 22, Harvard Apparatus, Inc., Holliston, Mass.) connected to a computer (Dell
laptop, Dell Inc., Austin, Tex.) by a serial cable (RS232) was used
to deliver either 10 mg/ml propofol (Propoflo, Abbott Laboratories,
North Chicago, Ill.) or propofol with 5 mg ketamine (Ketalar, Phoenix Scientific, Inc., St. Joseph, Mo.) added per ml of propofol. The
TCI computer program used for this study is called “STANPUMP.”
This shareware computer program is available free of charge from
its author (Steven L. Shafer, M.D., Anesthesiology Service [112A],
PAVAMC, 3801 Miranda Ave., Palo Alto, Calif. 94304). It uses
predetermined mathematical formulas to drive the infusion pump
and administers anesthetic drugs according to a three compartment
pharmacokinetic model. The computer program can target either
plasma concentration or effect site concentration of anesthetic. Effect site concentration was used for all of the anesthesia regimens in
the study. The initial computer setting for induction of anesthesia
(i.e., induction effect site concentration) was set at 10 μg/ml, which
produced a rapid, smooth induction for both TCI regimens. The
pharmacokinetic data set for propofol metabolism in the rat was
extrapolated from DePaepe and colleagues (3).
Induction using inhalation anesthesia. Animals were removed
from the restraint cone and placed in an induction chamber (IMPAC6, Vet Equip, Inc., Pleasanton, Calif.). Isoflurane inhalant (Isoflo,
Abbott Laboratories) was delivered to the chamber at 3% concentration in medical-grade air via an inhalant anesthesia machine (Excel
110SE, Datex-Ohmeda, Madison, Wis.) until they were rendered
unconscious. The oxygen flow rate was set at 1.5 liters/min for the
induction chamber and anesthesia-maintenance facemask.
Instrumenting, maintenance, and recovery. After induction of
anesthesia, the animal was removed from either the restraint cone
or induction chamber and placed on a warm platform (Prostation,
Summit Medical Equipment Co., Bend, Oreg.) attached to a warmwater recirculating pump (model TP-500, Gaymar T/Pump, Gaymar
Industries, Inc., Orchard Park, N.Y.). The platform was positioned
on a warm-air blanket (BAIR Hugger, Arizant Healthcare, Eden
Prairie, Minn.), and both devices assisted with thermoregulation
during anesthesia. Body temperature was maintained between 35 and
37°C. All animals used spontaneous ventilation during anesthesia.
Respiratory rates of 70 to 115 breaths per min were considered within
normal limits for a rat (18). Isoflurane was administered via facemask
(Rodent Mask #1, Vet Equip, Inc., Pleasanton, Calif.) in medicalgrade air by using a nonrebreathing (Bain) circuit. Animals then were
instrumented with a rectal temperature probe (#6076, Temperature
Probe and Cable, Heska Corp., Fort Collins, Colo.) and a pulse
oximeter probe (#4060, Small/Medium Lingual Clip, Heska Corp.)
placed on a hind foot. Both probes were connected to a pulse oximeter
(VetOx 4004, Heska Corp.), which provided continuous data on rectal
temperature, heart rate, and blood oxygen saturation (SpO2). Three
25-gauge needles were placed through the skin of the animal (one on
top of the head and one on each side of the thorax), and electrocardiography (EKG) leads were attached to them by alligator clips. EKG
was recorded using software (Chart for Windows, V4.1.1, ADInstruments, Colorado Springs, Colo.) on a laptop computer (Macintosh
Powerbook G3, Apple Computer, Cupertino, Calif.) connected to
a bioamplifier (ADInstruments PowerLab/16S, ADInstruments).
The EKG reading corroborated the heart rate information provided
by the pulse oximeter. Heart rates of 250 to 450 beats per min were
considered within normal limits for a rat (18).
Once the animal was instrumented, anesthesia was adjusted to the
starting concentration for each regimen: 2% for isoflurane, 10 μg/ml
Volume 44, No. 5 / September 2005
for propofol, and 9 μg/ml for propofol–ketamine. Each animal was
monitored for a 5-min stabilization period prior to data collection. A
tail-clamp response test then was administered to determine whether the
animal was at a surgical plane of anesthesia (2, 8, 20). A padded binder
clip (Boston Clip #4, Hunt Mfg. Co., Statesville, N.C.) was placed
on the tail of the rat and released slowly. The time to respond with a
forceful movement of any body part was assessed in seconds. Maximum
time for the clip to remain on the tail was 30 sec. All values between
0 and 15 sec after application of the clip defined a positive response;
values between 15 and 30 sec after application of the clip defined a
negative response. If the tail-clamp response was positive, anesthesia
was increased incrementally (0.5 μg/ml for TCI; 0.1% for isoflurane),
and the test repeated 2 min after the adjustment. Once a negative tailclamp response had been achieved, the surgical anesthetic plane was
maintained for 1 h. Anesthetic depth was monitored by repeating the
tail-clamp response test every 10 min. Heart rate, respiratory rate, rectal
temperature, and SpO2 were monitored continuously and recorded
every 10 min starting at time zero, which was set at the first negative
tail-clamp response test. If SpO2 fell below 80% for more than 1 min
for either of the TCI regimens after time zero, anesthesia was decreased
1 μg/ml without adjustment to the tail-clamp schedule. The isoflurane
concentration was decreased 0.1% if respiratory rate fell below 70
breaths per min and the tail-clamp response remained negative.
After 1 h, anesthesia was discontinued, and an injection cap was
affixed to the tail catheter for animals receiving chemical anesthesia.
The rat then was moved to a warmed recovery cage, where it was
placed in the center of a 6-in. circle drawn on a paper placed underneath the cage and monitored continuously until fully recovered
from anesthesia. Recovery time was defined as the total time elapsed
from when the anesthesia was discontinued to when the animal was
upright, making purposeful, coordinated movements, and had moved
its entire body (except the tail) out of the circle.
Data analysis and statistical methods. For each of the three anesthesia procedures, seven male and seven female rats were anesthetized
(total of 42). Heart rate and respiratory rate data were collected and
averaged for each animal and then for each anesthesia group. Recovery
time was averaged for each anesthesia group. Analysis of variance procedures were used at the first level to determine statistically significant
differences among anesthetic regimens. Models included anesthesia
alone, anesthesia plus gender, and anesthesia plus gender plus the
interaction between anesthesia and gender. A first-level determination of statistically significant differences between anesthesia groups
was followed by pair-wise comparisons between them. In general, a
P-value of < 0.05 was considered to be statistically significant. For all
pairwise comparisons, the family of three tests was evaluated at the
0.05-level by evaluating the individual tests at the 0.017 (0.05 ÷ 3)
level (Bonferroni approximation). Confidence intervals for survival
rate were determined in light of the exact binomial distribution. Statistical analyses were performed and data graphics produced utilizing
R Language for Statistical Computing (15).
Results
Body weight in kilograms was the only patient parameter required
to be input into the computer program for it to operate correctly.
The pulse oximeter provided SpO2, heart rate, and rectal temperature
information during anesthesia. The heart rate was verified by comparing the pulse oximeter readings with the computer program EKG
recording system. The average time lapsed from the start of induction
to the start of data collection (at time zero) was 12.1 min for the isoflurane group, 11.1 min for the propofol group, and 13.7 min for the
propofol–ketamine group.
Heart rate. The propofol–ketamine combination produced the
slowest average heart rate (mean ± the standard error of the mean)
Volume 44, No. 5 / September 2005
Figure 1. Average heart rate by gender for three anesthesia groups. The gender
difference for the propofol group was clinically apparent but not statistically
significant. As discussed, average heart rate was statistically different between
isoflurane and propofol–ketamine groups.
of 357.3 ± 3.1 beats per min (bpm). The average heart rate for the
propofol group was slightly faster at 368.3 ± 2.4 bpm. The isoflurane
group had the fastest average heart rate of 389.2 ± 1.6 bpm. Overall,
there was a statistically significant difference in heart rate between the
anesthesia groups (P = 0.017). The isoflurane group’s average heart
rate was determined to be statistically different (observed to be higher)
when compared with the average value for the propofol–ketamine
group (P = 0.0053) but not when compared with that for the propofol group (P = 0.055). There was no significant difference in average
heart rates between the propofol group and the propofol–ketamine
group (P = 0.32). There was no statistically significant difference in
average heart rate between genders (P = 0.30; Fig. 1).
Respiratory rate. Rats receiving different types of anesthesia exhibited a wide range of respiratory rates and efforts that were comparable
within groups. The TCI groups had faster, shallower breathing than
the isoflurane group throughout the anesthetic period. The propofol–ketamine group had the highest average respiratory rate—107
± 1.8 respirations per min (rpm)—with the lowest average SpO2
reading (80%). Animals maintained with propofol breathed slower,
averaging 93.5 ± 1.4 rpm, but showed better tissue oxygenation
(average SpO2 = 85.4%) than the propofol–ketamine group. The
isoflurane group had the lowest average respiratory rate, 62 ± 1 rpm,
and took noticeably deeper breaths, which were reflected in higher
tissue oxygenation readings (average SpO2 = 94.5%). Overall, there
was a statistically significant difference in respiratory rate between
anesthesia groups (P < 0.001). The isoflurane group’s average respiratory rate was determined to be statistically different (observed to be
lower) when compared with the average values for both the propofol–ketamine group (P < 0.001) and propofol group (P < 0.001).
The average respiratory rate for the propofol group was determined
to be statistically different (observed to be lower) from that of the
propofol–ketamine group (P < 0.001). Male rats had significantly
different (observed to be higher) respiratory rates than did female
rats for each anesthetic regimen (P < 0.001; Fig. 2).
Recovery time. Cessation of isoflurane anesthesia resulted in
period of immobility followed by a seemingly instantaneous return
to consciousness and awareness with subsequent hyperactivity for
30 to 90 sec. Half of the animals in the isoflurane anesthesia group
then seemed to renarcotize and not move at all for ≤ 10 min, prior
CONTEMPORARY TOPICS © 2005 by the American Association for Laboratory Animal Science
9
Figure 2. Average respiratory rate by gender for three anesthesia groups.
As discussed, all pair-wise comparisons between anesthesia groups showed
statistically significant differences and statistically significant differences were
found between genders.
to appearing normal and fully recovered. The other half of this
group remained conscious and aware from the moment they were
able to maintain an upright posture. Recovery from both chemical
anesthesia regimens was smooth and gradual, with animals exhibiting
slow, deliberate movements. Propofol–ketamine produced the longest
recovery times, with an average of 34.48 ± 1.27 min. Recovery from
propofol was faster than from propofol–ketamine, with an average time
of 19.08 ± 0.5 min. Isoflurane produced the quickest recovery times,
averaging 13.2 ± 0.35 min. Overall, there were statistically significant
differences in recovery time when comparing the inhalant anesthesia
group with the TCI groups (P < 0.001). The average recovery time for
the isoflurane group was determined to be statistically different (observed to be shorter) when compared with both the propofol–ketamine
group (P < 0.001) and the propofol group (P = 0.001). The average
recovery time for the propofol–ketamine group was not statistically
different from that of the propofol group (P = 0.092). There was no
statistically significant difference in recovery time between genders
(P = 0.50; Fig. 3).
Survival. Mortality occurred in both chemical anesthesia groups
but not in the volatile inhalant anesthesia group. Survival rates were
not statistically different among the three anesthesia regimens. The
isoflurane group had a 100% survival rate (14 of 14), with a 95%
confidence interval of 76.8 to 100%. The propofol group had a 92.8%
survival rate (13 of 14), with a 95% confidence interval of 66.1 to
99.8%. The propofol–ketamine group had a 78.6% survival rate (11
of 14), with a 95% confidence interval of 49.2 to 95.3%.
Discussion
TCI anesthesia with propofol or propofol combined with a low
dose of ketamine was comparable to an isoflurane inhalant regimen in ease of administration and control of the anesthetic event,
after the tail vein catheter had been placed, when used in rats for
procedures of one-hour duration. While average heart rates were
significantly higher for rats anesthetized using an isoflurane regimen
compared to TCI propofol–ketamine, all observed heart rates were
within the normal values published for rats. Average respiratory rates
were significantly different for each of the anesthesia groups, with
male rats having significantly higher respiratory rates than female
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CONTEMPORARY TOPICS © 2005 by the American Association for Laboratory Animal Science
Figure 3. Average recovery time by gender for three anesthesia groups. The
gender difference for the propofol–ketamine group was clinically apparent
but not statistically significant. As discussed, isoflurane was determined to
be statistically different when compared with both propofol–ketamine and
propofol.
rats. Average respiratory rates during TCI anesthesia were within
the normal values published for rats, but the shallow nature of the
breathing for the TCI regimens would likely affect blood and tissue
oxygenation. Endotracheal intubation and oxygen supplementation
would help reduce this effect. These procedures were not performed
during this study because many investigators that use bolus injection
anesthesia for relatively short duration procedures do not intubate
rats or provide oxygen supplementation. Isoflurane delivered through
a facemask resulted in average respiratory rates that were marginally
slower than normal published values for rats. The relatively positivepressure environment created by the 1.5 liters/min constant flow of
medical-grade air into the facemask resulted in a grossly decreased
respiratory effort with deeper breaths during inhalation. This factor,
combined with the respiratory depression associated with anesthesia,
could account for the observed slow respiratory rates. The high gas
flow rate was chosen to ensure that the anesthesia circuit was truly
nonrebreathing, because animals with normally high respiratory
rates can experience some rebreathing of exhaled gases, particularly
during peak inspiration (13). Rats anesthetized with isoflurane had
a significantly shorter average recovery time compared with that of
animals that received either TCI propofol–ketamine or TCI propofol.
There was not a statistically significant difference in average recovery
time for rats that received either TCI regimen; however, rats receiving the propofol–ketamine combination were clinically slower to
recover, which may be attributed to accumulation of ketamine at the
administered dose. Recovery time was not affected by gender for any
of the anesthesia regimens.
Survival analysis was performed in response to unexpected morbidity and mortality encountered in the TCI groups during the course
of the investigation. The propofol–ketamine group had three rats
that were adversely affected during the anesthetic event. Two of them
died suddenly: one at induction of anesthesia and the other during
recovery. The induction animal stopped breathing after the initial
bolus of propofol–ketamine was administered. Apnea, cardiovascular depression, and hypotension are known adverse effects of bolus
administration of propofol (13, 14, 18, 20) and so were considered
the cause of the condition. The low ketamine dose administered at
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induction did not counteract these effects. Attempts to revive the
animal were unsuccessful. The rat that died during recovery had a
relatively uneventful anesthesia. Physiologic parameters were maintained within the normal limits defined in the study. The average effect
site concentration of anesthetic for this animal was below the overall
average for the propofol–ketamine group. Average SpO2 was greater
than that for the overall average for the group but was < 90%. The
animal seemed to be recovering normally when it stopped breathing.
Attempts to revive it were unsuccessful. The cause of death was not
determined. The third affected rat in the propofol–ketamine group
also had a relatively uneventful anesthesia. Heart rate, respiratory
rate and body temperature were maintained within the normal limits
defined in the study. The average effect site concentration of propofol–ketamine for this animal during the anesthesia period was the
highest of the group (12 μg/ml), and the average SpO2 was < 80%.
When anesthesia was discontinued, the animal remained recumbent for > 1 h and was found to be unable to maintain an upright
posture. A subjective neurologic evaluation revealed the absence of
a righting reflex and poor forelimb and hindlimb grip strength. The
animal subsequently was euthanized. The clinical outcome of this rat
was attributed to CNS hypoxia, although the definitive cause of the
condition was not determined. Because six rats in the propofol-only
group received an average maintenance dose of anesthetic ≥ 12 μg/ml
with no noted clinical abnormalities at recovery, the dose was not
considered excessive during administration to the propofol–ketamine
animal. However, average SpO2 for the six propofol-only rats was
> 80%. The addition of 5 mg ketamine per ml of propofol infused
at an average effect site concentration of 12 μg/ml (of propofol) was
associated with an increased severity of hypoxia during anesthesia
and an unsatisfactory postanesthesia outcome.
The propofol group had one rat that was adversely affected during
anesthesia. The cause was determined to be iatrogenic. The animal was
administered a high average maintenance dose of propofol (20 μg/ml)
in an attempt to slow its respiratory rate to within the normal limits
used for the study. The manipulations resulted in an average SpO2 that
was < 80% for the anesthesia period. The overall effect was noted at
recovery, when the animal was unable to maintain an upright posture
and remained recumbent for > 1 h after anesthesia was discontinued.
A subjective neurologic evaluation revealed the absence of a righting
reflex and poor forelimb and hindlimb grip strength. The animal
subsequently was euthanized. All other incidents of hyperpnea were
noted during data collection, but no action was taken to alter the
condition if SpO2 remained within study guidelines.
Although not statistically significant, the morbidity and mortality
associated with the TCI groups was clinically relevant. Because pulse
oximetry may or may not be available to an investigator performing
surgery, it was not used as a primary monitor for adjusting anesthetic
administration. Instead, titration of anesthesia was based on maintaining a negative tail-clamp response; i.e., a surgical plane of anesthesia,
in conjunction with respiratory and heart rate parameters. Animals
were permitted to become relatively hypoxemic (SpO2 < 90%) during anesthesia as long as oxygen saturation did not fall below 80%
for longer than 1 min. Both TCI groups had overall average SpO2
readings below 90%, indicating relative hypoxemia. The condition
could inadvertently augment the pharmacologic effects of anesthetics,
resulting in increased anesthetic depth, prolonged recovery times, and
associated increases in morbidity and mortality. Oxygen supplementation would alleviate the hypoxemia and is recommended by the
authors, though it is not required for successful TCI anesthesia. The
isoflurane group maintained appropriate oxygenation despite slow
respiratory rates. This outcome was probably a benefit of the constant
positive air pressure provided by the high gas flow rate combined with
a properly sealed facemask.
Volume 44, No. 5 / September 2005
The procedures in this study were designed to mimic a “typical”
anesthesia that an investigator might use for a rodent surgery. Medical-grade air (21% oxygen) was used as part of the inhalant regimen
to simulate the oxygen content of inspired room air in the other
two anesthetic groups. The study design incorporated a chemical
anesthesia regimen that could be initiated and maintained with as
few adjustments to the equipment as possible in order to facilitate a
sole investigator’s performance of anesthesia and surgery on one or
multiple animals. Anesthesia was stopped abruptly at the 1-h time
period instead of being adjusted throughout and discontinued in anticipation of completing a procedure. Any controlled anesthesia (not
bolus injections) could be fine-tuned during surgery if an anesthetist
were present to control and monitor the equipment. This situation,
however, has not been the case for many rodent surgeries, where the
investigator performs preoperative patient preparation, anesthesia,
surgery, and postoperative monitoring duties. Proper aseptic technique precludes multiple adjustments to anesthesia equipment during
surgical procedures, so fewer changes are desirable.
We found that TCI was easy to perform, and adjustments to
anesthesia were simple and quick. The computer program provided
a variety of information to the user that is not readily apparent with
a manually controlled infusion using only a syringe pump or a gas
anesthesia machine. The program recorded all the data on each
animal that was input prior to anesthesia and then all concentration
manipulations that were performed during the anesthetic event.
Clock time from start to finish and total time were displayed and
recorded. Either plasma concentration or effect site concentration
could be selected as the desired target prior to starting the infusion,
and there was an option to change to a manually controlled infusion
if desired. When the infusion was initiated, the program displayed the
infusion rate, instantaneous target concentration, and set point target
concentration along with the total volume of drug that was infused.
If the set point target concentration was input as zero, the program
also displayed the amount of time required to reach a selected target
concentration from the current setting; e.g., the approximate time
required to decrease from 8 to 3 μg/ml. This feature assisted with
determining when the patient would start to show clinical signs of
recovery. All of the information was readily available and could be
exported from the computer and printed using a word processing
program. TCI satisfied one of the goals of this study; it provided an
intravenous anesthesia technique for rodent surgery that was performed and controlled comparable to an inhalation agent.
Propofol has been reported to have poor analgesic effects (9, 13,
14), so it does not meet all of the criteria for a “balanced” anesthetic
agent; i.e., providing amnesia, akinesia, and analgesia. Ketamine
has analgesic properties and unique cardiovascular effects, such as
stimulation, when compared with common opioid analgesics (9, 13,
14). These characteristics make it a viable candidate as an adjunct to
propofol anesthesia. Propofol infusion augmented with a low dose of
ketamine has been documented in both human and equine medicine,
where it provided stable intraoperative hemodynamics, along with
analgesia and an absence of respiratory depression postoperatively (4,
11, 12). The ketamine dose we chose for this study approximated what
has been administered to humans in a surgical propofol–ketamine
regimen (1). This is a subclinical dose for anesthesia in rats; the total
amount administered over the course of 1 h was approximately one
tenth of a published bolus dose of ketamine used for sedation in the
rat (20). The dosing regimen was considered low enough to exert
minimal effects on propofol pharmacokinetics–pharmacodynamics
during anesthesia.
TCI propofol–ketamine in rats did seem to provide a balanced
general anesthesia that would be suitable for surgery. However, the
ketamine dose did not appear to be low enough to avoid some tisCONTEMPORARY TOPICS © 2005 by the American Association for Laboratory Animal Science
11
sue accumulation. As a result, rats receiving the combination had a
significant increase in average recovery time compared with that for
isoflurane. They also were notably slower to recover, compared with
animals receiving propofol alone, even though this difference was not
statistically significant. This drug combination has shown promise
in other species as a surgical anesthesia regimen (4, 11). Future
investigations in the rat that involve a lower dose of ketamine than
what we used potentially could yield an anesthesia regimen with all
of the desirable traits that were sought in this study. We therefore
recommend using propofol instead of the currently reported dose of
propofol–ketamine for TCI in rats.
Intravenous anesthesia offers unique advantages when compared
with volatile anesthesia. It allows unencumbered access to the airway
and head, no concern over waste gas scavenging or operating room
pollution, and uses agents that can provide postoperative analgesia.
TCI provided an anesthetic event that was as easy to induce and
control as inhalant anesthesia in the rat once venous access was accomplished. The equipment needed to perform TCI is uncomplicated,
relatively inexpensive, and easily transportable, making it a viable
option for use in the operating room, the laboratory, and, with a
battery-operated syringe pump, in a field setting. The technique
is being used successfully in human medicine as an alternative to
volatile inhalants for general anesthesia. As pharmacokinetic data for
the metabolism of different drugs in different species are calculated
and published, TCI likely will prove to be a viable anesthetic option
in veterinary medicine.
Acknowledgments
The authors would like to thank Patty Chen, DVM, and Wayne Koller
for their technical support and assistance in the performance of this study.
Opinions, interpretations, conclusions, and recommendations are those of
the authors and are not necessarily endorsed by the U.S. Army.
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Volume 44, No. 5 / September 2005