Objective—To describe the effects of changes in circuit
volume and oxygen inflow rate on inspired oxygen
concentration for a large animal circle anesthetic
Study Population—A large animal circle anesthetic
system, a 10 L/min flowmeter, and 20- and 40-L
Procedure—Circuit volume was determined by a carbon
dioxide dilution technique. Oxygen flow rates of
3, 6, and 10 L/min were delivered to the circuit with
the large breathing bag, and a flow rate of 6 L/min
was used with the small bag. Gas samples were collected
during a 20-minute period. The time constant
(τ) and half-time (T1/2) were calculated and compared
with measured values.
Results—Mean ± SEM volume of the breathing circuit
with a 20- and 40-L breathing bag was 32.97 ±
0.91 L and 49.26 ± 0.58 L, respectively. The from
measurements was 11.97, 6.10, and 3.60 minutes at
oxygen flow rates of 3, 6, and 10 L/min, respectively,
for the large breathing bag and 3.73 minutes at a flow
rate of 6 L/min for the small breathing bag. The T1/2
was 8.29, 4.22, and 2.49 minutes at oxygen flow
rates of 3, 6, and 10 L/min, respectively, for the large
breathing bag and 2.58 minutes for the small breathing
Conclusions and Clinical Relevance—This study
emphasizes that there are delays in the rate of
increase in the inspired oxygen concentration that
accompany use of conventional large animal circle
anesthetic systems and low rates of inflow for fresh
oxygen. (Am J Vet Res 2005;66:1675–1678)
Objective—To determine the effect of 6 plasma ketamine concentrations on the minimum alveolar concentration (MAC) of isoflurane in dogs.
Procedure—In experiment 1, the MAC of isoflurane was measured in each dog and the pharmacokinetics of ketamine were determined in isoflurane-anesthetized dogs after IV administration of a bolus (3 mg/kg) of ketamine. In experiment 2, the same dogs were anesthetized with isoflurane in oxygen. A target-controlled IV infusion device was used to administer ketamine and to achieve plasma ketamine concentrations of 0.5, 1, 2, 5, 8, and 11 μg/mL by use of parameters obtained from experiment 1. The MAC of isoflurane was determined at each plasma ketamine concentration, and blood samples were collected for ketamine and norketamine concentration determination.
Results—Actual mean ± SD plasma ketamine concentrations were 1.07 ± 0.42 μg/mL, 1.62 ± 0.98 μg/mL, 3.32 ± 0.59 μg/mL, 4.92 ± 2.64 μg/mL, 13.03 ± 10.49 μg/mL, and 22.80 ± 25.56 μg/mL for target plasma concentrations of 0.5, 1, 2, 5, 8, and 11 μg/mL, respectively. At these plasma concentrations, isoflurane MAC was reduced by 10.89% to 39.48%, 26.77% to 43.74%, 25.24% to 84.89%, 44.34% to 78.16%, 69.62% to 92.31%, and 71.97% to 95.42%, respectively. The reduction in isoflurane MAC was significant, and the response had a linear and quadratic component. Salivation, regurgitation, mydriasis, increased body temperature, and spontaneous movements were some of the adverse effects associated with the high plasma ketamine concentrations.
Conclusions and Clinical Relevance—Ketamine appears to have a potential role for balanced anesthesia in dogs. (Am J Vet Res 2006;67:21–25)
Objective—To characterize the effects of ketamine
administration on the cardiovascular and respiratory
systems and on acid-base balance and to record
adverse effects of ketamine in isoflurane-anesthetized
Animals—6 healthy adult mongrel dogs.
Procedure—Dogs were anesthetized with isoflurane
(1.25 times the individual minimum alveolar concentration)
in oxygen, and ketamine was administered IV
to target pseudo–steady-state plasma concentrations
of 0, 0.5, 1, 2, 5, 8, and 11 µg/mL. Isoflurane concentration
was reduced to an equipotent concentration.
Cardiovascular, respiratory, and acid-base variables;
body temperature; urine production; and adverse
effects were recorded before and during noxious
stimulation. Cardiac index, stroke index, rate-pressure
product, systemic vascular resistance index, pulmonary
vascular resistance index, left ventricular
stroke work index, right ventricular stroke work index,
arterial oxygen concentration, mixed-venous oxygen
concentration, oxygen delivery, oxygen consumption,
oxygen extraction ratio, alveolar-arterial oxygen partial
pressure gradient, and venous admixture were calculated.
Plasma ketamine and norketamine concentrations
Results—Overall, ketamine administration improved
ventilation, oxygenation, hemodynamics, and oxygen
delivery in isoflurane-anesthetized dogs in a dosedependent
manner. With the addition of ketamine,
core body temperature was maintained or increased
and urine production was maintained at an acceptable
amount. However, at the higher plasma ketamine
concentrations, adverse effects such as spontaneous
movement and profuse salivation were observed.
Myoclonus and dysphoria were observed during
recovery in most dogs.
Conclusions and Clinical Relevance—Infusion of
ketamine appears to be a suitable technique for balanced
anesthesia with isoflurane in dogs. Plasma ketamine
concentrations between 2 to 3 µg/mL elicited
the most benefits with minimal adverse effects.
(Am J Vet Res 2005;66:2122–2129)
Objective—To determine whether withholding of food affects autonomic nervous system balance by analysis of heart rate (HR), HR variability (HRV), and frequency of second-degree atrioventricular block in horses.
Animals—5 healthy Thoroughbreds.
Procedures—For two 24-hour periods in a crossover study, food was withheld from horses or horses were maintained on their regular feeding schedule (control conditions) in their stalls and Holter monitor ECG recordings were obtained. The ECGs were analyzed by use of fast-Fourier transformation, and power spectrum densities were calculated for low-frequency (0.01 to 0.07 Hz) and high-frequency (0.07 to 0.6 Hz) variations in HR. Serum cortisol and plasma ACTH, norepinephrine, and glucose concentrations were measured at predetermined time points.
Results—Withholding of food resulted in significantly lower HR and more frequent second-degree atrioventricular block (the frequency of which was inversely related to the HR), compared with findings for control conditions. Circadian rhythms were similar during food-withholding and control conditions; peak HR was detected from 7:00 pm to 8:00 pm, and the lowest HR was detected in the early morning. During food-withholding conditions, the low-frequency and high-frequency components of HRV were significantly higher, and the low-frequency-to-high-frequency ratio was lower than during control conditions. Serum cortisol concentration was higher and plasma glucose concentration was lower at 6:00 pm in horses when food was withheld, compared with findings during control conditions.
Conclusions and Clinical Relevance—Indices of HRV seemed to be sensitive to changes in autonomic nervous activity and may be useful as clinical indices of the neuroendocrine response to stressors in horses.
Objective—To evaluate the effects of dorsal versus lateral recumbency on the cardiopulmonary system during isoflurane anesthesia in red-tailed hawks (Buteo jamaicensis).
Animals—6 adult 1.1- to 1.6-kg red-tailed hawks.
Procedures—A randomized, crossover study was used to evaluate changes in respiratory rate, tidal volume, minute ventilation, heart rate, mean arterial and indirect blood pressures, and end-tidal Pco2 measured every 5 minutes plus Paco2 and Pao2 and arterial pH measured every 15 minutes throughout a 75-minute study period.
Results—Respiratory rate was higher, tidal volume lower, and minute ventilation not different in lateral versus dorsal recumbency. Position did not affect heart rate, mean arterial blood pressure, or indirect blood pressure, although heart rate decreased during the anesthetic period. Birds hypoventilated in both positions and Paco2 differed with time and position × time interaction. The Petco2 position × time interaction was significant and Petco2 was a mean of 7 Torr higher than Paco2. The Paco2 in dorsal recumbency was a mean of 32 Torr higher than in lateral recumbency. Birds in both positions developed respiratory acidosis.
Conclusions and Clinical Relevance—Differences in tidal volume with similar minute ventilation suggested red-tailed hawks in dorsal recumbency might have lower dead space ventilation. Despite similar minute ventilation in both positions, birds in dorsal recumbency hypoventilated more yet maintained higher Pao2, suggesting parabronchial ventilatory or pulmonary blood flow distribution changes with position. The results refute the hypothesis that dorsal recumbency compromises ventilation and O2 transport more than lateral recumbency in red-tailed hawks.