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Abstract

Objective—To characterize halothane and sevoflurane anesthesia in spontaneously breathing rats.

Animals—16 healthy male Sprague-Dawley rats.

Procedure—8 rats were anesthetized with halothane and 8 with sevoflurane. Minimum alveolar concentration (MAC) was determined. Variables were recorded at anesthetic concentrations of 0.8, 1.0, 1.25, and 1.5 times the MAC of halothane and 1.0, 1.25, 1.5, and 1.75 times the MAC of sevoflurane.

Results—Mean (± SEM) MAC for halothane was 1.02 ± 0.02% and for sevoflurane was 2.99 ± 0.19%. As sevoflurane dose increased from 1.0 to 1.75 MAC, mean arterial pressure (MAP) decreased from 103.1 ± 5.3 to 67.9 ± 4.6 mm Hg, and PaCO2 increased from 58.8 ± 3.1 to 92.2 ± 9.2 mm Hg. As halothane dose increased from 0.8 to 1.5 MAC, MAP decreased from 99 ± 6.2 to 69.8 ± 4.5 mm Hg, and PaCO2 increased from 59.1 ± 2.1 to 75.9 ± 5.2 mm Hg. Respiratory rate decreased in a dose-dependent fashion from 88.5 ± 4.5 to 58.5 ± 2.7 breaths/min during halothane anesthesia and from 42.3 ± 1.8 to 30.5 ± 4.5 breaths/min during sevoflurane anesthesia. Both groups of rats had an increase in eyelid and pupillary aperture with an increase in anesthetic dose.

Conclusions and Clinical Relevance—An increase in PaCO2 and a decrease in MAP are clinical indicators of an increasing halothane and sevoflurane dose in unstimulated spontaneously breathing rats. Increases in eyelid aperture and pupil diameter are reliable signs of increasing depth of halothane and sevoflurane anesthesia. Decreasing respiratory rate is a clinical indicator of an increasing dose of halothane. (Am J Vet Res 2003;64:470–474)

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in American Journal of Veterinary Research

Abstract

Objective—To quantitate dose- and time-related magnitudes of interactive effects of morphine (MOR) and isoflurane (ISO) in horses and to characterize pharmacokinetics of MOR in plasma and the ventilatory response to MOR during administration of ISO.

Animals—6 adult horses.

Procedure—Horses were anesthetized 3 times to determine the minimum alveolar concentration (MAC) of ISO in O2 and then to characterize the change in anesthetic requirement as defined by the alteration in ISO MAC following IV administration of saline (0.9% NaCl) solution and 2 doses of MOR (low dose, 0.25 mg/kg; high dose, 2.0 mg/kg). Arterial blood samples were obtained before and after MOR and analyzed.

Results—Mean ± SD baseline ISO MAC was 1.43 ± 0.06%. The ISO MAC did not change with time after administration of saline solution. Effects of MOR on ISO MAC varied. Maximal change in MAC ranged from –20.2 to +28.3% and –18.9 to +56.2% after low and high doses of MOR, respectively. Typical half-life of MOR in plasma was 40 to 60 minutes and related to dose. Mean PaCO2 increased from 70 mm Hg before MOR to 88 to 102 mm Hg for 30 to 240 minutes after the high dose of MOR. Recovery from anesthesia after administration of the high dose of MOR was considered undesirable and dangerous.

Conclusions and Clinical Relevance—Our results do not support routine clinical use of MOR administered IV at dosages of 0.25 or 2.0 mg/kg as an adjuvant to anesthesia in horses administered ISO. (Am J Vet Res 2003;64:166–175)

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in American Journal of Veterinary Research

Abstract

Objective—To describe the effects of changes in circuit volume and oxygen inflow rate on inspired oxygen concentration for a large animal circle anesthetic system.

Study Population—A large animal circle anesthetic system, a 10 L/min flowmeter, and 20- and 40-L breathing bags.

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 bag.

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)

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in American Journal of Veterinary Research

Abstract

Objective—To determine whether infusion of xylazine and ketamine or xylazine and propofol after sevoflurane administration in horses would improve the quality of recovery from anesthesia.

Animals—6 healthy adult horses.

Procedures—For each horse, anesthesia was induced by administration of xylazine, diazepam, and ketamine and maintained with sevoflurane for approximately 90 minutes (of which the last 60 minutes were under steady-state conditions) 3 times at 1-week intervals. For 1 anesthetic episode, each horse was allowed to recover from sevoflurane anesthesia; for the other 2 episodes, xylazine and ketamine or xylazine and propofol were infused for 30 or 15 minutes, respectively, after termination of sevoflurane administration. Selected cardiopulmonary variables were measured during anesthesia and recovery. Recovery events were monitored and subjectively scored.

Results—Cardiopulmonary variables differed minimally among treatments, although the xylazine-propofol infusion was associated with greater respiratory depression than was the xylazine-ketamine infusion. Interval from discontinuation of sevoflurane or infusion administration to standing did not differ significantly among treatments, but the number of attempts required to stand successfully was significantly lower after xylazine-propofol infusion, compared with the number of attempts after sevoflurane alone. Scores for recovery from anesthesia were significantly lower (ie, better recovery) after either infusion, compared with scores for sevoflurane administration alone.

Conclusions and Clinical Relevance—Xylazine-ketamine or xylazine-propofol infusion significantly improved quality of recovery from sevoflurane anesthesia in horses. Xylazine-ketamine or xylazine-propofol infusions may be of benefit during recovery from sevoflurane anesthesia in horses for which a smooth recovery is particularly critical. However, oxygenation and ventilation should be monitored carefully.

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in American Journal of Veterinary Research

Abstract

Objective—To verify the isoflurane anesthetic minimum alveolar concentration (MAC)-sparing effect of a previously administered target plasma fentanyl concentration of 16 ng/mL and characterize an anticipated further sparing in isoflurane MAC associated with higher target plasma fentanyl concentrations.

Animals—8 horses.

Procedures—Horses were assigned 2 of 3 target plasma fentanyl concentrations (16, 24, and 32 ng/mL), administered in ascending order. Following determination of baseline MAC, horses received a loading dose of fentanyl followed by a constant rate infusion; MAC determination was performed in triplicate at baseline and at each fentanyl concentration. Venous blood samples were collected throughout the study for determination of actual plasma fentanyl concentrations. Recovery from anesthesia was monitored, and behaviors were rated as excellent, good, fair, or poor.

Results—Mean ± SD fentanyl plasma concentrations were 13.9 ± 2.6 ng/mL, 20.1 ± 3.6 ng/mL, and 24.1 ± 2.4 ng/mL for target concentrations of 16, 24, and 32 ng/mL, respectively. The corresponding changes in the MAC of isoflurane were −3.28%, −6.23%, and +1.14%. None of the changes were significant. Recovery behavior was variable and included highly undesirable, potentially injurious excitatory behavior.

Conclusions and Clinical Relevance—Results of the study did not verify an isoflurane-sparing effect of fentanyl at a plasma target concentration of 16 ng/mL. Furthermore, a reduction in MAC was not detected at higher fentanyl concentrations. Overall, results did not support the routine use of fentanyl as an anesthetic adjuvant in adult horses.

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in American Journal of Veterinary Research

Abstract

Objective—To qualitatively and quantitatively evaluate the characteristics of desflurane with regard to the induction of and recovery from anesthesia in cats.

Animals—6 cats.

Procedure—Anesthesia was induced and maintained with desflurane in oxygen. Individual minimum alveolar concentration (MAC) values were determined; anesthesia was maintained at 1.25 × MAC for a total anesthesia time (including MAC determination) of 5 hours. Cats were allowed to recover from anesthesia. Induction and recovery periods were video recorded and later scored by use of a grading scale from 0 to 100 (100 being the best outcome). Timing of events was recorded.

Results—The MAC of desflurane was 10.27 ± 1.06%, and mean dose was 5.6 ± 0.2 MAC-hours. Times to loss of coordination, recumbency, and endotracheal intubation were 1.3 ± 0.4, 2.3 ± 0.3, and 6.4 ± 1.1 minutes, respectively. Median score for quality of anesthetic induction was 93 (range, 91 to 94). Times to first movement, extubation, standing, and ability to jump and land with coordination were 2.8 ± 1.0, 3.8 ± 0.5, 14.3 ± 3.9, and 26.4 ± 5.1 minutes, respectively. Alveolar washout of desflurane was rapid. Median score for quality of anesthetic recovery was 94 (range, 86 to 96).

Conclusions and Clinical Relevance—Desflurane was associated with rapid induction of and recovery from anesthesia in cats; assessors rated the overall quality of induction and recovery as excellent. Results appear to support the use of desflurane for induction and maintenance of anesthesia in healthy cats. (Am J Vet Res 2004;65:748–751)

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in American Journal of Veterinary Research

Abstract

Objective—To quantitate the dose and time-related effects of morphine sulfate on the anesthetic sparing effect of xylazine hydrochloride in halothane-anesthetized horses and determine the associated plasma xylazine and morphine concentration-time profiles.

Animals—6 healthy adult horses.

Procedure—Horses were anesthetized 3 times to determine the minimum alveolar concentration (MAC) of halothane in O2 and characterize the anesthetic sparing effect (ie, decrease in MAC of halothane) by xylazine (0.5 mg/kg, IV) administration followed immediately by IV administration of saline (0.9% NaCl) solution, low-dose morphine (0.1 mg/kg), or high-dose morphine (0.2 mg/kg). Selected parameters of cardiopulmonary function were also determined over time to verify consistency of conditions.

Results—Mean (± SEM) MAC of halothane was 1.05 ± 0.02% and was decreased by 20.1 ± 6.6% at 49 ± 2 minutes following xylazine administration. The amount of MAC reduction in response to xylazine was time dependent. Addition of morphine to xylazine administration did not contribute further to the xylazine-induced decrease in MAC (reductions of 21.9 ± 1.2 and 20.7 ± 1.5% at 43 ± 4 and 40 ± 4 minutes following xylazine-morphine treatments for low-and high-dose morphine, respectively). Overall, cardiovascular and respiratory values varied little among treatments. Kinetic parameters describing plasma concentration-time curves for xylazine were not altered by the concurrent administration of morphine.

Conclusions and Clinical Relevance—Administration of xylazine decreases the anesthetic requirement for halothane in horses. Concurrent morphine administration to anesthetized horses does not alter the anesthetic sparing effect of xylazine or its plasma concentration-time profile. (Am J Vet Res 2004; 65:519–526)

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in American Journal of Veterinary Research

Abstract

Objective—To evaluate whether guaifenesin can prevent adverse anesthetic induction events caused by propofol and whether a guaifenesin-propofol induction combination has brief cardiovascular effects commensurate with rapid drug washout.

Animals—8 healthy adult horses.

Procedures—Guaifenesin was administered IV for 3 minutes followed by IV injection of a bolus of propofol (2 mg/kg). Additional propofol was administered if purposeful movement was detected. Anesthesia was maintained for 2 hours with isoflurane or sevoflurane at 1.2 times the minimum alveolar concentration with controlled normocapnic ventilation. Normotension was maintained via a dobutamine infusion. Plasma concentrations of propofol and guaifenesin were measured every 30 minutes.

Results—Mean ± SD guaifenesin and propofol doses inducing anesthesia in half of the horses were 73 ± 18 mg/kg and 2.2 ± 0.3 mg/kg, respectively. No adverse anesthetic induction events were observed. By 70 minutes, there was no significant temporal change in the dobutamine infusion rate required to maintain normotension for horses anesthetized with isoflurane or sevoflurane. Mean plasma guaifenesin concentrations were 122 ± 30μM, 101 ± 33μM, 93 ± 28μM, and 80 ± 24μM at 30, 60, 90, and 120 minutes after anesthetic induction, respectively. All plasma propofol concentrations were below the limit of quantitation.

Conclusions and Clinical Relevance—Guaifenesin prevented adverse anesthetic induction events caused by propofol. Guaifenesin (90 mg/kg) followed by propofol (3 mg/kg) should be sufficient to immobilize > 99% of calm healthy adult horses. Anesthetic drug washout was rapid, and there was no change in inotrope requirements after anesthesia for 70 minutes.

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in American Journal of Veterinary Research

Abstract

Objective—To quantitate the effects of desflurane and mode of ventilation on cardiovascular and respiratory functions and identify changes in selected clinicopathologic variables and serum fluoride values associated with desflurane anesthesia in horses.

Animals—6 healthy adult horses.

Procedure—Horses were anesthetized on 2 occasions: first, to determine the minimum alveolar concentration (MAC) of desflurane in O2 and second, to characterize cardiopulmonary and clinicopathologic responses to 1×, 1.5×, and 1.75× desflurane MAC during both controlled and spontaneous ventilation.

Results—Mean ± SEM MAC of desflurane in horses was 8.06 ± 0.41%; inhalation of desflurane did not appear to cause airway irritation. During spontaneous ventilation, mean PaCO2 was 69 mm Hg. Arterial blood pressure, stroke volume, and cardiac output decreased as the dose of desflurane increased. Conditions of intermittent positive pressure ventilation and eucapnia resulted in further cardiovascular depression. Horses recovered quickly from anesthesia with little transient or no clinicopathologic evidence of adverse effects. Serum fluoride concentration before and after administration of desflurane was below the limit of detection of 0.05 ppm (2.63µM/L).

Conclusions and Clinical Relevance—Results indicate that desflurane, like other inhalation anesthetics, causes profound hypoventilation in horses. The magnitude of cardiovascular depression is related to dose and mode of ventilation; cardiovascular depression is less severe at doses of 1× to 1.5× MAC, compared with known effects of other inhalation anesthetics under similar conditions. Desflurane is not metabolized to an important degree and does not appear to prominently influence renal function or hepatic cellular integrity or function. ( Am J Vet Res 2005;66:669–677)

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in American Journal of Veterinary Research