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Abstract

Objective

To study the effects of inhalation anesthetic agents on the response of horses to 3 hours of hypoxemia.

Design

Controlled crossover study.

Animals

Five healthy adult horses.

Procedure

Horses were anesthetized twice: once with halothane, and once with isoflurane in O2. Anesthetized horses were positioned in left lateral recumbency. Constant conditions for the study began at 2 hours of anesthesia. A constant agent dose of 1.2 minimum alveolar concentration, PaO2 of 50 ± 5 mm of Hg, and PaCO2 of 45 ± 5 mm of Hg were maintained for 3 hours. Circulatory measurements were made at 0.5, 1, 2, and 3 hours of hypoxemia (anesthesia hours 2.5, 3, 4, and 5). Blood was collected from horses for biochemical analyses before anesthesia, within a few minutes after standing, and at 1, 2, 4, and 7 days after anesthesia.

Results

Cardiac index was greater (P = 0.018) during isoflurane than halothane anesthesia. Cardiac index remained constant during the 3 hours of hypoxemia during halothane anesthesia, whereas it decreased from the baseline during isoflurane anesthesia. Marginally nonsignificant P values for an agent difference were detected for arterial O2 content (P = 0.051), and oxygen delivery (P = 0.057). Serum activities of aspartate transaminase (P = 0.050) and sorbitol dehydrogenase (P = 0.017) were higher in halothane-anesthetized horses than in isoflurane-anesthetized horses. Circulatory function was better in hypoxemic horses anesthetized with isoflurane than with halothane. Isoflurane resulted in less muscular injury in hypoxemia horses than did halothane anesthesia. Halothane anesthesia and hypoxemia were associated with hepatic insult.

Conclusion

Isoflurane is better than halothane for hypoxemic horses.(Am J Vet Res 1996;57:351-360)

Free access
in American Journal of Veterinary Research

Abstract

Objective

To characterize isoflurane (ISO)-induced anesthesia in ferrets and rats.

Animals

8 ferrets (Mustela putorius furo) and 8 Sprague-Dawley rats.

Procedure

Ferrets and rats were anesthetized in a similar manner, using ISO in oxygen. Minimum alveolar concentration (MAC) was determined, using the tail-clamp method. Immediately thereafter, assessments were recorded for 0.8, 1.0, 1.5, and 2.0 MAC (order randomized) of ISO.

Results

MAC of ISO was (mean ± SEM) 1.74 ± 0.03 and 1.58 ± 0.05% for ferrets and rats, respectively. Mean arterial blood pressure (MAP) was 75.0 ± 4.3 and 107.9 ± 2.7 mm Hg at 0.8 MAC for ferrets and rats, respectively, and decreased in a parallel dose-dependent manner. Respiratory frequency decreased in rats as ISO dose increased; however, respiratory frequency increased in ferrets as ISO dose increased from 0.8 to 1.5 MAC but then decreased at 2.0 MAC. At 0.8 MAC, hypoventilation was much greater in ferrets (Paco2 = 71.4 ± 3.5 mm Hg), compared with rats (Paco2 = 57.7 ± 1.9 mm Hg). In both species, Paco2 progressively increased as anesthetic dose increased. Eyelid aperture of ferrets increased in a dose-dependent manner. Pupil diameter in ferrets and rats increased as ISO dose increased.

Conclusions and Clinical Relevance

The MAP and Paco2 in ferrets and rats and eyelid aperture in ferrets consistently and predictably changed in response to changes in anesthetic dose of ISO. Magnitude of respiratory depression was greater in ferrets than rats. Changes in MAP and Paco2 in ferrets and rats and eyelid aperture in ferrets are consistent guides to changes in depth of ISO-induced anesthesia. (Am J Vet Res 1999;60:1577–1583)

Free access
in American Journal of Veterinary Research

Abstract

Objective

To characterize variables used to monitor rabbits during inhalation anesthesia.

Animals

8 male New Zealand White rabbits.

Procedure

Rabbits were similarly anesthetized with halothane (HAL) or isoflurane (ISO) in a crossover study; half received HAL followed by ISO, and the protocol was reversed for the remaining rabbits. After induction, minimum alveolar concentration (MAC) was determined for each agent, using the tail-clamp method, and variables were recorded at 0.8, 1.0, 1.5, and 2.0 MAC (order randomized).

Results

Mean ± sem mac was 1.42 ± 0.05 and 2.07 ± 0.09% for hal and iso, respectively. Directly measured auricular mean arterial blood pressure was 52.8 ± 5.6 and 54.8 ± 6.1 mm Hg at 0.8 mac for hal and iso, respectively, and decreased from these values in a parallel dose-dependent manner. Respiratory frequency remained constant (range, 69 to 78 breaths/min) over the range of hal doses but incrementally decreased from a mean of 53 (at 0.8 mac) to 32 breaths/min (at 2.0 mac) for iso. The Paco2 was similar at 0.8 mac for hal and iso and progressively increased with increasing doses of both agents; Paco2 at 2.0 mac for iso was significantly greater than that at 2.0 mac for hal (79.8 ± 13.7 vs 54.9 ± 4.0 mm Hg, respectively). Eyelid aperture consistently increased in a dose-dependent manner for both anesthetics.

Conclusions

Arterial blood pressure, Paco2, and eyelid aperture consistently and predictably changed in rabbits in response to changes in anesthetic doses. The magnitude of respiratory depression was greater for iso than for hal. (Am J Vet Res 1999;60:1189–1195)

Free access
in American Journal of Veterinary Research

Summary

To study behavioral and cardiopulmonary characteristics of horses recovering from inhalation anesthesia, 6 nonmedicated horses were anesthetized under laboratory conditions on 3 different days, with either halothane or isoflurane in O2. Anesthesia was maintained at constant dose (1.5 times the minimum alveolar concentration [mac]) of halothane in O2 for 1 hour (H1), halothane in O2 for 3 hours (H3), or isoflurane in O2 for 3 hours (I3). The order of exposure was set up as a pair of Latin squares to account for horse and trial effects. Circulatory (arterial blood pressure and heart rate) and respiratory (frequency, PaCO2 , PaO2 , pHa) variables were monitored during anesthesia and for as long as possible during the recovery period. End-tidal percentage of the inhaled agent was measured every 15 seconds by automated mass spectrometry, then by hand-sampling after horses started moving. Times of recovery events, including movement of the eyelids, ears, head, and limbs, head lift, chewing, swallowing, first sternal posture and stand attempts, and the number of sternal posture and stand attempts, were recorded.

The washout curve or the et ratio (end-tidal percentage of the inhaled agent at time t to end-tidal percentage of the inhaled agent at the time the anesthesia circuit was disconnected from the tracheal tube) plotted against time was similar for H1 and H3. The slower, then faster (compared with halothane groups) washout curve of isoflurane was explainable by changes in respiratory frequency as horses awakened and by lower blood/gas solubility of isoflurane. The respiratory depressant effects of isoflurane were marked and were more progressive than those for halothane at the same 1.5 mac dose. During the first 15 minutes of recovery, respiratory frequency for group-I3 horses increased significantly (P < 0.05), compared with that for the halothane groups. For all groups, arterial blood pressure increased throughout the early recovery period and heart rate remained constant.

Preanesthesia temperament of horses and the inhalation agent used did not influence the time of the early recovery events (movement of eyelids, ears, head, and limbs), except for head lift. For events that occurred at anesthetic end-tidal percentage < 0.20, or when horses were awake, temperament was the only factor that significantly influenced the nature of the recovery (chewing P = 0.04, extubation P = 0.001, first stand attempt P = 0.008, and standing P = 0.005). The quality of the recoveries did not differ significantly among groups (H1, H3, I3) or horses; however 5 of 6 horses recovering from the H1 exposure had ideal recovery. During recovery, the anesthetic end-tidal percentage did not differ significantly among groups. However, when concentrations were compared on the basis of anesthetic potency (ie, mac multiple) a significantly (P < 0.05) lower MAC multiple of isoflurane was measured for the events ear movement, limb movement, head lift, and first attempt to sternal posture, compared with that for horses given halothane, indicating that isoflurane may be a more-potent sedative than halothane in these horses.

Free access
in American Journal of Veterinary Research

Abstract

Objective—To test the hypothesis that head-down positioning in anesthetized horses increases intracranial pressure (ICP) and decreases cerebral and spinal cord blood flows.

Animals—6 adult horses.

Procedures—For each horse, anesthesia was induced with ketamine hydrochloride and xylazine hydrochloride and maintained with 1.57% isoflurane in oxygen. Once in right lateral recumbency, horses were ventilated to maintain normocapnia. An ICP transducer was placed in the subarachnoid space, and catheters were placed in the left cardiac ventricle and in multiple vessels. Blood flow measurements were made by use of a fluorescent microsphere technique while each horse was in horizontal and head-down positions. Inferential statistical analyses were performed via repeated-measures ANOVA and Dunn-Sidak comparisons.

Results—Because 1 horse developed extreme hypotension, data from 5 horses were analyzed. During head-down positioning, mean ± SEM ICP increased to 55 ± 2 mm Hg, compared with 31 ± 2 mm Hg during horizontal positioning; cerebral perfusion pressure was unchanged. Compared with findings during horizontal positioning, blood flow to the cerebrum, cerebellum, and cranial portion of the brainstem decreased significantly by approximately 20% during head-down positioning; blood flows within the pons and medulla were mildly but not significantly decreased. Spinal cord blood flow was low (9 mL/min/100 g of tissue) and unaffected by position.

Conclusions and Clinical Relevance—Head-down positioning increased heart-brain hydrostatic gradients in isoflurane-anesthetized horses, thereby decreasing cerebral blood flow and, to a greater extent, increasing ICP. During anesthesia, CNS regions with low blood flows in horses may be predisposed to ischemic injury induced by high ICP.

Full access
in American Journal of Veterinary Research

Abstract

OBJECTIVE To measure concentrations of trazodone and its major metabolite in plasma and urine after administration to healthy horses and concurrently assess selected physiologic and behavioral effects of the drug.

ANIMALS 11 Thoroughbred horses enrolled in a fitness training program.

PROCEDURES In a pilot investigation, 4 horses received trazodone IV (n = 2) or orally (2) to select a dose for the full study; 1 horse received a vehicle control treatment IV. For the full study, trazodone was initially administered IV (1.5 mg/kg) to 6 horses and subsequently given orally (4 mg/kg), with a 5-week washout period between treatments. Blood and urine samples were collected prior to drug administration and at multiple time points up to 48 hours afterward. Samples were analyzed for trazodone and metabolite concentrations, and pharmacokinetic parameters were determined; plasma drug concentrations following IV administration best fit a 3-compartment model. Behavioral and physiologic effects were assessed.

RESULTS After IV administration, total clearance of trazodone was 6.85 ± 2.80 mL/min/kg, volume of distribution at steady state was 1.06 ± 0.07 L/kg, and elimination half-life was 8.58 ± 1.88 hours. Terminal phase half-life was 7.11 ± 1.70 hours after oral administration. Horses had signs of aggression and excitation, tremors, and ataxia at the highest IV dose (2 mg/kg) in the pilot investigation. After IV drug administration in the full study (1.5 mg/kg), horses were ataxic and had tremors; sedation was evident after oral administration.

CONCLUSIONS AND CLINICAL RELEVANCE Administration of trazodone to horses elicited a wide range of effects. Additional study is warranted before clinical use of trazodone in horses can be recommended.

Full access
in American Journal of Veterinary Research

Abstract

Objective—To quantitate effects of dose of sevoflurane and mode of ventilation on cardiovascular and respiratory function in horses and identify changes in serum biochemical values associated with sevoflurane anesthesia.

Animals—6 healthy adult horses.

Procedure—Horses were anesthetized twice: first, to determine the minimum alveolar concentration (MAC) of sevoflurane and second, to characterize cardiopulmonary and serum biochemical responses of horses to 1.0, 1.5, and 1.75 MAC multiples of sevoflurane during controlled and spontaneous ventilation.

Results—Mean (± SEM) MAC of sevoflurane was 2.84 ± 0.16%. Cardiovascular performance during anesthesia decreased as sevoflurane dose increased; the magnitude of cardiovascular depression was more severe during mechanical ventilation, compared with spontaneous ventilation. Serum inorganic fluoride concentration increased to a peak of 50.8 ± 7.1 µmol/L at the end of anesthesia. Serum creatinine concentration and sorbitol dehydrogenase activity reached their greatest values (2.0 ± 0.8 mg/dL and 10.2 ± 1.8 U/L, respectively) at 1 hour after anesthesia and then returned to baseline by 1 day after anesthesia. Serum creatine kinase, aspartate aminotransferase, and alkaline phosphatase activities reached peak values by the first (ie, creatine kinase) or second (ie, aspartate aminotransferase and alkaline phosphatase) day after anesthesia.

Conclusions and Clinical Relevance—Sevoflurane causes dose-related cardiopulmonary depression, and mode of ventilation further impacts the magnitude of this depression. Except for serum inorganic fluoride concentration, quantitative alterations in serum biochemical indices of liver- and muscle-cell disruption and kidney function were considered clinically unremarkable and similar to results from comparable studies of other inhalation anesthetics. (Am J Vet Res 2005;66:606–614)

Full access
in American Journal of Veterinary Research

Abstract

Objective—To determine pharmacokinetics and selected cardiopulmonary effects of fentanyl in isoflurane-anesthetized rhesus monkeys.

Animals—6 adult male rhesus monkeys.

Procedure—Fentanyl (8 mg/kg of body weight, IV) was administered to 6 monkeys anesthetized with isoflurane. End-tidal isoflurane concentration and esophageal temperature were kept constant, and ventilation was mechanically assisted. Heart rate, rhythm, aortic blood pressure, and blood pH, gas, and fentanyl concentrations were determined before and for 8 hours after administration of fentanyl. Pharmacokinetics of fentanyl were derived by use of noncompartmental methods based on statistical moment theory.

Results—Heart rate and mean arterial pressure decreased transiently following fentanyl administration. Maximal decreases were observed 5 to 15 minutes after administration. Arterial pH, PaCO2, and PaO2 ranged from 7.46 ± 0.04 to 7.51 ± 0.05 units, 29.2 ± 3 to 34.6 ± 4.4 mm Hg, and 412.6 ± 105.3 to 482.9 ± 71.2 mm Hg, respectively. The clearance, volume of distribution area, volume of distribution steady state, mean residence time, area under the curve, elimination rate constant, and half-life were 32.5 ± 2.48 ml/kg/min, 9.04 ± 1.91 L/kg, 7.0 ± 1.2 L/kg, 218.5 ± 35.5 min, 0.247 ± 0.019 mg/ml/min, 0.004 ± 0.001/min, and 192.0 ± 33.5 min, respectively.

Conclusions and Clinical Relevance—Transient but potentially clinically important decreases in heart rate and mean arterial pressure were observed following fentanyl administration. Distribution and clearance data were similar to those reported for dogs and humans. (Am J Vet Res 2000;61:931–934)

Full access
in American Journal of Veterinary Research

Abstract

Objective—To quantitate the dose- and time-related effects of IV administration of xylazine and detomidine on urine characteristics in horses deprived of feed and water.

Animals—6 horses.

Procedure—Feed and water were withheld for 24 hours followed by IV administration of saline (0.9% NaCl) solution, xylazine (0.5 or 1.0 mg/kg), or detomidine (0.03 mg/kg). Horses were treated 4 times, each time with a different protocol. Following treatment, urine and blood samples were obtained at 15, 30, 60, 120, and 180 minutes. Blood samples were analyzed for PCV and serum concentrations of total plasma solids, sodium, and potassium. Urine samples were analyzed for pH and concentrations of glucose, proteins, sodium, and potassium.

Results—Baseline (before treatment) urine flow was 0.30 ± 0.03 mL/kg/h and did not significantly change after treatment with saline solution and low-dose xylazine but transiently increased by 1 hour after treatment with high-dose xylazine or detomidine. Total urine output at 2 hours following treatment was 312 ± 101 mL versus 4,845 ± 272 mL for saline solution and detomidine, respectively. Absolute values of urine concentrations of sodium and potassium also variably increased following xylazine and detomidine administration.

Conclusions and Clinical Relevance—Xylazine and detomidine administration in horses deprived of feed and water causes transient increases in urine volume and loss of sodium and potassium. Increase in urine flow is directly related to dose and type of α2-adrenergic receptor agonist. Dehydration in horses may be exacerbated by concurrent administration of α2-adrenergic receptor agonists. (Am J Vet Res 2004;65:1342–1346)

Full access
in American Journal of Veterinary Research