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Summary

Effects of phenylbutazone (pbz) and furosemide (fur) on the respiratory tract of horses were evaluated, focusing on bronchial responsiveness. Four healthy Thoroughbreds were used and data were analyzed by use of a Latin square design. Histamine provocation tests (0.5, 1, 2, and 4 µg/kg/min, iv) were done: (1) without prior treatment with pbz or fur, (2) 30 minutes after administration of pbz (8 mg/kg, iv), (3) 1 hour after administration of fur (1 mg/kg, iv), and (4) after administration of pbz plus fur. Pulmonary function tests (dynamic compliance, resistance, respiratory frequency, and tidal volume) and heart rate were monitored throughout the experiments. Phenylbutazone did not influence basal pulmonary function test results, whereas fur caused a significant (P < 0.05) increase in dynamic compliance and decrease in resistance. Histamine infusion resulted in a dose-dependent decrease in dynamic compliance and a dose-dependent increase in resistance, respiratory frequency, and heart rate. Phenylbutazone administration significantly (P < 0.05) attenuated most of the changes induced by histamine, whereas fur had less protective action. Administration of pbz plus fur before administration of histamine was less effective than administration of pbz alone.

Free access
in American Journal of Veterinary Research

Summary

The effect of 3 plasma concentrations of alfentanil on the minimum alveolar concentration (mac) of halothane in horses was evaluated. Five healthy geldings were anesthetized on 3 occasions, using halothane in oxygen administered through a mask. After induction of anesthesia, horses were instrumented for measurement of blood pressure, airway pressure, and end-tidal halothane concentrations. Blood samples, for measurement of pH and blood gas tensions, were taken from the facial artery. Positive pressure ventilation was begun, maintaining PaCO 2, at 49.1 ± 3.3 mm of Hg and airway pressure at 20 ± 2 cm of H2O. The mac was determined in triplicate, using a supramaximal electrical stimulus of the oral mucous membranes. Alfentanil infusion was then begun, using a computer-driven infusion pump to achieve and maintain 1 of 3 plasma concentrations of alfentanil. Starting at 30 minutes after the beginning of the infusion, mac was redetermined in duplicate. Mean ± sd measured plasma alfentanil concentration during the infusions were 94.8 ± 29.0, 170.7 ± 29.2 and 390.9 ± 107.4 ng/ml. Significant changes in mac were not observed for any concentration of alfentanil. Blood pressure was increased by infusion of alfentanil and was dose-related, but heart rate did not change. Pharmacokinetic variables of alfentanil were determined after its infusion and were not significantly different among the 3 doses.

Free access
in American Journal of Veterinary Research

Summary

The diffusing capacity for carbon monoxide (dl co ) and the functional residual capacity (frc) of the lung were measured in 5 healthy Thoroughbreds before and after instillation of autologous blood into their lungs, in an attempt to develop a method to quantitate extravascular blood in the lungs of horses with exercise-induced pulmonary hemorrhage. Mean (± SD) baseline values of dl co and frc were 333.8 ± 61.9 ml/min/mm of Hg and 21.464 ± 4.156 L, respectively. Blood instillation resulted in decreases in dl co and frc. The paradoxic decrease in dl co (we were expecting to find an increase owing to blood in the airspaces, as has been reported in people) appears to be associated with the bronchoscopic procedure and with presence of blood in the airways. We concluded that rebreathing dl co measurements were not effective for detecting blood introduced bronchoscopically into the lungs of horses.

Free access
in American Journal of Veterinary Research

SUMMARY

Cardiovascular and respiratory functions were serially characterized in 7 healthy, spontaneously breathing, adult horses (from which food had been withheld) during 5 hours of constant 1.06% alveolar halothane (end-expired halothane concentration of 1.06%; equivalent to 1.2 times the minimal alveolar anesthetic concentration for horses). To enable comparison of temporal results in relation to 2 body postures, horses were studied in lateral recumbency (lr) and dorsal recumbency (dr) on separate occasions. Temporal changes in results of measures of circulation previously reported from this laboratory for horses in lr were confirmed (ie, a time-related increase in systemic arterial blood pressure, cardiac output, stroke volume, and pcv). During dr, systemic arterial blood pressure was initially significantly (P < 0.05) greater and pulmonary artery pressure less than results at comparable periods during lr. Differences ceased to exist with duration of anesthesia. Except for a greater heart rate at hour 5 of dr, no other significant differences in circulation were found between lr and dr. In general, except for PaO2 measures of ventilation did not change with time in either lr or dr. The PaO2 was significantly greater during lr, compared with dr, but the average did not change significantly with time in either body posture.

Free access
in American Journal of Veterinary Research

Abstract

Objectives

To evaluate the role of nitric oxide (NO), vasoactive intestinal peptide (VIP), and a transmitter acting through an apamin-sensitive mechanism in mediating inhibitory transmission in the equine jejunal circular muscle, and to determine the distribution of VIP-and NO-producing nerve fibers in the myenteric plexus and circular muscle.

Procedure

Circular muscle strips were suspended in tissue baths containing an oxygenated modified Krebs solution and attached to isometric force transducers. Responses to electrical field stimulation (EFS), tetrodotoxin, the NO antagonists l-N-nitro-arginine-methyl-ester (L-NAME) and N-nitro-l-arginine, apamin, VIP, authentic NO, and the NO donar sodium nitroprusside were tested. Immunostaining for VIP-like and NADPH diaphorase histochemical staining were performed on paraformaldehyde-fixed tissue.

Results

Subpopulations of myenteric neurons and nerve fibers in the circular muscle were positive for NADPH diaphorase and VIP-like staining. EFS caused a frequency-dependent inhibition of contractile activity. Tetrodotoxin prevented the EFS-induced inhibition of contractions. L-NAME (200 μM) and apamin 0.3 μM) significantly (P < 0.01) reduced EFS-stimulated inhibition of contractile activity at most frequencies tested. The effects of L-NAME and apamin were additive. In their combined presence, EFS induced excitation instead of inhibition (196.7% increase at 5 Hz, n = 28, P < 0.01). Inhibition of contractile activity by EFS was mimicked by sodium nitroprusside. Authentic NO (3-6 μM) abolished contractile activity. VIP induced a dose-dependent inhibition of contractile activity (89.1 ± 6.3% reduction at approximately 0.3 μM, n = 16). Antagonism of NO synthesis did not alter the response to VIP.

Conclusion

NO, VIP, and a substance acting through an apamin-sensitive mechanism appear to comediate inhibitory transmission in the equine jejunal circular muscle.

Clinical Relevance

These findings may suggest new therapeutic targets for motility disorders, such as agents that inhibit the synthesis or actions of NO. (Am J Vet Res 1996;57:1206-1213)

Free access
in American Journal of Veterinary Research

SUMMARY

The effects of different arterial carbon dioxide tensions (PaCO2 ) on cerebrospinal fluid pressure (csfp) and intraocular pressure (iop) were studied in 6 male halothane-anesthetized horses positioned in left lateral recumbency. Steady-state anesthetic conditions (1.06% end-tidal halothane concentration) commenced 60 minutes following anesthetic induction with only halothane in oxygen. During atracurium neuromuscular blockade, horses were ventilated, and respiratory rate and peak inspiratory airway pressure were maintained within narrow limits. The csfp and iop were measured at 3 different levels of PaCO2 (approx 40, 60, and 80 mm of Hg). The PaCO2 sequence in each horse was determined from a type of switchback design with the initial PaCO2 (period 1), established 30 minutes after the commencement of steady-state anesthesia, being repeated in the middle (period 3) and again at the end (period 5) of the experiment. Measurements taken from the middle 3 periods (2, 3, and 4) would form a Latin square design replicated twice. The interval between each period was approximately 45 minutes.

Data from periods 2, 3, and 4 indicated that csfp (P < 0.05) and mean systemic arterial pressure increased significantly (P < 0.05) with high PaCO2 . Mean central venous pressure, heart rate, and iop did not change significantly during these same conditions. Measurements taken during periods 1, 3, and 5 were compared to assess the time-related responses to anesthesia and showed a significant increase in csfp, a significant decrease in mean central venous pressure, and a small (but not statistically significant) increase in mean systemic arterial pressure.

Free access
in American Journal of Veterinary Research

SUMMARY

Microvascular permeability of the jejunum of clinically normal equids and microvascular permeability associated with 60 minutes of ischemia (25% baseline blood flow) and subsequent reperfusion were investigated. Eight adult horses were randomly allotted to 2 equal groups: normal and ischemic/reperfusion injury. Lymphatic flow rates, mesenteric blood flow, and lymph and plasma protein concentrations were determined at 15-minute intervals throughout the study. Microvascular permeability was determined by estimates of the osmotic reflection coefficient, which was determined when the ratio of lymphatic protein to plasma protein concentration reached a constant minimal value as lymph flow rate increased (filtration-independent lymph flow rate), which occurred at venous pressure of 30 mm of Hg. Full-thickness jejunal biopsy specimens were obtained at the beginning and end of each experiment, and were prepared for light microscopy to estimate tissue volume (edema) and for transmission electron microscopy to evaluate capillary endothelial cell morphology.

The osmotic reflection coefficient for normal equine jejunum was 0.19 ± 0.06, and increased significantly (P < 0.0001) to 0.48 ± 0.05 after the ische- mia/reperfusion period. Microscopic evaluation revealed a significant increase (P < 0.0001) in submucosal and serosal volume and capillary endothelial cell damage in horses that underwent ischemia/reperfusion injury. Results indicate that ischemia/re-perfusion of the equine jejunum caused a significant increase in microvascular permeability.

Free access
in American Journal of Veterinary Research

Summary

Cardiovascular and respiratory changes that accompany markedly long periods (12 hours) of halothane anesthesia were characterized. Eight spontaneously breathing horses were studied while they were positioned in left lateral recumbency and anesthetized only with halothane in oxygen maintained at a constant end-tidal concentration of 1.06% (equivalent to 1.2 times the minimal alveolar concentration for horses). Results of circulatory and respiratory measurements during the first 5 hours of constant conditions were similar to those previously reported from this laboratory (ie, a time-related significant increase in systemic arterial blood pressure, cardiac output, stroke volume, left ventricular work, pcv, plasma total solids concentration, and little change in respiratory system function). Beyond 5 hours of anesthesia, arterial blood pressure did not further increase, but remained above baseline. Cardiac output continued to increase, because heart rate significantly (P < 0.05) increased. Peak inspiratory gas flow increased significantly (P < 0.05) in later stages of anesthesia. There was a significant decrease in inspiratory time beginning at 4 hours. Although PaO2 and PaCO2 did not significantly change during the 12 hours of study, P v ̄ O 2 increased significantly (P < 0.05) and progressively with time, beginning 6 hours after the beginning of constant conditions. Metabolic acidosis increased with time (significantly [P < 0.05] starting at 9 hours), despite supplemental iv administered NaHCO3, Plasma concentrations of eicosanoids: 6-ketoprostaglandin F (pgf a stable metabolite of pgi 1), pgf , pge, and thromboxane (TxB2, a stable metabolite of TxA2) were measured in 5 of the 8 horses before and during anesthesia. Significant changes from preanesthetic values were not detected. Dynamic thoracic wall and lung compliances decreased with time.

Free access
in American Journal of Veterinary Research