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Summary

The effect of xylazine on the arrhythmogenic dose of epinephrine (ade) was studied in 9 horses. Anesthesia was induced by administration of guaifenesin (50 mg/kg of body weight, iv) followed by thiamylal (4 to 6 mg/kg, iv) and was maintained at 1 minimal alveolar concentration (mac) of halothane (0.89%). Base apex ecg and facial artery pressure were recorded. Epinephrine was infused in a sequence of arithmetically spaced increasing rates (initial rate 0.25 µg/kg/min) for a maximum of 10 minutes. The ade was defined as the lowest epinephrine infusion rate to the nearest 0.25 µg/kg/min at which at least 4 premature ventricular depolarizations occurred in a 15- second period. Xylazine (1.1 mg/kg, iv) was administered after the control ade was determined. Xylazine did not significantly alter the ade (control, 1.12 ± 0.38 µg/kg/ min; xylazine, 1.21 ± 0.46 µg/kg/min). Blood pressure increased transiently for 8 minutes after xylazine administration. Baseline systolic and diastolic arterial pressures and heart rate were not significantly different from control baseline pressures and heart rate 15 minutes after xylazine administration. Blood pressure and heart rate increased significantly during control and xylazine ade determinations. Significant differences in pH, Pao2 , Paco2 , or base excess were not observed between baseline and ade in the control or xylazine groups. One horse developed atrial fibrillation, and 2 horses developed ventricular fibrillation during ade determinations.

Free access
in American Journal of Veterinary Research

Summary:

An in-circuit vaporizer for delivery of isoflurane was evaluated. The isoflurane concentration within an isolated circle breathing circuit was determined for 1 hour in 6 in-the-circuit vaporizers with the wicks removed. A mechanical ventilator and artificial lung were connected to the circuit. Isoflurane concentration increased as vaporizer setting increased, and delivered concentration (%) at 60 minutes ( X ¯ ± sem) ranged from 0.46 ± 0.10 at tap setting 1 to 3.67 ± 0.30 at setting 5. Temperature of the isoflurane did not change.

Cardiovascular and respiratory function were maintained within a clinically acceptable range in 6 dogs anesthetized with thiamylal and maintained with 1.87% end-tidal isoflurane delivered from the in-circuit vaporizer during spontaneous ventilation, controlled ventilation, and closed-circuit anesthesia. The range of vaporizer tap settings ( X ¯ ± sem) was lower during closed-system anesthesia (2.5 ± 0.1 to 3.5 ± 0.6) and during controlled ventilation (2.6 ± 0.2 to 3.3 ± 0.2) than during semi-closed system anesthesia (5.4 ± 0.3 to 6.8 ± 0.4).

The in-circuit vaporizer was used to deliver isoflurane to 36 dogs anesthetized for a variety of surgical and medical procedures. Ventilation was spontaneous, assisted, and in 1 instance, controlled. Cardiovascular function, respiratory function, and recovery times were within clinically acceptable ranges. The initial vaporizer tap setting ( X ¯ ± sem) was 8.2 ± 0.4, and this corresponded to an end-tidal isoflurane concentration of 3.5 ± 0.6. The range of vaporizer settings during the maintenance phase ( X ¯ ± sem) was 2.8 ± 0.5 to 4.6 ± 1.9. This corresponded to an end-tidal isoflurane concentration of 1.2 ± 0.1 to 1.8 ± 0.1%.

This study documents that when appropriate guidelines are followed and limitations understood, the in-circle vaporizer is suitable for delivery of isoflurane to dogs undergoing a variety of surgical and medical procedures. Guidelines include removal of the wick, attention to the relatively rapid increase of anesthetic depth during the first 5 minutes of anesthesia, and the need to decrease the setting of the vaporizer control lever if assisted or controlled ventilation is used, or if closed system flow rates are used. Limitations include unpredictability of output with changing ambient temperature and difficulty adapting its use with semi-open breathing systems such as the t-piece or Bain coaxial circuit.

Free access
in Journal of the American Veterinary Medical Association

SUMMARY

The effect of hypercapnia on the arrhythmo-genic dose of epinephrine (ADE) was investigated in 14 horses. Anesthesia was induced with guaifenesin and thiamylal sodium and was maintained at an end-tidal halothane concentration between 0.86 and 0.92%. Base-apex ECG, cardiac output, and facial artery blood pressure were measured and recorded. The ADE was determined at normocapnia (arterial partial pressure of carbon dioxide [Paco2 = 35 to 45 mm of Hg), at hypercapnia (Paco2 = 70 to 80 mm of Hg), and after return to normocapnia. Epinephrine was infused at arithmetically spaced increasing rates (initial rate = 0.25 μg/kg of body weight/min) for a maximum of 10 minutes. The ADE was defined as the lowest epinephrine infusion rate, to the nearest 0.25 μg/kg/min, at which 4 premature ventricular complexes occurred in a 15-second period. The ADE (mean ± SD) during hypercapnia (1.04 ± 0.23 μg/kg/min) was significantly (P < 0.05) less than the ADE at normocapnia (1.35 ± 0.38 μg/kg/min), whereas the ADE after return to normocapnia (1.17 ± 0.22 μg/kg/min) was not significantly different from those during normocapnia or hypercapnia. Baseline systolic and diastolic arterial pressures and cardiac output decreased after return to normocapnia. Significant differences were not found in arterial partial pressure of O2 (PaO2) or in base excess during the experiment. Two horses developed ventricular fibrillation and died during normocapnic determinations of ADE. Hypercapnia was associated with an increased risk of developing ventricular arrhythmias in horses anesthetized with guaifenesin, thiamylal sodium, and halothane.

Free access
in American Journal of Veterinary Research

SUMMARY

Programmed electrical stimulation techniques were used to evaluate the effects of halothane and isoflurane on induction of atrial fibrillation in anesthetized dogs. Experiments were performed in 16 dogs anesthetized with α-chloralose. Critically timed premature stimuli were applied to the right atrial appendage and Bachmann bundle to determine the atrial fibrillation threshold, defined as the minimal current required to induce rapid, irregular atrial electrical activity of at least 8 seconds' duration, Atrial fibrillation thresholds were determined at baseline (0.0% inhalational anesthetic), 0.5 minimal alveolar concentration (mac), and 1.0 mac of halothane (n = 8) and isoflurane (n = 8).

In the absence of inhalation anesthetic, it was significantly (P < 0.01) easier to induce atrial fibrillation at the Bachmann bundle vs the right atrial appendage. Atrial fibrillation threshold at the Bachmann bundle was not affected by increasing concentrations of halothane, but was increased by 1.0 mac of isoflurane (P < 0.05). It was concluded that at 1.0 mac isoflurane, but not halothane, has antifibrillatory effects in atrial tissue.

Free access
in American Journal of Veterinary Research

SUMMARY

Eight adult female cattle (6 Holstein, 1 Jersey, 1 Brown Swiss) were used to determine the antagonistic effects of tolazoline, an α2-adrenoceptor antagonist, on xylazine-induced (via caudal epidural administration) depression of cns, respiratory, and cardiovascular activity and rumen motility. A 2% solution of xylazine HCl was injected into the epidural space at the first coccygeal interspace, using a dosage of 0.05 mg/kg of body weight, diluted to a 5-ml volume with sterile water, and administered at a rate of approximately 1 ml/30 s. Eight minutes after xylazine injection, either tolazoline (0.3 mg/kg) or saline solution (4 ml) was administered iv. All 8 cattle were treated, using both regimens in a random sequence; at least 1 week elapsed between treatments. Epidurally administered xylazine induced caudal analgesia (S3 to coccyx), as evaluated by no response to superficial and deep muscular pinprick, and induced sedation, cardiopulmonary depression, and inhibition of rumen motility, but all cattle remained standing. Tolazoline effectively reversed xylazine-induced rumen hypomotility, and partially antagonized xylazine-induced cardiopulmonary depression without affecting sedation and desirable local (S3 to coccyx) analgesic effects.

Free access
in American Journal of Veterinary Research

SUMMARY

The cardiopulmonary effects of 4 positions (standing, right lateral, left lateral, and dorsal recumbency) were evaluated in conscious cattle in which no sedatives or anesthetic drugs were given. Each position was maintained for 30 minutes, during which time there were no significant changes in heart rate, respiratory rate, mean arterial blood pressure, arterial pH, PaCO2 , arterial base excess, or venous blood gas values. Significant decreases in PaO2 developed when cattle were in lateral positions and dorsal recumbency. Cardiac index was unchanged in all positions, except in dorsal recumbency at 30 minutes, when it was significantly decreased.

Free access
in American Journal of Veterinary Research

Abstract

Objective—To determine the effects of perzinfotel, butorphanol, and their combination on the minimal alveolar concentration (MAC) of isoflurane in cats.

Animals—7 healthy sexually intact cats (4 males and 3 females), aged 12 to 17 months and weighing 2.8 to 4.6 kg.

Procedures—In a crossover design, saline (0.9% NaCl) solution, perzinfotel (2.5 to 15 mg/kg; IV, IM, and SC), butorphanol tartrate (0.2 mg/kg, IM), or a combination of 5 mg of perzinfotel/kg and 2 mg of butorphanol tartrate/kg (both IM) was administered to 6 cats before 7 separate episodes of anesthesia with isoflurane in oxygen. Heart rate, arterial blood pressure, bispectral index (BIS), and inspiration and expiration concentrations of isoflurane were continuously monitored. The isoflurane MAC was determined twice during anesthesia.

Results—IV, IM, and SC administration of perzinfotel at 2.5 to 15 mg/kg resulted in a significant decrease in mean isoflurane MAC by 43.3% to 68.0%. The BIS significantly increased after perzinfotel administration via the same routes at 2.5 to 15 mg/kg and after perzinfotelbutorphanol administration IM. Blood pressure was significantly higher after perzinfotel was administered at 5 mg/kg, IM; 10 mg/kg, IV; and 10 mg/kg, SC than after saline solution administration.

Conclusions and Clinical Relevance—Perzinfotel administration decreased the isoflurane MAC and increased several BIS and blood pressure values in anesthetized cats. Administration of perzinfotel prior to isoflurane anesthesia may improve anesthetic safety by reducing inhalant anesthetic requirements and improving cardiovascular function during anesthesia. (Am J Vet Res 2010;71:1270–1276)

Full access
in American Journal of Veterinary Research

Objective

To test the hypothesis that small volumes of hypertonic saline-dextran (HSD) solution can be used to effectively resuscitate dogs in shock induced by gastric dilatation-volvulus (GDV). and, compared with administration of large volumes of lactated Ringer's solution (LRS), can be used to limit the overall volume of fluid needed for resuscitation.

Design

Prospective, clinical study.

Animals

15 dogs with GDV-induced shock.

Procedure

Initially, HSD solution (5 ml/kg of body weight) or LRS (60 to 90 ml/kg) was administered. All dogs then received a maintenance administration (20 ml/kg/h) of LRS. Cardiorespiratory, blood gas, and serum biochemical analyses were performed over a 4-hour period after initiation of treatment.

Results

Systolic arterial and central venous pressures and plasma volume increased more rapidly in dogs in the HSD + LRS group. The cumulative dose of fluids administered to dogs in the HSD + LRS group was significantly less than that administered to dogs in the LRS group. Serum sodium and chloride concentrations and osmolality increased significantly in dogs in the HSD + LRS group, but not in dogs in the LRS group. Ventricular arrhythmias were detected in both groups of dogs, but did not appear to be induced by either form of fluid therapy.

Clinical Implications

Administration of HSD rapidly restored cardiorespiratory function and induced resuscitation equivalent to administration of large volumes of LRS. Use of HSD solutions to treat dogs in GDV-induced shock may be more efficient than use of isotonic fluids. Administration of HSD solution was not associated with noticeable complications.

Free access
in Journal of the American Veterinary Medical Association

Abstract

Objective—To determine the effects of IV administration of enalaprilat on cardiorespiratory and hematologic variables as well as inhibition of angiotensin converting enzyme (ACE) activity in exercising horses.

Animals—6 adult horses.

Procedure—Horses were trained by running on a treadmill for 5 weeks. Training was continued throughout the study period, and each horse also ran 2 simulated races at 120% of maximum oxygen consumption. Three horses were randomly selected to receive treatment 1 (saline [0.9% NaCl] solution), and the remaining 3 horses received treatment 2 (enalaprilat; 0.5 mg/kg of body weight, IV) before each simulated race. Treatment groups were reversed for the second simulated race. Cardiorespiratory and hematologic data were obtained before, during, and throughout the 1-hour period after each simulated race. Inhibition of ACE activity was determined during and after each race in each horse.

Results—Exercise resulted in significant increases in all hemodynamic variables and respiratory rate. The pH and PO2 of arterial blood decreased during simulated races, whereas PCO2 remained unchanged. Systemic and pulmonary blood pressure measurements and arterial pH, PO2, and PCO2 returned to baseline values by 60 minutes after simulated races. Enalaprilat inhibited ACE activity to < 25% of baseline activity without changing cardiorespiratory or blood gas values, compared with horses administered saline solution.

Conclusions and Clinical Relevance—Enalaprilat administration almost completely inhibited ACE activity in horses without changing the hemodynamic responses to intense exercise and is unlikely to be of value in preventing exercise-induced pulmonary hemorrhage. (Am J Vet Res 2001;62:1008–1013)

Full access
in American Journal of Veterinary Research

Abstract

Objective—To measure QT interval duration and QT dispersion in Boxers and to determine whether QT variables correlate with indices of disease severity in Boxers with familial ventricular arrhythmias, including the number of ventricular premature complexes per day, arrhythmia grade, and fractional shortening.

Animals—25 Boxers were evaluated by ECG and echocardiography.

Procedure—The QT interval duration was measured from 12-lead ECG and corrected for heart rate (QTc), using Fridericia's formula. The QT and QTc were calculated for each lead, from which QT and QTc dispersion were determined. Echocardiography and 24-hour ambulatory ECG were performed to evaluate for familial ventricular arrhythmias. Total number of ventricular premature complexes, arrhythmia grade, and fractional shortening were determined and used as indices of disease severity.

Results—There was no correlation between any QT variable and total number of ventricular premature complexes, arrhythmia grade, or fractional shortening. No difference between QT dispersion and QTc dispersion was identified, and correction for heart rate did not affect the results.

Conclusions and Clinical Relevance—QT interval duration and dispersion did not correlate with indices of disease severity for familial ventricular arrhythmias. Heart rate correction of the QT interval did not appear to be necessary for QT dispersion calculation in this group of dogs. QT dispersion does not appear to be a useful noninvasive diagnostic tool in the evaluation of familial ventricular arrhythmias of Boxers. Identification of affected individuals at risk for sudden death remains a challenge in the management of this disease. (Am J Vet Res 2001;62:1481–1485)

Full access
in American Journal of Veterinary Research