Arrhythmias in dogs associated with epinephrine and thiamylal anesthesia.

Sinus tachycardia bigeminy, ventricular rhythm, ventricular tachycardia, and ventricular fibrillation are produced by relatively small intravenous doses of epinephrine in nonanesthetized dogs and in dogs anesthetized with thiamylal sodium. Origin of the abnormal beat in coupled bigeminal rhythms generated from the bundle o of His or above. Increases in arterial blood pressure may predispose to arrhythmia formation during thiamylal anesthesia.     

Treatment of sustained monomorphic narrow‐complex ventricular tachycardia in a 16‐year‐old Arab mare with a constant rate of infusion of lidocaine and oral propranolol

A 16‐year‐old Arab mare was referred for treatment of a tachydysrhythmia detected on electrocardiogram by the referring veterinarian. Monomorphic, narrow‐complex ventricular tachycardia (VT) was confirmed by electro‐cardiogram in a normal base‐apex configuration. Subsequent diagnostic testing, including echocardiography and measurement of plasma cardiac troponin concentration, did not reveal a definitive cause for the dysrhythmia. Conversion of VT to ventricular bigeminy occurred following treatment with magnesium sulfate and lidocaine hydrochloride, administered intravenously. Treatment with orally administered propranolol subsequently led to conversion to sinus rhythm with intermittent ventricular premature complexes (VPCs). Oral prednisolone treatment was also initiated to address the possibility that VT resulted from underlying myocarditis. The mare was discharged with oral propranolol, a tapering course of prednisolone, and exercise restriction. Follow‐up cardiac examination 6 weeks later revealed persistent, intermittent VPCs and reversion to VT occurred during exercise. Sinus rhythm with intermittent VPCs was re‐established following the administration of lidocaine. The mare was again discharged with oral propranolol therapy, which was subsequently replaced with oral sotalol. Sudden death occurred 4 weeks later but a post mortem examination was not performed.     

Evaluation of cardiac rhythm in dogs treated with levamisole hydrochloride using 24-hour ambulatory electrocardiography

The objective of this study was to evaluate the electrocardiographic alterations in the cardiac rhythm in dogs treated with levamisole hydrochloride over a period of 24 hours. Thirty-six mixed-breed dogs, both male and female, all clinically healthy, were used in the experiment. The dogs were divided into 6 groups with 6 dogs in each group, according to dosage and route of administration. The Holter test was initiated immediately after the treatment, and was maintained for 24 hours. In the group treated with 10 mg/kg by way of subcutaneous injection, one of them showed ventricular premature complexes, sometimes isolated and other times in pairs, and ventricular tachycardia, concentrated mainly in the first hour after administration of the drug. In the group of 6 animals treated subcutaneously with 25mg/kg, four showed isolated ventricular premature complexes, ventricular bigeminy and trigeminy, mainly during the first 2 hours after administration of the drug. All the animals in the other groups showed sinus arrhythmia followed by sinus arrest. The disturbances in the cardiac rhythm observed in clinically healthy animals treated with levamisole hydrochloride, indicate that it is preferable to avoid subcutaneous administration of levamisole hydrochloride and that the oral administration of the drug should be done with caution.     

Computer use and applications in veterinary medicine.

An 8-year-old neutered male cat with a history of intermittent collapse and dyspnea was evaluated. Hypertrophic cardiomyopathy was diagnosed on the basis of findings from physical examination, radiography, and echocardiography. Cardiac arrhythmias were not recorded during routine electrocardiography. Continuous ambulatory electrocardiography documented severe ventricular arrhythmias (ventricular premature complexes, ventricular bigeminy, and paroxysmal ventricular tachycardia). Continuous ambulatory electrocardiography can detect intermittent and potentially life-threatening cardiac arrhythmias.     

Analgesic and cardiopulmonary effects of premedication with tramadol in calves anesthetized with the infusion of guaifenesin and thiamylal

This study examined the analgesic and cardiopulmonary effects of intravenous (IV) tramadol during general intravenous anesthesia in calves. Calves were premedicated with diazepam (0.2 mg/kg, IV) with tramadol (2 mg/kg, IV) (group T) or saline (group S). Anesthesia was induced by thiamylal sodium (4 mg/kg, IV) and maintained with an infusion (2 ml/kg/hr) of 5% guaifenesin containing thiamylal sodium (2 mg/ml). Additional thiamylal sodium (1–2 mg/kg, IV) was administered when interference from the calves was observed during surgery. The total counts of additional thiamylal sodium administration, analgesia score using a visual analog scale, recovery time, and cardiopulmonary function in the different groups were assessed and compared. Group T showed significantly fewer counts of additional drug administration and a significantly higher analgesia score. Tramadol may provide adequate analgesia with minimal cardiopulmonary changes in calves during general anesthesia.     

Effects of xylazine and acetylpromazine upon induced ventricular fibrillation in dogs anesthetized with thiamylal and halothane.

Ventricular arryhythmias including ventricular fibrillation were produced with epinephrine in dogs induced to an anesthetic state with thiamylal and maintained with halothane. In dogs given (premedicated) xylazine 20 minutes prior to anesthesia, ventricular arrhythmias, including ventricular fibrillation, were induced with much smaller doses of epinephrine than in nonpremedicated dogs. Dogs premedicated with acetylpromazine 20 minutes prior to anesthesia with thiamylal and halothane displayed protection from epinephrine-induced arrhythmias. Caution is advised from using xylazine in the presence of halothane if epinephrine is to be administered.     

Effect of xylazine on the arrhythmogenic dose of epinephrine in thiamylal/halothane-anesthetized horses.

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 micrograms/kg/min) for a maximum of 10 minutes. The ADE was defined as the lowest epinephrine infusion rate to the nearest 0.25 micrograms/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 micrograms/kg/min; xylazine, 1.21 +/- 0.46 micrograms/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.     

Comparison of the cardiopulmonary effects of etomidate and thiamylal in dogs.

The cardiopulmonary effects of etomidate, a nonbarbiturate, short-acting, IV anesthetic, were compared and contrasted with those of thiamylal sodium in chronically instrumented conscious dogs. Etomidate, when administered IV at dosages of 1.5 and 3.0 mg/kg of body weight, produced anesthesia lasting from 8 +/- 5 and 21 +/- 9 minutes, respectively. Heart rate, aortic blood pressure, left ventricular peak pressure, left ventricular end diastolic pressure, left ventricular contractile force, and myocardial oxygen consumption were unchanged after administration of either dose of etomidate; however, the dosage of 1.5 mg/kg produced significant (P less than 0.05) increases in respiratory rate and decreases in tidal volume. The minute volume remained unchanged from base-line values. Significant (P less than 0.05) decreases in tidal volume, arterial pH, and partial pressure of oxygen were produced, and minute volume remained unchanged when 3.0 mg of etomidate/kg of body weight was administered. Thiamylal sodium (8.0 mg/kg of body weight; given IV) produced anesthesia lasting for 14 +/- 5 minutes. Significant increases (P less than 0.05) in heart rate, arterial blood pressure, left ventricular peak pressure, and myocardial oxygen consumption were observed after IV administration. Left ventricular contractility was significantly (P less than 0.05) decreased. Respiratory rate was not significantly (P less than 0.05) affected by thiamylal although tidal volume and minute volume were decreased. These respiratory alterations resulted in significant (P less than 0.05) increases in the arterial partial pressure of carbon dioxide and decreases in pH and the partial pressure of oxygen. On the basis of cardiopulmonary function, etomidate offered rapid, safe, short duration anesthesia superior to that of thiamylal sodium.     

Pharmacokinetics of pentobarbital and thiamylal as combined anesthetics in sheep

This study was performed to investigate the possible mechanisms underlying prolongation of anesthesia times in sheep caused by the sequential administration of thiamylal and pentobarbital. Sodium thiamylal was injected as an intravenous bolus dose (13.2 mg/kg) followed in 7 min by sodium pentobarbital (14.3 mg/kg) by the same route to seven sheep. Separate studies were conducted for each of the two drugs administered separately to the same animals at the same doses. Mean anesthesia times (to the return of the palpebral reflex) were 7.89 min (thiamylal), 5.39 min (pentobarbital) and 34.1 min (the sequential combination). The kinetic parameters Vd(area), Vd(ss), t 1/2 beta, and ClB for either drug were not affected by the other when given in combination. The t 1/2 alpha was shorter, and the Vc was smaller, for pentobarbital when administered with thiamylal, while there were no changes in thiamylal disposition for the combination regimen. Computer-generated curves, associated with the two-compartment open model showing the fraction of dose in each compartment as a function of time, illustrated that pentobarbital rapidly achieved higher concentrations in the peripheral compartment after prior thiamylal administration. Protein-binding studies showed that this could not be attributed to displacement of pentobarbital from plasma albumin by thiamylal. Calculation of total and free drug concentrations at the time of awakening showed that, when the drugs were combined, the concentration of each drug was less than half of that observed at awakening when they were studied separately. It can be concluded that the prolonged sleeping times associated with the sequential combination of the two agents were not due to an alteration in kinetic parameters of either drug caused by the other, but rather to an additive effect of the subanesthetic concentrations of the two drugs when combined. The fact that sleeping times were supra-additive is attributed to a shift of awakening time from the distribution (alpha) phase, when given independently, to the elimination (beta) phase when administered in combination.     

Effects of phenylbutazone on thiamylal disposition and anaesthesia in ponies

Phenylbutazone given during the perisurgical period has been reported to increase the intensity and duration of thiamylal anaesthesia in horses. A possible mechanism of competitive plasma protein binding has been suggested. The purpose of the present study was to experimentally reproduce the phenomenon of increased intensity and/or duration of thiamylal anaesthesia and to determine if there is competitive displacement of plasma protein bound thiamylal by phenylbutazone. Six ponies each received one of three treatments, 11 mg/kg intravenous (i.v.) thiamylal; 8.8 mg/kg i.v. phenylbutazone; and 11 mg/kg i.v. thiamylal with 8.8 mg/kg i.v. phenylbutazone given 9 min later. Thirteen blood samples were collected from 0 time through 600 min following drug administration and plasma drug concentrations quantified by high performance liquid chromatography. The pharmacokinetics of thiamylal and phenylbutazone were best described by three- and two-compartment models, respectively. There were no significant differences in pharmacokinetic parameters for thiamylal in the presence of phenylbutazone. However, there were differences in phenylbutazone pharmacokinetics when preceded by thiamylal administration. Unbound phenylbutazone concentrations were increased at 171, 231 and 351 min when given with thiamylal, accompanied by decreases in per cent bound phenylbutazone (P < 0.05). There were also significant (P < 0.05) changes in per cent plasma protein binding of thiamylal and phenylbutazone between 120 and 360 min, when in combination. No changes in intensity or duration of anaesthesia were observed.     

Effect of midazolam preanesthetic administration on thiamylal induction requirement in dogs.

The thiamylal sparing effect of midazolam was studied in 30 healthy Beagle and mixed-breed dogs. Using a replicated Latin square design, all dogs were given placebo (saline solution) and 0.025, 0.05, 0.1, and 0.2 mg of midazolam/kg of body weight prior to IV administration of thiamylal sodium. The 0.1 and 0.2 mg/kg dosages significantly decreased the amount of thiamylal required to obtund swallowing reflex and easily achieve endotracheal intubation. Midazolam at 0.1 and 0.2 mg/kg reduced thiamylal requirement by 16.4% and 18.9%, respectively, whereas the 0.05 mg/kg dosage decreased thiamylal requirement by only 6.8%. The 0.2 mg/kg dosage did not further decrease thiamylal requirement beyond that achieved with the 0.1 mg/kg dosage of midazolam. This study demonstrates that the preanesthetic IV administration of midazolam reduces the thiamylal dose necessary to accomplish intubation. The optimal preanesthetic dosage (lowest dosage with significant effect) was 0.1 mg/kg.     

Arrhythmias and transient changes in cardiac function after topical administration of one drop of phenylephrine 10% in an adult cat undergoing conjunctival graft

HistoryA 2-year-old, entire female, Somali cat weighing 3.8 kg was admitted for a conjunctival graft on the right eye, for treatment of an acute descemetocele. Medetomidine 4.2 μg kg^?1 and methadone 0.2 mg kg^?1 were administered by intramuscular injection as preanaesthetic medication. Anaesthesia was induced using diazepam 0.26 mg kg^?1 and propofol 4 mg kg^?1 administered by intravenous (IV) injection. Following endotracheal intubation, anaesthesia was maintained with isoflurane delivered in oxygen (1 L minute^?1) and nitrous oxide (2 L minute^?1) via a non-rebreathing system. Twenty minutes after induction of anaesthesia, one drop of a 10% phenylephrine hydrochloride solution was administered topically to the right eye.Physical examinationAfter phenylephrine administration, a decrease in heart rate (from 95 to 80 beats minute^?1) and an increase in arterial blood pressure occurred. The pulse then became difficult to palpate manually and multifocal ventricular premature contractions were observed on the electrocardiogram.ManagementNitrous oxide was discontinued and the isoflurane vaporizer setting was decreased from 1.5% to 0.5%. Lidocaine 1 mg kg^?1 IV was administered, this resulted in ventricular bigeminy. The quality of the femoral pulse improved and was regular in rhythm and character. Surgery was completed as fast as possible. The bigeminy progressively disappeared and before disconnecting the cat from the breathing system, there was a normal sinus rhythm with a heart rate of 85 beats minute^?1.Follow-upEchocardiography was performed during recovery and showed mitral and aortic valve insufficiency and dilation of the left ventricle, suggesting a reduction in systolic function. Echocardiography was repeated the following day and was normal.ConclusionsIn order to diminish the potential for cardiovascular sequelae associated with systemic absorption of ocular phenylephrine, less concentrated solutions, smaller drop size or different instillation techniques should be considered for topical use in small patients.     

Thiamylal-and halothane-sparing effect of diazepam in dogs

The thiamylal- and halothane-sparing effect of diazepam was studied in two experiments using 32 conditioned dogs. Twenty-four dogs received 0.05, 0.1 or 0.2 ml/kg diazepam or 0.9% saline (placebo) prior to the administration of thiamylal sodium i.v. Eight dogs received 0.1 or 0.2 mg/kg diazepam i.v. or placebo prior to or during halothane anesthesia. All three doses of diazepam significantly decreased the amount of thiamylal required to allow orotracheal intubation. The 0.2 mg/kg i.v. dose of diazepam produced the most significant effects. Premedication of dogs with diazepam did not reduce the concentration of halothane required to maintain anesthesia. The administration of 0.1 and 0.2 mg/kg diazepam i.v. during halothane anesthesia decreased the concentration of halothane required to maintain anesthesia. These studies demonstrate that diazepam reduces the amount of thiamylal required for orotracheal intubation, and when given intra-operatively reduces the concentration of halothane required to maintain anesthesia.     

Thiamylal-Sparing Effect of Midazolam for Canine Endotracheal Intubation A Clinical Study of 118 Dogs

One hundred eighteen dogs were studied at three veterinary teaching hospitals after the administration of midazolam (0.1 mg/kg, intravenously [IV]) or a placebo. Midazolam and placebo treatments were randomized and blinded to the investigators. The dose of thiamylal required for tracheal intubation 3 to 5 minutes after midazolam or placebo was calculated. The dose of thiamylal at the three hospitals was 10.6,9.8, and 10.1 mg/kg IV after midazolam, and 12.1,11.2, and 11.6 mg/kg IV after placebo. Pooled data from the three hospitals yielded a significant (p < .001) decrease in mean IV thiamylal dose after midazolam (10.2 mg/kg) compared with placebo (11.6 mg/kg). Overall, there was a 12% decrease in the dose of thiamylal required for tracheal intubation after midazolam compared to that after the placebo. The thiamylal dose was significantly (p < .001) decreased after midazolam compared with placebo for dogs weighing more than 15 kg but not for dogs weighing less than 15 kg.     

Evaluation of xylazine as a sedative and preanesthetic agent in horses.

Xylazine administered intramuscularly (IM) to horses at the dose level of 2 mg/kg was an effective sedative and preanesthetic for thiamylal sodium narcosis or thiamylal sodium and halothane anesthesia. Evaluation of response of cardiovascular, respiratory, and hepatic function did not indicate serious untoward effects, although cardiac and respiratory rate decreased, calculated vigor of left ventricular contraction decreased, calculated peripheral vascular resistance increased, and transient innocuous cardiac arrhythmias occurred. Effects of the anesthetics used on respiratory function (blood gases and pH), using xylazine as a preanesthetic, were comparable with those observed when promazine was used. The onset of action of xylazine given IM was at least as rapid as that occurring when promazine was given intravenously; e.g., 5 minutes for first observable effects, and 15 to 20 minutes for maximal effect. Recovery, times from anesthesia when using xylazine administered IM as a preanesthetic agent were comparable with those reported after promazine was given intravenously; moreover, horses given xylazine were more calm during recovery and seldom tried to stand before they were able.     

Fatal interaction between thiamylal sodium and a proprietary antidiarrheal preparation in the dog

A drug interaction involving thiamylal sodium and a proprietary antidiarrheal preparation caused fatal ventricular fibrillation in a dog. Trials involving 4 dogs were conducted to recreate the interaction, and 1 of the 4 dogs died after ventricular fibrillation developed. The nature of the interaction was hypothesized to be mediated through the anticholinergic drug diphemanil methylsulfate, producing an autonomic imbalance.     

Epinephrine-induced ventricular arrhythmias in dogs anesthetized with halothane: potentiation by thiamylal and thiopental

Epinephrine-induced ventricular arrhythmias were studied in 8 dogs anesthetized at weekly intervals with halothane (1.09% end-tidal concentration) preceded by thiamylal or thiopental (20 mg/kg of body weight). Lead II, bundle of His and high right atrial electrograms, and femoral artery and airway pressures were recorded. Epinephrine was infused in logarithmically spaced increasing rates (initial rate = 0.25 micrograms/kg/min) for a maximum of 2.5 minutes. The maximal (greater than or equal to 4 ventricular premature depolarizations within 15 s of each other) and minimal (all other ventricular or junctional rhythms) arrhythmogenic doses were calculated (infusion rate X time to arrhythmia). The mean (+/- SD) minimal arrhythmogenic dosages for the thiamylal-halothane, thiopental-halothane, and halothane-only groups were 1.84 +/- 0.66, 1.83 +/- 0.64, and 3.69 +/- 1.32 micrograms/kg, respectively; the mean (+/- SD) maximal arrhythmogenic dosages were 2.32 +/- 0.77, 3.37 +/- 1.30, and 8.86 +/- 4.40 micrograms/kg, respectively, with no change after 4 hours of anesthesia. During infusion of the maximal arrhythmogenic dosages, the mean infusion of the maximal arrhythmogenic dosages, the mean percentage increase in serum K+ for thiamylal-halothane, thiopental-halothane, and halothane-only groups was 33 +/- 14%, 31 +/- 13%, and 38 +/- 18%, respectively.     

Hemodynamic effects of high-frequency oscillatory ventilation in halothane-anesthetized dogs

Hemodynamic effects of spontaneous ventilation, intermittent positive-pressure ventilation (IPPV), and high-frequency oscillatory ventilation (HFOV) were compared in 6 dogs during halothane anesthesia. Anesthesia was induced with IV thiamylal Na and was maintained with halothane (end-tidal concentration, 1.09%). During placement of catheters, dogs breathed spontaneously through a conventional semiclosed anesthesia circuit. Data were collected, and dogs were mechanically ventilated, using IPPV or HFOV in random order. Ventilation was adjusted to maintain PaCO2 between 38 and 43 mm of Hg during IPPV and HFOV. Cardiac index, aortic blood pressure, and maximum rate of increase of left ventricular pressure were significantly (P less than 0.05) less during HFOV than during spontaneous ventilation, whereas right atrial and pulmonary artery pressure were significantly greater during HFOV than during spontaneous ventilation. During IPPV, only the maximum rate of increase of left ventricular pressure was significantly less than that during spontaneous ventilation.     

Cardiovascular and respiratory effects of three rapidly acting barbiturates in dogs

The cardiovascular and respiratory effects of 3 rapidly acting barbiturates, thiopental sodium, thiamylal sodium, and methohexital sodium, were studied in dogs from completion of injection until 12.5 minutes after injection. The doses administered were 19.4 mg of thiopental/kg of body weight, 18.4 mg of thiamylal/kg, and 9.7 mg of methohexital/kg, which were chosen as equipotent doses necessary to inhibit the laryngoscopic reflex in 50% of the population. To determine the cardiovascular and respiratory effects for each drug, the values at each measurement time following injection were compared with baseline values (T0). At the 15- and 30-second measurement times following thiopental administration, stroke volume (SV) decreased; heart rate (HR), left atrial pressure, and mean pulmonary arterial pressure increased; and cardiac index (CI), myocardial contractility, and systemic and pulmonary vascular resistances were not different from baseline values. Mean arterial pressure (MAP) was not different from the baseline value at 15 seconds, but was increased from 30 seconds to 2 minutes. All values except HR had returned to baseline values by 7.5 minutes. At all measurement times, arterial oxygen tension and arterial pH were decreased, and arterial carbon dioxide tension increased from baseline values. Although the cardiovascular and respiratory changes following administration of thiamylal and methohexital were similar to those described for thiopental, some differences were found. Following thiamylal administration, systemic vascular resistance increased at 1 minute, pulmonary vascular resistance increased at 1 and 2 minutes, and myocardial contractility increased at 1 and 2 minutes. Following methohexital administration, MAP decreased at 15 seconds, and SV decreased at all measurement times.(ABSTRACT TRUNCATED AT 250 WORDS)     

Resection of Subvalvular Aortic Stenosis Surgical and Perioperative Management in Seven Dogs

Open heart surgery was performed during cardiopulmonary bypass (CPB) to surgically correct subvalvular aortic stenosis in seven dogs. After initiation of total CPB, cardiac arrest was induced by antegrade and retrograde administration of blood cardioplegia. The subvalvular fibrous stenosis was resected through a transverse aortotomy. Intraoperatively and postop-eratively, dobutamine, nitroprusside, lidocaine, blood(-products), and crystalloid solutions were used to manage hypotension and optimize cardiac index.
Aortic cross-clamp time varied from 73 to 166 minutes, and duration of CPB varied from 130 to 210 minutes, latrogenic incision into the mitral valve in two dogs was the most significant introperative complication. Postoperative complications included: hypoproteinemia (n = 7), premature ventricular depolarization (n = 6), increased systemic vascular resistance index (n = 5), increased O₂ extraction (n = 3), pulmonary edema (n = 2), and decreased cardiac index (n = 1). All seven dogs were discharged alive and in stable condition. Six dogs are alive and in stable condition after a mean follow up of 15.8 months.
This is the first detailed report of CPB in a series of clinical veterinary patients. Using the techniques described in this paper, open heart surgery of considerable duration can be performed successfully in dogs with significant myocardial hypertrophy and endomyocardial fibrosis secondary to subvalvular aortic stenosis.