Quinidine is a medication used in equine medicine primarily for the treatment of atrial fibrillation in horses. It functions as a class Ia antiarrhythmic agent, working by altering the conduction of electrical impulses within the heart to restore normal rhythm. Quinidine is typically administered orally and is known for its ability to convert atrial fibrillation to normal sinus rhythm. The pharmacokinetics, dosage, and potential side effects of quinidine are important considerations for its use in veterinary practice. This page compiles peer-reviewed research studies and scholarly articles that explore the pharmacological properties, therapeutic applications, and safety profile of quinidine in equine cardiac care.
Muir WW, Reed SM, McGuirk SM.Intravenous administration of quinidine gluconate converted atrial fibrillation (AF) to sinus rhythm in 9 of 12 horses. Twelve horses that were diagnosed by ECG to have AF were administered up to 11 mg of quinidine gluconate/kg of body weight in 1.0- to 1.5-mg/kg bolus injections every 10 to 15 minutes. The total dose of quinidine administered IV ranged from 1.8 to 5.8 g. Increased ventricular rate, apprehension, and mild depression were observed during treatment. Other signs of toxicosis were not observed. One horse was successfully treated with IV administered quinidine gluconate on 3 occasi...
Collatos C, Clark ES, Reef VB, Morris DD.A foal with vegetative bacterial endocarditis affecting the wall of the left atrium was treated successfully with cefotaxime, erythromycin, and rifampin. Bacterial isolates included Escherichia coli from blood and Rhodococcus equi from a P-type osteomyelitic lesion of the left third metatarsal bone and from synovial fluid from the left metatarsophalangeal joint. Cardiac complications included cardiomegaly and atrial fibrillation, which responded to treatment with digoxin and quinidine sulfate. Cardiac function was considered normal 18 months after treatment. Bacteriologic cure of osteoarthriti...
Stewart GA, Fulton LJ, McKellar CD.Atrial fibrillation is described in a champion pacer which earlier had been named Australian Harness Horse of the Year as a 3-year-old in 1986-87. Prior to conversion atrial fibrillation had been present for at least 6 weeks, during which the horse had not raced. Successful treatment was achieved with two 10g doses of quinidine sulphate per oesophageal tube, after slow digitalisation with intravenous digoxin over 4d. Four hours after commencement of quinidine therapy the arrhythmia had regressed to atrial flutter and converted to sinus rhythm 10 min later. Considering his age, standard of raci...
Guthrie AJ, Nichas E, Viljoen FV, Hartmann AM, Killeen VM.A case of sustained supraventricular tachycardia of unknown aetiology in a two-year-old Thoroughbred filly is reported. The cardiac dysrhythm was successfully treated by the oral administration of quinidine sulphate. Conversion of the dysrhythm to sinus rhythm occurred approximately 80 min after the initial dose of 5 g of quinidine sulphate. The horse returned to training approximately 2 months after treatment and has since successfully returned to racing.
Reef VB, Levitan CW, Spencer PA.Sixty-seven horses presented with atrial fibrillation (AF) from January 1, 1980 to August 1, 1986. All horses were evaluated for the type and severity of the underlying cardiac disease and the probable duration of the arrhythmia. Fifty-two (78%) of the horses were treated with quinidine sulfate and/or digoxin. The response to treatment was assessed in each horse. Horses were followed for periods extending from 8 months to 7 years. Standardbreds, young horses, and males predominated in the study group. There were more male horses (stallions and geldings) than mares. Most horses with AF had no e...
Bertone JJ, Traub-Dargatz JL, Wingfield WE.Atrial fibrillation in a pregnant, lactating, 15-year-old mare nursing a 70-day-old foal was converted to normal sinus rhythm, using quinidine sulfate. The maximum concentration of quinidine was 4.3 mg/L in the mare's milk and was 2.6 mg/L in the mare's serum. Treatment with quinidine did not interrupt the pregnancy. Six months after treatment, the mare developed acute volvulus of the large colon and died. At necropsy, the mare did not have macroscopic or microscopic cardiac lesions. The fetus was macroscopically and histologically normal.
Muir WW, McGuirk S.Knowledge of the dosage, rate and route of administration, and potential side effects of drugs used to treat cardiac disease in horses has been refined. The judicious use of these drugs can increase exercise capacity, improve health, and potentially prolong life. Currently, antiarrhythmics (quinidine, lidocaine), positive inotropies (digoxin), and diuretics (furosemide) are the primary agents used to treat cardiovascular disease in horses. The development of newer drugs (verapamil, milrinone, bumetanide) and their usefulness in therapy for horses with cardiovascular disease require further inv...
Holmes JR, Henigan M, Williams RB, Witherington DH.The paper describes five cases of atrial fibrillation detected after racing. In four of them, the arrhythmia disappeared spontaneously within 24 h and they were regarded as paroxysmal in type. In the fifth case, which won its race, the arrhythmia persisted for at least 45 h after racing. It was therefore regarded as an example of persistent atrial fibrillation. It was then treated with quinidine sulphate which restored sinus rhythm. It would seem that paroxysmal atrial fibrillation may be a cause of sudden decrease in racing performance.
McGuirk SM, Muir WW.Cardiac arrhythmias are probably more common in horses than in any other domestic animal species. The most frequent clinical complaint associated with cardiac arrhythmias is exercise intolerance. Physical examination is characterized by auscultation abnormalities such as fast or slow heart rate, irregular rhythm, extra sounds, long pauses, or abnormal heart sounds. The electrocardiogram is used to make a definitive diagnosis of the dysrhythmia. Other laboratory and cardiac function tests are employed to determine the etiology and to assess the significance of the arrhythmia. Antiarrhythmic the...
Muir WW, McGuirk SM.The rational therapy of cardiovascular disease in horses requires a thorough knowledge of the pharmacology and pharmacokinetics of several specific drugs (digitalis, digoxin). Calcium solutions, dopamine, and dobutamine are frequently used to treat congestive heart failure in horses. Quinidine, procainamide, lidocaine, and propranolol are used to treat a variety of supraventricular and ventricular arrhythmias. Furosemide, a highly potent loop diuretic, is used to eliminate edema and promote diuresis. A thorough understanding of the applied pharmacology, dosage recommendations, toxicity, and pr...
Meijler FL, Kroneman J, van der Tweel I, Herbschleb JN, Heethaar RM, Borst C.RR interval sequences during spontaneous atrial fibrillation in eight horses were analyzed as in previous studies in patients and dogs using histograms and serial auto-correlograms. In patients and dogs with spontaneous atrial fibrillation, ventricular rhythms were always random. In the horses, the histograms were skewed with median RR intervals of approximately 1,000 ms. A striking finding in these animals was the presence of long RR intervals up to 5,000 ms in duration. The shortest RR intervals lasted 400 to 600 ms. In contrast to findings in dogs and patients, the serial autocorrelograms s...
Muir WW, McGuirk SM.Hemodynamic measurements were obtained from 17 horses with atrial fibrillation or flutter. Eight horses had atrial fibrillation of unknown duration. In 7 horses, atrial fibrillation developed during anesthesia. Atrial flutter developed during recovery from anesthesia in 2 horses. Hemodynamic measurements were considered normal in 7 of 8 conscious horses with atrial fibrillation. Cardiac output was decreased and pulmonary arterial blood pressure and right atrial pressure were increased in 1 conscious horse which had signs of congestive heart failure. Arterial blood pressure decreased in 5 of 7 ...
Morris DD, Fregin GF.Seventy-seven horses with atrial fibrillation (AF) were treated orally with quinidine sulfate (QS) at the University of Pennsylvania, School of Veterinary Medicine, Thirty-seven horses (48%) had adverse reactions to QS, the most common of which were nasal mucosal edema, anorexia, colic, and diarrhea. The mean highest daily QS dose was slightly greater in horses showing toxicity (P less than 0.1) and their conversion rate was less (P less than 0.05). A schedule of gradually increasing QS dose was time consuming, costly and not associated with fewer adverse reactions. Horses were compared by the...
Beech J.An 18-month-old male Quarter Horse was referred for evaluation of a tic that had started after injury to the right forelimb 4 weeks earlier. The right forelimb appeared paretic and had constant regular twitches of variable intensity that were usually sufficiently forceful to move the trunk, neck, and head. The horse frequently threw the limb forward. The twitch persisted during sleep but disappeared during general anesthesia and following sedation with xylazine. It was unaffected by acetylpromazine, diphenylhydantoin, diazepam, carbamazepine, trimethadione, procainamide, quinidine, propranolol...
McGuirk SM, Muir WW, Sams RA.A pharmacokinetic study was made, using 7 healthy adult horses (weighing between 400 and 560 kg) given quinidine gluconate IV and quinidine sulfate orally. The apparent volume of distribution of quinidine base was 3.10 +/- 0.79 L/kg, total body clearance was 5.49 +/- 2.40 ml/minute/kg, and plasma half-life was 6.65 +/- 3.00 hours. The systemic availability of quinidine sulfate after oral administration of a 10 mg/kg dose was 48.5 +/- 20.4%. Oral administrations of quinidine sulfate in doses of 10 mg/kg and 10 g produced peak plasma concentrations of 0.79 microgram/ml at 146 minutes and 1.47 mi...
Button C, Scrutchfield WL, Clark RG, Knauer KW, Schmitz DG.A variety of atrial dysrhythmias including paroxysmal atrial tachycardia, atrial tachycardia with 2nd-grade atrioventricular block, atrial fibrillation, and atrial flutter developed in a 5-year-old Quarter Horse gelding. Quinidine and propranolol were not successful in restoring normal sinus rhythm. Sinus rhythm was re-established during digoxin therapy, but later reverted to atrial dysrhythmia. At necropsy, multiple, discrete pale areas were found on both atria and the interatrial myocardium. Histologic examination of these lesions demonstrated myocytolysis and replacement by fibrous connecti...
Wingfield WE, Miller CW, Voss JL, Bennett DG, Breukels J.Echocardiography is a non-invasive, safe means of assessing intracardiac structures and their motion. In the normal motion of the mitral valve there is anterior motion noted during atrial systole. In 3 horses with atrial fibrillation the mitral valve motion had no anterior motion to correspond with atrial contractions. Quinidine returned the electrocardiogram of one horse to a sinus rhythm after which echocardiography demonstrated the reappearance of the anterior motion of the mitral valve.
Rose RJ, Davis PE.Three young Standardbred pacers with atrial fibrillation were treated with quinidine sulphate (QS) by stomach tube. They were given 10g QS every 2 hours until cardioversion was achieved. Total doses varied between 20 and 30 g. No premedication was given nor any follow up treatment after return to sinus rhythm. All horses were given 3 months rest after the treatment, and when electrocardiographed then, and at 6 months, showed normal sinus rhythm. As all 3 horses won races after cardioversion and showed subsequent normal electrocardiograms it seems likely that atrial fibrillation can occur witho...
Tobin T, Blake JW, Valentine R.The plasma half-life of phenylbutazone in horses was not increased after pretreatment with chloramphenicol or quinidine, but was increased after oxyphenbutazone. This increased plasma half-life after oxyphenbutazone is consistent with observations in other species and suggests that oxyphenbutazone inhibits the metabolism of phenylbutazone in horses. Lack of inhibition of phenylbutazone metabolism in the horse by chloramphenicol and quinidine is inconsistent with results obtained in other species.
Buntenkötter S, Deegen E.Cardioversion through varying quinidine sulphate treatments was achieved in 3 riding horses with auricular fibrillation. The horses were subjected to continual telemetric ECG registration before and after reversion in a uniform exercise-tolerance-test. A comparison of the heart rate curves showed a strong decline of the heart beat frequency during the exercise phase after synchronisation. The decrease in heart rate during exercise was greater than 30% in all 3 patients. Anti-arrhythmic treatment is, therefore, recommended for horses with auricular fibrillation and without concurrent heart dise...
Muir WW, McGuirk S.Knowledge of the dosage, rate and route of administration, and potential side effects of drugs used to treat cardiac disease in horses has been refined. The judicious use of these drugs can increase exercise capacity, improve health, and potentially prolong life. Currently, antiarrhythmics (quinidine, lidocaine), positive inotropies (digoxin), and diuretics (furosemide) are the primary agents used to treat cardiovascular disease in horses. The development of newer drugs (verapamil, milrinone, bumetanide) and their usefulness in therapy for horses with cardiovascular disease require further inv...
Van Loon G, Jordaens L, Muylle E, Nollet H, Sustronck B.A five-year-old warmblood mare with atrial fibrillation was treated with quinidine sulphate. The atrial rhythm changed to atrial flutter and, because there were toxic effects, the treatment was discontinued. Seven months after the occurrence of the atrial flutter, treatment with a rapid atrial pacing technique restored a normal sinus rhythm. One year after the pacing therapy the horse was still in sinus rhythm and had been brought back into training.
Stadler P, Deegen E, Kroker K.Echocardiographic evaluation of 45 horses (43 warm-blooded horses, one Thoroughbred and one Standardbred) with atrial fibrillation was performed, using M-mode, B-mode and pulsed-wave Doppler echocardiography. Seventeen (38%) of these horses had a history of poor performance. An increase of left ventricular shortening fraction was found in 25 horses (56%). Atrial dilatation was seen in 37 horses (82%). Eight horses (18%) had normal atrial dimensions. In nine horses only left atrium and in six horses only right atrium was enlarged. In the other 22 horses both atria were dilated. Correlation was ...
Muir WW, McGuirk SM.The rational therapy of cardiovascular disease in horses requires a thorough knowledge of the pharmacology and pharmacokinetics of several specific drugs (digitalis, digoxin). Calcium solutions, dopamine, and dobutamine are frequently used to treat congestive heart failure in horses. Quinidine, procainamide, lidocaine, and propranolol are used to treat a variety of supraventricular and ventricular arrhythmias. Furosemide, a highly potent loop diuretic, is used to eliminate edema and promote diuresis. A thorough understanding of the applied pharmacology, dosage recommendations, toxicity, and pr...
Tobin T, Blake JW, Valentine R.The plasma half-life of phenylbutazone in horses was not increased after pretreatment with chloramphenicol or quinidine, but was increased after oxyphenbutazone. This increased plasma half-life after oxyphenbutazone is consistent with observations in other species and suggests that oxyphenbutazone inhibits the metabolism of phenylbutazone in horses. Lack of inhibition of phenylbutazone metabolism in the horse by chloramphenicol and quinidine is inconsistent with results obtained in other species.
McGuirk SM, Muir WW, Sams RA.A pharmacokinetic study was made, using 7 healthy adult horses (weighing between 400 and 560 kg) given quinidine gluconate IV and quinidine sulfate orally. The apparent volume of distribution of quinidine base was 3.10 +/- 0.79 L/kg, total body clearance was 5.49 +/- 2.40 ml/minute/kg, and plasma half-life was 6.65 +/- 3.00 hours. The systemic availability of quinidine sulfate after oral administration of a 10 mg/kg dose was 48.5 +/- 20.4%. Oral administrations of quinidine sulfate in doses of 10 mg/kg and 10 g produced peak plasma concentrations of 0.79 microgram/ml at 146 minutes and 1.47 mi...
Collatos C, Clark ES, Reef VB, Morris DD.A foal with vegetative bacterial endocarditis affecting the wall of the left atrium was treated successfully with cefotaxime, erythromycin, and rifampin. Bacterial isolates included Escherichia coli from blood and Rhodococcus equi from a P-type osteomyelitic lesion of the left third metatarsal bone and from synovial fluid from the left metatarsophalangeal joint. Cardiac complications included cardiomegaly and atrial fibrillation, which responded to treatment with digoxin and quinidine sulfate. Cardiac function was considered normal 18 months after treatment. Bacteriologic cure of osteoarthriti...
Garber JL, Reef VB, Reimer JM, Evans LH.A 2-year-old Thoroughbred racehorse developed ventricular tachycardia after elective laryngoplasty and ventriculectomy were performed while anesthesia was maintained with halothane. During surgery, the horse became febrile and developed transient mild hypercarbia. The horse was treated with an IV infusion of quinidine gluconate. Continuous electrocardiographic monitoring was used to evaluate cardiac rhythm during treatment, and conversion was achieved after 12 hours of IV infusion. The inciting cause for the arrhythmia was not determined.
Stewart GA, Fulton LJ, McKellar CD.Atrial fibrillation is described in a champion pacer which earlier had been named Australian Harness Horse of the Year as a 3-year-old in 1986-87. Prior to conversion atrial fibrillation had been present for at least 6 weeks, during which the horse had not raced. Successful treatment was achieved with two 10g doses of quinidine sulphate per oesophageal tube, after slow digitalisation with intravenous digoxin over 4d. Four hours after commencement of quinidine therapy the arrhythmia had regressed to atrial flutter and converted to sinus rhythm 10 min later. Considering his age, standard of raci...
Rose RJ, Davis PE.Three young Standardbred pacers with atrial fibrillation were treated with quinidine sulphate (QS) by stomach tube. They were given 10g QS every 2 hours until cardioversion was achieved. Total doses varied between 20 and 30 g. No premedication was given nor any follow up treatment after return to sinus rhythm. All horses were given 3 months rest after the treatment, and when electrocardiographed then, and at 6 months, showed normal sinus rhythm. As all 3 horses won races after cardioversion and showed subsequent normal electrocardiograms it seems likely that atrial fibrillation can occur witho...
Eberhardt C, Schwarzwald CC.In horses, persistent atrial dysfunction is a prognostic indicator of recurrence of atrial fibrillation (AF). The objective of this study was to quantify left atrial (LA) contractile function in Warmblood horses after successful cardioversion of AF to sinus rhythm, using standard two-dimensional echocardiography (2DE) and two-dimensional speckle tracking (2DST) analyses, and to provide proof-of-concept for use of 2DST to detect LA contractile dysfunction in horses. Methods: Nineteen Warmblood horses with AF and successful transvenous electrical cardioversion (TVEC) or medical cardioversion usi...
Stern JA, Doreste YR, Barnett S, Lahmers SM, Baumwart RD, Seino KK, Bonagura JD.Sustained narrow-QRS tachycardia of three months duration and left ventricular systolic dysfunction were identified in a fifteen-year-old Quarter Horse. No underlying cause for the tachyarrhythmia was found and no predisposing structural cardiac lesions were evident by echocardiography. Intravenous diltiazem and lidocaine were administered without achieving successful conversion of the arrhythmia. Oral quinidine therapy converted the tachyarrhythmia to sinus rhythm. Ventricular systolic dysfunction and chamber dilatation subsequently resolved. As with other species, echocardiographic features ...
Scheck JL.A stallion was presented for surgical treatment of limbal squamous cell carcinoma. Excision with a conjunctival pedicle flap was done to prevent ocular invasion and preserve vision. A complication was the diagnosis of atrial fibrillation, which was not converted to sinus rhythm due to the potential complication of treatment with quinidine sulfate. Traitement d’un carcinome épidermoïde oculaire chez un étalon atteint de fibrillation auriculaire. Un étalon a été présenté pour une chirurgie d’un carcinome épidermoïde limbique. Après l’excision, un lambeau à pédicule conjonctiva...
Guthrie AJ, Nichas E, Viljoen FV, Hartmann AM, Killeen VM.A case of sustained supraventricular tachycardia of unknown aetiology in a two-year-old Thoroughbred filly is reported. The cardiac dysrhythm was successfully treated by the oral administration of quinidine sulphate. Conversion of the dysrhythm to sinus rhythm occurred approximately 80 min after the initial dose of 5 g of quinidine sulphate. The horse returned to training approximately 2 months after treatment and has since successfully returned to racing.
Bertone JJ, Traub-Dargatz JL, Wingfield WE.Atrial fibrillation in a pregnant, lactating, 15-year-old mare nursing a 70-day-old foal was converted to normal sinus rhythm, using quinidine sulfate. The maximum concentration of quinidine was 4.3 mg/L in the mare's milk and was 2.6 mg/L in the mare's serum. Treatment with quinidine did not interrupt the pregnancy. Six months after treatment, the mare developed acute volvulus of the large colon and died. At necropsy, the mare did not have macroscopic or microscopic cardiac lesions. The fetus was macroscopically and histologically normal.
Button C, Scrutchfield WL, Clark RG, Knauer KW, Schmitz DG.A variety of atrial dysrhythmias including paroxysmal atrial tachycardia, atrial tachycardia with 2nd-grade atrioventricular block, atrial fibrillation, and atrial flutter developed in a 5-year-old Quarter Horse gelding. Quinidine and propranolol were not successful in restoring normal sinus rhythm. Sinus rhythm was re-established during digoxin therapy, but later reverted to atrial dysrhythmia. At necropsy, multiple, discrete pale areas were found on both atria and the interatrial myocardium. Histologic examination of these lesions demonstrated myocytolysis and replacement by fibrous connecti...
Parraga ME, Kittleson MD, Drake CM.The aim of this study was to determine if quinidine administration increases steady state serum digoxin concentration in horses. Digoxin (0.01 mg/kg q. 12 h per os) was administered to 6 horses for 7 days. Steady state was confirmed by identifying statistically indistinguishable peak and trough serum digoxin concentrations on Days 4, 5, and 6. On Day 6, serum digoxin concentration was measured at baseline and 0.25, 0.5, 1, 2, 4, 6, 8 and 12 h after digoxin administration. On Day 7, quinidine (20 g at baseline and 10 g at 2, 4 and 6 h) was administered per os and serum digoxin concentration was...
Wingfield WE, Miller CW, Voss JL, Bennett DG, Breukels J.Echocardiography is a non-invasive, safe means of assessing intracardiac structures and their motion. In the normal motion of the mitral valve there is anterior motion noted during atrial systole. In 3 horses with atrial fibrillation the mitral valve motion had no anterior motion to correspond with atrial contractions. Quinidine returned the electrocardiogram of one horse to a sinus rhythm after which echocardiography demonstrated the reappearance of the anterior motion of the mitral valve.
Beech J.An 18-month-old male Quarter Horse was referred for evaluation of a tic that had started after injury to the right forelimb 4 weeks earlier. The right forelimb appeared paretic and had constant regular twitches of variable intensity that were usually sufficiently forceful to move the trunk, neck, and head. The horse frequently threw the limb forward. The twitch persisted during sleep but disappeared during general anesthesia and following sedation with xylazine. It was unaffected by acetylpromazine, diphenylhydantoin, diazepam, carbamazepine, trimethadione, procainamide, quinidine, propranolol...
Burns JJ, MacMillan KM, John EE.Atrial fibrillation (AF) is the most clinically relevant cardiac arrhythmia identified in the Standardbred racehorse. However, there is limited literature regarding athletic ability in Standardbred racehorses following AF conversion. To investigate this issue, the objectives of this review were to: i) determine the success rate in using quinidine sulphate to treat AF in a population of hospitalized equine patients in Atlantic Canada between January 2008 and December 2019; and ii) measure return to athletic ability (using racetrack earnings and top-3 finishes) following cardioversion with quini...
Wijnberg ID, van der Kolk JH.Atrial fibrillation is a disorder of cardiac rhythmicity, and its importance in the horse depends on the underlying cause and the function of the horse. Before the decision is taken to start treatment, it has first to be ascertained whether treatment is worthwhile and whether the horse is an appropriate candidate for treatment. This article gives a short overview of current opinion on the cause and treatment of atrial fibrillation in the horse. The most used treatment at the moment, oral chinidine sulphate, is discussed. The hemodynamic consequences of atrial fibrillation and the response of a...
Kuroda T, Minamijima Y, Kinman CK, Takahashi Y, Ebisuda Y, Inoue K, Ishikawa H, Mita H, Tamura N, Nukada T, Toutain PL, Ohta M.Quinidine (QND) sulfate is an effective treatment for atrial fibrillation (AF) in horses, and several dosage regimens have been proposed to address its wide variability in response and potential adverse effects. The purpose of this study was to analyze the variability in plasma quinidine concentrations using population pharmacokinetics to determine an effective and safe dosage regimen for Thoroughbred horses. Unassigned: Six healthy Thoroughbred horses were treated with 20 mg/kg quinidine sulfate dihydrate (16.58 mg/kg QND base) administered PO or 5 mg/kg quinidine hydrochloride monohydr...
