title: Characterization of chronotropic and dysrhythmogenic effects of atropine in dogs with bradycardia.
abstract: OBJECTIVE
  To characterize the magnitude, character, and time course of chronotropic and dysrhythmogenic responses of dogs with vagally mediated bradycardia to atropine sulfate.
  
  
  METHODS
  Latin square design.
  
  
  METHODS
  Seven clinically normal adult mixed-breed dogs.
  
  
  METHODS
  Vagally mediated bradycardia was induced with morphine and fentanyl citrate. Atropine (0.02 mg/kg of body weight) was administered i.v., s.c., or i.m.. Electrocardiograms were recorded continuously for 5 minutes before and for 35 minutes after atropine administration or until a sustained parasympatholytic response was observed. Data were digitized, analyzed independently for changes in atrial and ventricular rate, and compared between different routes of administration.
  
  
  RESULTS
  All dogs developed second-degree atrioventricular (AV) block after i.v. administration of atropine, and 71% of dogs developed AV block after s.c. or i.m. administration. The AV block arose and resolved more rapidly with i.v. administration than with s.c. or i.m. administration. The AV block was principally attributable to an increase in the atrial rate prior to increases in the ventricular rate. Atropine, regardless of route of administration, potentiated baseline ventricular bradycardia in 62% of the experiments (mean heart rate decrease of 16 beats/min; decreased to < 20 beats/min in 2 dogs for < or = 10 seconds). Duration of the bradycardic potentiation was longer with s.c. administration (9.1 minutes, s.c., vs 1.4 minutes, i.v., and 4.6 minutes, i.m.). Parasympatholytic rate was higher for i.v. than s.c. or i.m. administration (128 beats/min vs 92 beats/min and 101 beats/min). Two dogs given atropine s.c. failed to resolve the AV block and attain sinus rhythm.
  
  
  CONCLUSIONS
  Administration of 0.02 mg of atropine/kg by i.v., i.m., and s.c. routes for vagally mediated bradycardia in dogs consistently induces AV block and occasional brief potentiation of ventricular bradycardia.
  
  
  CONCLUSIONS
  Parasympathomimetic effects occur and resolve most rapidly and consistently, and the stable parasympatholytic effect is of greatest magnitude after i.v. administration. Thus, vagally mediated bradycardia in clinically normal dogs appears to be best abolished by i.v. administration of atropine.
authors.affiliation: Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman 99164-7010, USA.
authors.forename: M
authors.initials: M
authors.lastname: Rishniw
authors.forename: A H
authors.initials: AH
authors.lastname: Tobias
authors.forename: B K
authors.initials: BK
authors.lastname: Slinker
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D001285:Atropine / Q000494:pharmacology*; D004285:Dogs; D004562:Electrocardiography; D005283:Fentanyl / Q000494:pharmacology*; D006325:Heart Atria; D006327:Heart Block; D006339:Heart Rate / Q000187:drug effects*; D006352:Heart Ventricles; D009020:Morphine / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D009020:Morphine; D001285:Atropine; D005283:Fentanyl