Sammy, a 12-year-old, male, neutered Beagle presented to Cornell University's Hospital for Animals on October 24, 2001 as a referral for an episode of collapse that occurred the previous day. Upon presentation to Cornell, Sammy was quiet, alert, and responsive, but was very lethargic. Physical examination revealed severe bradycardia with audible S4 heart sounds and a grade III/VI left apical systolic murmur. Jugular pulses were present, and femoral pulses were strong bilaterally. An electrocardiogram (ECG) revealed third degree AV block with an atrial rate of 160 bpm and a ventricular escape rhythm of 20-22 bpm. Thoracic radiographs taken at the referring veterinarian revealed mild cardiomegaly with normal pulmonary vasculature. Echocardiography was performed and showed mild to moderate dilation of all four cardiac chambers. The left ventricle was hyperdynamic with an increased fractional shortening (43%). There was moderate mitral insufficiency during both systole and diastole. Severe thickening and prolapsing of the mitral valve was diagnosed as presumptive valvular endocardiosis. The treatment of choice for complete heart block, whether the animal is symptomatic or not, is transvenous pacemaker implantation. That same afternoon, a transvenous permanent pacemaker was surgically implanted in Sammy. Due to the inherent instability of Sammy's ventricular escape rhythm, it was decided that a temporary pacemaker would be placed prior to the implantation of a permanent pacemaker. The pacemaker used in Sammy's case was a single chamber rate responsive pacemaker with a bipolar lead placed in the endocardium of the right ventricle.