A 13-year-old female spayed border collie cross presented for pericardial effusion, arrhythmia, and a suspected cardiac mass. Echocardiogram revealed severe thickening and hypokinesis of the interventricular septum with a heterogenous, cavitated myocardium, concerning for neoplasia. Electrocardiogram revealed predominantly accelerated idioventricular rhythm with frequent periods of nonsustained ventricular tachycardia. Occasional prolonged PR intervals terminating in an aberrantly conducted QRS complex were present. These beats were postulated to represent either first-degree atrioventricular block with aberrant QRS conduction or atrioventricular dissociation. Cytology of the pericardial effusion revealed atypical, suspected neoplastic, mast cells. The patient was euthanized, and post-mortem examination confirmed full thickness infiltration of the interventricular septum by a mast cell tumor, with metastasis to the tracheobronchial lymph node and spleen. Given the anatomic location of the mass, the observed atrioventricular nodal conduction delay may represent neoplastic infiltration of the atrioventricular node. Neoplastic infiltration of the ventricle was suspected to cause the accelerated idioventricular rhythm and ventricular tachycardia. To the authors’ knowledge, this is the first reported case of a primary cardiac mast cell tumor causing arrhythmia and pericardial effusion in a dog.

一只 13 岁雌性绝育边境牧羊犬因心包积液、心律失常和疑似心脏肿块就诊。超声心动图显示室间隔严重增厚和运动功能减退，伴有异质性空洞心肌，与肿瘤有关。心电图显示主要是室性心律加快，伴有频繁的非持续性室性心动过速。偶尔出现延长的 PR 间期，终止于异常传导的 QRS 波群。这些搏动被假定代表具有异常 QRS 传导的一度房室传导阻滞或房室分离。心包积液的细胞学检查显示非典型、疑似肿瘤性肥大细胞。病人被安乐死，尸检证实肥大细胞瘤对室间隔全层浸润，并转移至气管支气管淋巴结和脾脏。考虑到肿块的解剖位置，观察到的房室结传导延迟可能代表房室结的肿瘤浸润。怀疑心室肿瘤浸润导致室性心律加速和室性心动过速。据作者所知，这是第一例报道原发性心脏肥大细胞肿瘤导致狗心律失常和心包积液的病例。观察到的房室结传导延迟可能代表房室结的肿瘤浸润。怀疑心室肿瘤浸润导致室性心律加速和室性心动过速。据作者所知，这是第一例报道原发性心脏肥大细胞肿瘤导致狗心律失常和心包积液的病例。观察到的房室结传导延迟可能代表房室结的肿瘤浸润。怀疑心室肿瘤浸润导致室性心律加速和室性心动过速。据作者所知，这是第一例报道原发性心脏肥大细胞肿瘤导致狗心律失常和心包积液的病例。