Pericardial malignant mesothelioma diagnosed in a dog by immunocytochemistry of the pericardial fluid: a case report.

BACKGROUND: Pericardial effusions are one of the most common cardiac diseases in dogs. Common causes of haemorrhagic pericardial effusions include neoplasia, such as hemangiosarcoma, mesothelioma, chemodectoma, and ectopic thyroid tumours, and benign idiopathic pericardial effusion. Distinguishing among reactive mesothelial cells, malignant mesothelioma, and adenocarcinoma in body effusions is a diagnostic challenge. Therefore, the author aimed to discover whether the observed cells were reactive mesothelial, mesothelioma, or adenocarcinoma cells through immunocytochemistry using five markers (cytokeratin, vimentin, desmin, E-cadherin, and calretinin) in a canine patient. CASE PRESENTATION: A 2.1 kg, spayed female, 10-year-old Yorkshire Terrier dog presented to a local hospital with dyspnoea and was evaluated for pericardial effusion. The presence of pericardial fluid was confirmed, and she was referred to our hospital for further evaluation. In cytological evaluation, cells shed individually or in clusters were observed, along with numerous non-degenerative neutrophils and macrophages. The cells showed binucleation, anisocytosis, anisokaryosis, abnormal nucleoli, abundant basophilic cytoplasm, high nuclear-cytoplasmic ratio, and coarse chromatin. Large atypical multinucleate cells were also observed. Erythrophagia was observed, indicating chronic haemorrhage. Immunocytochemistry using pericardial fluid was positive for cytokeratin, vimentin, desmin, E-cadherin, and calretinin. Therefore, malignant mesothelioma was diagnosed. CONCLUSIONS: Immunocytochemistry is a very useful diagnostic technique because it can determine whether several fluorescent markers are simultaneously expressed in the same cell. Further, E-cadherin and calretinin can be used for the differential diagnosis of reactive mesothelial cells, malignant mesothelioma, and adenocarcinoma in dogs.

Subtotal Pericardiectomy and Epicardiectomy for Treatment of Septic Pericarditis and Constrictive Epicarditis in a Dog.

A 1 yr old, 1.7 kg, spayed female Chihuahua was presented for respiratory distress and an enlarged cardiac silhouette as seen on thoracic radiographs. Echocardiogram revealed pericardial effusion and cardiac tamponade. Computed tomography revealed marked pleural and pericardial effusion, thickening of the pericardium caudally, and a mass along the mediastinum. Pericardial fluid obtained via pericardiocentesis showed suppurative inflammation with mixed anaerobic bacteria isolated on culture. Subtotal pericardiectomy and partial lung lobectomy was performed to treat septic pericarditis. Postoperative echocardiogram showed increased right-sided pressures consistent with constrictive epicarditis, and 10 days after surgery, the dog was re-presented for right-sided heart failure. An epicardectomy was performed. A definitive source of infection was not identified, although a penetrating foreign body (e.g., grass awn) was suspected. The dog recovered and 10 yr follow up revealed no evidence of constrictive pathology on echocardiogram. This case report demonstrates the successful treatment of septic pericarditis and constrictive epicarditis via subtotal pericardiectomy and epicardiectomy.

Multidetector-row CT features and histopathological analysis of pericardial recesses in dogs.

At the level of pericardial reflections and near the great thoracic vessels, pericardial recesses (PRs) are present, where fluid can collect to increase the pericardial reserve volume. To date, these structures have not been described in vivo in veterinary patients. The aims of this observational and descriptive study were to describe the location and appearance of PRs in dogs, as seen with multidetector-row CT (MDCT), and to develop a dedicated imaging technique for their best visualization. Dogs who underwent MDCT examination of the whole body were included in the study and CT data were retrospectively evaluated. Dogs with any thoracic abnormality were excluded. MDCT analysis of the PR's was compared with the pathological features of PRs. PRs were identified as fluid-attenuating (10-30 HU), non-enhancing structures showing varied appearance. Two types of PRs were identified at the level of the transverse sinus of the pericardium and classified on the basis of their anatomic location: the aortic recess and the pulmonic recess. A third pericardial fluid-containing structure was seen in a little number of cases, at the level of the termination of the caudal vena cava into the right atrium. A dorsal, slight oblique multiplanar section through the aortic bulb resulted the best technique for visualization of all the recesses. Anatomo-pathological evaluation confirmed the location and the presence of pocket-like reflections of the pericardium identified with 3D-CT models. Knowledge of the CT appearance of the pericardial recesses is necessary in order to avoid their misinterpretation and subsequent unnecessary invasive investigations.

Echocardiographic, thoracic ultrasonographic, and CTA diagnosis of pericardial neoplasia in a dog.

A 12-year-old male neutered Yorkshire Terrier presented for coughing and respiratory distress. Transthoracic echocardiography initially misdiagnosed the patient with pericardial effusion; repeat echocardiography increased suspicion for neoplasia. A definitive diagnosis was not apparent. Findings on thoracic computed tomography and thoracic ultrasound were consistent with a diffusely thickened, heterogenous, hypoechoic soft tissue structure surrounding the heart. Fine needle aspirates were obtained using ultrasound guidance and routine cytology of the intrapericardial mass was consistent with neoplasia, with pericardial mesothelioma most likely. These novel findings highlight the importance of thoracic ultrasound and potential limitations of echocardiography in diagnosis of pericardial neoplasia.

DEGENERATIVE CARDIAC DISEASE IN TWO SPECIES OF TORTOISE (CHELONOIDIS NIGRA COMPLEX, CENTROCHELYS SULCATA).

Cardiac lesions in tortoises are incompletely described in the literature. This retrospective case series includes 11 cases of degenerative cardiac disease in young tortoises from two species in human care: Galápagos tortoise complex (Chelonoidis nigra complex) (n = 9) and sulcata tortoises (Centrochelys sulcata) (n = 2). Eight tortoises were male, two were female, and sex was undetermined for one individual. The age range at the time of death was 10-32 yr with a mean of 19 yr. The most common clinical signs noted prior to death were peripheral edema, lethargy, and inappetence. Common necropsy findings included generalized edema and pericardial effusion. All cases had ventricular myocardial fibrosis and several cases had epicardial adhesions. Additional common findings included hepatic lesions (hepatic lipidosis, hepatic fibrosis, and hepatitis) and pulmonary lesions (pulmonary edema, pulmonary fibrosis, and pneumocytic hypertrophy). A definitive cause for degenerative cardiac disease was not identified in this case series, but the young age distribution of the tortoises suggests that inappropriate environmental parameters, husbandry, and diet should be investigated as possible underlying contributing factors.

DETERMINATION OF ECHOCARDIOGRAPHIC TECHNIQUES FOR CLINICAL ASSESSMENT OF GALAPAGOS (CHELONOIDIS NIGRA COMPLEX) AND ALDABRA (ALDABRACHELYS GIGANTEA) TORTOISES.

The first objective of this study was to establish clinically relevant techniques for cardiac echocardiography in nonanesthetized Galapagos (Chelonoidis nigra complex) and Aldabra (Aldabrachelys gigantea) tortoises. A second objective was to establish guidelines for determining normal echocardiographic anatomy and function in both species. Select echocardiographic reference values were defined for 17 healthy Galapagos tortoises and 27 healthy Aldabra tortoises. Tortoises were either placed in ventral recumbency on an elevated surface or allowed to stand in a natural position by using food distraction. An ultrasound probe was applied in the left or right cervicobrachial window and was positioned in two long axis views to evaluate the three chambers of the heart and the associated great vessels, the presence of pericardial effusion, the atrioventricular inflow velocities, and pulmonic and aortic outflow velocities. The heart rate was 28 ± 12 (median ± SD) bpm, and the ejection fraction was 60.5 ± 10%. Thirty-four of 44 tortoises had identifiable physiologic pericardial effusion. All tortoises were successfully imaged using the techniques described, with consistent identification of cardiac structure and assessment of function. This study provides echocardiographic reference intervals for the clinical evaluation of suspected cardiac disease in captive-managed Galapagos and Aldabra tortoises.

Treatment and outcomes of five dogs with intrathoracic migration of porcupine quills.

OBJECTIVE: To describe the perioperative findings and outcomes in dogs that underwent exploratory thoracotomy following porcupine quill migration and report (1) the use of traction alone for the removal of superficial quills in the pulmonary parenchyma, and (2) the use of exploratory pericardiotomy for the incidental identification of cardiac quills. ANIMALS: Five client-owned dogs. STUDY DESIGN: Short case series. METHODS: Medical records of dogs with pneumothorax or pericardial effusion secondary to porcupine quill migration from January 2019 to January 2022 were reviewed. Advanced imaging and a median sternotomy were performed in each dog. Demographics, preoperative findings, surgical treatment, and outcome were recorded. RESULTS: Pulmonary quills were treated with gentle traction or lung lobectomy. The intraoperative palpation or appearance of the pericardium prompted pericardiotomy in three dogs, exposing epicardial and intravascular quills. No evidence of these quills was detected on preoperative advanced imaging or on gross appearance of the pericardium. Quills were removed with a combination of gentle traction, purse-string sutures, and mattress sutures. Pneumothorax and pericardial effusion resolved postoperatively in all dogs. All dogs survived to discharge. CONCLUSION: Traction allowed removal of quills that were superficially attached to the pulmonary parenchyma without the need for subsequent lobectomy. Epicardial and myocardial quills were detected following pericardiotomy in three dogs despite the absence of quills grossly penetrating the pericardium. Preoperative imaging did not allow consistent detection of quills.

Systematic review of the treatment options for pericardial effusions in dogs.

OBJECTIVE: To evaluate the evidence for the conservative and surgical management of pericardial effusions for neoplastic and idiopathic etiologies in dogs. STUDY DESIGN: Systematic review. SAMPLE POPULATION: Peer-reviewed English-language articles describing the treatment and outcome of naturally occurring pericardial effusion in domestic dogs. METHODS: A literature search was performed with PubMed, Cab Abstracts, Scopus, and Agricola in August 2019 for articles describing pericardial effusion treatment in dogs. Inclusion criteria were applied, and articles were evaluated for reported outcome and level of evidence by using The Oxford 2011 Levels of Evidence, a previously described hierarchical system, and GRADE (Grading of Recommendations, Assessment, Development and Evaluation). RESULTS: One hundred eight of the 641 unique articles that were identified and evaluated met inclusion criteria. Most articles included were case studies (68.2%) or retrospective case series (25.2%), with all articles providing a low level of evidence. The articles had inconsistent inclusion criteria, outcome measures, and follow-up, making comparison of outcomes difficult. CONCLUSION: Because of the low quality of evidence of the studies included in this systematic review and the variability of the outcomes, there is not sufficient evidence to recommend one treatment option rather than another. CLINICAL SIGNIFICANCE: There is a requirement for higher quality evidence such as randomized controlled trials and prospective comparative cohort studies. Standardization of outcome measures reported for each treatment option and disease process studied will allow for better comparison of outcomes between studies.

PERICARDIAL MESOTHELIOMA AND ASSOCIATED PERICARDIAL EFFUSION IN A TIGER RAT SNAKE (SPILOTES PULLATUS) TREATED WITH PERICARDIOCENTESIS.

A 1.5 kg, male, wild-caught tiger rat snake (Spilotes pullatus) presented with an externally visible distension of the body wall at the level of the heart. Ultrasound examination showed marked pericardial effusion. Pericardial fluid showed no bacterial or fungal growth, few leukocytes, and few suspected reactive mesothelial or neoplastic cells. Therapeutic pericardiocentesis was successfully performed, removing most of the fluid from the pericardial sac. The snake had mild lethargy and weakness immediately after the procedure but returned to normal behavior within 2 wk. Repeat pericardiocentesis was performed 6 mo after the initial presentation when moderate refilling of the pericardial sac was seen. The snake died 4 days after the second procedure with acute hemorrhage. Pericardial mesothelioma was diagnosed by histopathology after postmortem examination. This report provides the first documented case of mesothelioma in a tiger rat snake and the first description of the disease in colubrids.

Outcomes of dogs with recurrent idiopathic pericardial effusion treated with a 3-port right-sided thoracoscopic subtotal pericardiectomy.

OBJECTIVE: To describe a 3-port right-sided thoracoscopic subtotal pericardiectomy (TSP) to treat dogs with recurrent idiopathic pericardial effusion (RIPE) and report their long-term survival. STUDY DESIGN: Retrospective case series. ANIMALS: Sixteen client-owned dogs. METHODS: Medical records were searched for dogs with idiopathic pericardial effusion that recurred after 1 or more pericardiocenteses and treated with a 3-port right-sided technique (1 subxiphoid camera port and 2 instrument ports on the right side of the dog). Follow-up consisted of direct examination or communication with referring veterinarians or owners. RESULTS: Thoracoscopic subtotal pericardiectomy was successfully completed in all dogs, with no surgical complications. The median duration of postoperative follow-up was 191.5 days (range, 5-1345). The median survival time (MST) after surgery was 365 days (range, 5-1345); MST of dogs with a histopathological diagnosis of neoplasia (n = 4) was 76 days, whereas dogs with no evidence of neoplasia had an MST of 367 days (n = 12, P = .14). Recurrent pleural effusion was the ultimate cause of death or reason for euthanasia in 8 of 16 dogs. CONCLUSION: The technique described here resulted in similar surgical times and outcomes for dogs undergoing TSP for RIPE compared with previous studies. Neoplasia was identified in 4 of 16 dogs. CLINICAL SIGNIFICANCE: Thoracoscopic subtotal pericardiectomy can be readily performed with only 2 instrument ports, both on the same side of the dog, and without 1-lung ventilation. Recurrent pleural effusion led to the death of half of the dogs in this series.

Outcome in dogs with presumptive idiopathic pericardial effusion after thoracoscopic pericardectomy and pericardioscopy.

OBJECTIVE: To report the outcome of dogs with presumptive echocardiographic idiopathic pericardial effusion treated with thoracoscopic pericardectomy and pericardioscopy. STUDY DESIGN: Multi-institutional retrospective study (2011-2017). ANIMALS: Eighteen dogs. METHODS: Records were searched for dogs with pericardial effusion and no identified cause by preoperative echocardiography and subsequent thoracoscopic pericardectomy and pericardioscopy. Collected data included presenting complaint, physical examination, laboratory results, imaging, and operative findings. Follow-up was obtained via telephone interview and/or recheck examination. RESULTS: No evidence of mass lesions or cause for the effusion was identified in any of the dogs by preoperative echocardiography. Nine dogs had unremarkable pericardioscopic examination results. Nine dogs had pericardioscopic abnormalities consistent with masses, nodules, or adhesions. Median survival time (MST) for the 9 dogs with abnormalities identified by pericardioscopy was 66 days, whereas MST for the 9 dogs with unremarkable pericardioscopic examination results was not reached (P = .0067). Median survival time for dogs based on histopathologic diagnosis alone was not different between dogs with a diagnosis of neoplasia and dogs with a diagnosis of pericarditis (P = .1056). Among dogs with lesions identified during pericardioscopy, MST did not differ between those with a diagnosis of malignancy and those with a diagnosis of pericarditis (P = .78). CONCLUSION: Dogs with presumptive idiopathic pericardial effusion without evidence of masses, nodules, and/or adhesions during thoracoscopic pericardectomy and pericardioscopy lived longer than dogs in which abnormalities were identified during pericardioscopy. CLINICAL SIGNIFICANCE: Thoracoscopic pericardectomy/pericardioscopy and targeted biopsy of the pericardium and pleura are recommended in dogs with echocardiographic idiopathic pericardial effusion.

Hydropericardium syndrome outbreak caused by fowl adenovirus serotype 4 in China in 2015.

There have been many outbreaks of hydropericardium syndrome (HPS), which is characterized by pericardial effusion and hepatitis, in Chinese chicken farms since June 2015. HPS was mainly found in miscellaneous meat-type chickens, Ma chickens, layer chicks and Three-yellow chickens, while it was occasionally found in white broilers. To determine the specific causative pathogen and pathogenicity of HPS in chickens, we collected 25 suspected cases and performed clinical pathology and aetiology analyses. The results showed that the 25 cases exhibited multifocal hepatitis with intra-nuclear inclusion bodies and 70 nm-latticed viral particles in the cell nuclei. All samples were positive for fowl adenovirus (FAdV), and sequencing results showed that the hexon gene shared the highest nucleotide similarities with the hexon gene of group 1 serotype 4 (FAdV-4). FAdV-4 was highly pathogenic to embryos and specific pathogen-free chickens, causing 100 and 70 % mortality rates, respectively. Thus, FAdV-4 is associated with HPS outbreaks in China.

Acute fatal pericardial effusion induced by accidental ingestion of cigarette butts in a dog.

A dog was referred for collapse and tachypnea after ingesting cigarette butts. Thoracic radiography and echocardiography indicated pericardial effusion, and an electrocardiogram showed tachycardia, variable QRS complexes, and ventricular premature complexes. This is the first description of fatal pericardial effusion associated with cigarette butt ingestion in a veterinary patient.

Effusion péricardique aiguë mortelle induite par l'ingestion accidentelle de mégots de cigarette chez un chien. Un chien a été recommandé pour un collapsus et une tachypnée après l'ingestion de mégots de cigarette. Une radiographie thoracique et une échocardiographie ont indiqué une effusion péricardique et un électrocardiogramme a montré la tachycardie, des complexes QRS variables et des complexes ventriculaires prématurés. Il s'agit de la première description d'une effusion péricardique mortelle associée à l'ingestion de mégots de cigarettes chez un patient vétérinaire.(Traduit par Isabelle Vallières).

Clinical presentation, echocardiographic findings, treatment strategies, and prognosis of dogs with myxomatous mitral valve disease presented with pericardial effusion due to suspected left atrial tear: a retrospective case-control study.

INTRODUCTION/OBJECTIVES: Left atrial tear (LAT) is a life-threatening complication in dogs with myxomatous mitral valve disease (MMVD). The study objective was to describe clinical presentation, echocardiographic findings, treatment strategies, and survival in dogs with LAT compared to a control group of dogs with a similar stage of MMVD but no LAT. ANIMALS AND MATERIALS AND METHODS: Two-center retrospective case-controlled study including 15 dogs with and 15 dogs without LAT was conducted. Clinical and echocardiographic data were reviewed, and survival information were collected. RESULTS: Nine dogs in each group were in stage C of MMVD, while the remaining were in stage B2. No differences between groups were found regarding age, body weight, sex, kidney values, and echocardiography-derived cardiac dimensions. Most reported clinical signs associated with LAT included weakness, respiratory signs, and syncope. Treatment varied and was mainly focused on the management of congestive heart failure. Three dogs with LAT received a pericardiocentesis. All 15 dogs with LAT had died of cardiac causes, five dogs during the first seven days after admission. The median survival time for all 15 dogs with LAT was 52 days compared to 336 days in the control group (P=0.103). When excluding five dogs with LAT that died during the first seven days, the median survival increased to 407 days, not different compared to the control group (P=0.549). CONCLUSIONS: Dogs with MMVD and LAT have a high short-term mortality; however, when surviving the acute phase, the long-term prognosis may not differ from dogs with a similarly advanced degree of MMVD but without LAT.

Detection of Anaplasma phagocytophilum in an inflammatory pericardial effusion of a dog.

An 11-year-old female spayed German Wirehaired Pointer with a 1-week history of lethargy, hyporexia, diarrhea, and coughing presented with pericardial effusion causing cardiac tamponade. An echocardiogram revealed no structural cause for pericardial effusion. The pericardial effusion was an exudate with mixed macrophagic and neutrophilic inflammation. Morulae occasionally were found within neutrophils. The pericardial fluid and blood were qPCR and cPCR positive for Anaplasma phagocytophilum (NC State University, Vector-borne Disease Diagnostic Laboratory, Raleigh, NC). The dog's blood was negative by ELISA (Vetscan Flex4 Rapid Test, Zoetis, Parsippany, NJ) for A. phagocytophilum antibodies at initial presentation and subsequently positive (SNAP4DxPlus, IDEXX, Westbrook, ME) 7 days later. After pericardiocentesis and administration of doxycycline (5 mg/kg PO q12h for 14 days), a repeat echocardiogram performed 1 month later showed no recurrence of pericardial effusion.

Hydropericardium syndrome: current state and future developments.

Hydropericardium syndrome (HPS) is a highly infectious disease caused by fowl adenovirus serotype 4 (FAV-4) affecting poultry, especially broiler birds. The disease was initially reported from Angara Goth, Pakistan, and then from India during 1994, in the poultry belt of Jammu and Kashmir, and thereafter, from almost all parts of the country, causing heavy economic losses to the poultry industry. The disease occurs predominantly in broilers of the age group of 3-5 weeks, characterized by sudden onset of high mortality up to 80 %. The causative agent of HPS is fowl adenovirus 4, which is a member of the species Fowl Adenovirus C, genus Aviadenovirus, family Adenoviridae [60]. FAV-4 is non-enveloped and icosahedral in shape, measuring 70-90 nm in size and containing a linear dsDNA of approximately 45 kb in size as its genome. The livers of affected birds show necrotic foci and basophilic intranuclear inclusion bodies in the hepatocytes. The disease can be diagnosed from its gross and microscopic changes in the liver and by various serological tests, such as agar gel immunodiffusion, counterimmunoelectrophoresis, indirect haemagglutination, fluorescent antibody techniques, and ELISA. In the past few years, PCR has been used as a rapid diagnostic tool for the detection of fowl adenoviruses. The disease has been brought under control by the use of formalin-inactivated, attenuated or live vaccines in experimentally infected birds. Advancement in the field of computational immunology accelerates knowledge acquisition and simultaneously reduces the time and effort involved in screening potential epitopes, leading toward the development of epitope-based vaccines.

Cardiac MRI findings in a dog with a diffuse pericardial mesothelioma and pericardial effusion.

Veterinary cardiac MRI (cMRI) is a relatively new technique. A dog with recurrent pericardial effusion and a questionable right atrial mass lesion on echocardiography underwent cMRI. cMRI provided excellent anatomic information about the heart and surrounding structures and helped to rule out the presence of a focal mass. A diffuse thickening and enhancement of the pericardium was detected. Pericardiectomy was performed and histopathology revealed a diffuse pericardial mesothelioma. This case illustrates the potential of cMRI in the management of patients with pericardial effusion when echocardiographic findings are equivocal and illustrates cMRI findings in a case of diffuse pericardial mesothelioma.

Epidemiological, clinical, radiographic, echocardiographic findings and outcome in client-owned guinea pigs (Cavia porcellus) with cardiac disease: 80 cases (2010-2021).

OBJECTIVE: To characterize epidemiological, clinical, radiographic, and echocardiographic features of cardiac diseases in guinea pigs examined at a referral exotics center. ANIMALS: 80 guinea pigs. PROCEDURES: Medical records of guinea pigs that had echocardiography performed between June 2010 and January 2021 were reviewed. RESULTS: The percentage of guinea pig patients with cardiovascular disease was 2.8%. Clinical signs included dyspnea (46/80), lethargy (18/80), and anorexia (10/80). The most common physical examination finding was heart murmur (10/80). Radiographic abnormalities included subjective cardiomegaly (37/67), pleural effusion (21/67), and increased lung opacity (40/67). Median (range) vertebral heart score on right lateral (48/67) and ventrodorsal (39/67) projections was 9.0 vertebrae (6.6 to 13.2 vertebrae) and 10.8 vertebrae (7.9 to 13.2 vertebrae), respectively. The most common echocardiographic diagnosis was cardiomyopathy (30/80), categorized as restrictive (11/30), hypertrophic (10/30), or dilated (9/10). Other cardiac diseases included cor pulmonale (21/80), pericardial effusion (18/80), congenital heart disease (6/80), acquired valvular disease (3/80), and cardiovascular mass (2/80). Congestive heart failure was present in 36 of 80. Median survival time from diagnosis was 2.5 months (95% CI, 1.1 to 6.2 months). Animals that died from heart disease had a significantly shorter survival time than those that died from a noncardiac disease (P = .02). CLINICAL RELEVANCE: On radiographs, cardiomegaly, pleural effusion, and alveolar or interstitial lung pattern should be considered as indications for echocardiography in guinea pigs. Cardiomyopathy (restrictive, hypertrophic, or dilated), cor pulmonale, and pericardial effusion were the most common echocardiographic diagnoses. Further studies on diagnosis and treatment of cardiovascular diseases in guinea pigs are needed.

Presumed primary cardiac mast cell tumor, pericardial effusion, and arrhythmia in a dog.

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 postmortem 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.