Introduction. Enhancement of cardiac disease diagnostics in animals is particularly important due to the increasing incidence of pericardial pathologies in dogs of various breeds and sizes. In recent years, significant progress has been made in diagnostics of cardiac diseases in animals, including diagnostics of diseases accompanied by pericardial effusion inducing compression of the heart chambers and hemodynamic disorders. This condition causes reduction of cardiac filling and emptying, provokes venous congestion, and can lead to cardiogenic shock and death of a patient [1–3].

Diagnostic tests for pericardial pathologies accompanied by pericardial effusion include echocardiography, electrocardiography, radiography, computed tomography, cytomorphological examination of effusion, histological examination of the pericardium and neoplasms of the heart and pericardium. Each method has its advantages and limitations, but echocardiography (EchoCG) is the preferred first-line diagnostic test [2]. This visual diagnostic method is highly informative, as it allows one to determine the size of the heart and evaluate its function, the presence and volume of pericardial effusion, the presence of intracardiac thrombi and cardiac and pericardial tumors [3][5][6]. EchoCG is the leading method for dynamic monitoring of pericardial effusion during conservative therapy [7]. An important advantage of EchoCG is its non-invasiveness and the absence of the need for animal sedation.

A cytological analysis of pericardial effusion is a necessary diagnostic method for determining the etiology of the pathological process and patient’s treatment strategy. In some cases, this test is crucial and reduces the amount of diagnostic procedures required for the patient.

A number of ultrasound criteria for diagnosing canine pericardial diseases accompanied by effusions have been identified in the available national and international literature. However, the relationship between the clinical symptoms and echocardiographic signs that enables diagnosing disease severity and prognosticating its course remains poorly studied, the same as the correlation of pathological changes with the body weight and age of the patients. Furthermore, the cytological characteristics of effusions, which determine effusion causes, are insufficiently described. The aim of the study is to establish clinical, echocardiographic, and cytological relationships in dogs with pericardial pathologies accompanied by pericardial effusions.

Materials and Methods. The study was conducted at the Department of Veterinary Surgery of Moscow State Academy of Veterinary Medicine and Biotechnology – MVA named after K. I. Skryabin and at the Veterinary Research Center “Biocontrol” in the period from 2018 to 2023. The objects of the study were sexually mature dogs of various breeds, ages, and sexes (n=200). The following dog breeds were included in the study: mixed breeds (n=38), French Bulldogs (n=19), Labrador Retrievers (n=12), Chihuahuas (n=11), Yorkshire Terriers (n=10), Golden Retrievers (n=8), Smooth-haired Dachshunds (n=6), American Staffordshire Terriers (n=5), Jack Russell Terriers (n=5), Cane Corso (n=5), German Shepherd Dogs (n=4), Bullmastiffs (n=4), Dogue de Bordeaux (n=4), Siberian Huskies (n=4), German Spitz (n=3), Staffordshire Bull Terriers (n=3), Shiba Inu (n=2), Central Asian Shepherd Dogs (n=2), Russian Spaniels (n=2), Pomeranian Spitz (n=2), German Boxers (n=2), Chinese Crested (n=2), Ca de Bou (n=2), Caucasian Shepherd Dogs (n=2), Dobermans (n=2), East European Shepherd Dogs (n=2), Weimaraners (n=2), Boerboels (n=2), Bernese Mountain Dogs (n=2), American Pit Bull Terriers (n=2), English Bulldogs (n=2), Biewer Yorkshire Terriers (n=2). The following breeds included in the study were represented by a single animal: English Cocker Spaniel, Alaskan Malamute, Dogo Argentino, Beauceron, West Highland White Terrier, Dalmatian, Irish Setter, Keeshond, Maremma Sheepdog, Miniature Poodle, Moscow Toy Terrier, Pug, Neapolitan Mastiff, Pointer, Petit Brabancon, Rhodesian Ridgeback, Russian Hound, Russian Toy Terrier, Samoyed, Skye Terrier, Scottish Terrier, Olde English Bulldogge, Whippet, Fox Terrier, Miniature Pinscher, Shar Pei, Shih Tzu, Entlebucher Mountain Dog, Japanese Chin (Fig. 1).

The distribution of animals by age groups was as follows: Group 1 — from 1 to 5 years (n = 14), Group 2 — from 6 to 9 years (n = 71), Group 3 — from 10 to 15 years (n = 107), Group 4 — from 16 to 17 years (n = 8). Distribution of animals by sex — 134 males and 66 females (Table 1).

For a reliable assessment of the heart structures, the animals were divided into groups depending on body weight: Group 1 — up to 10 kg (n=54), Group 2 — 10–25 kg (n=58), Group 3 — 26–45 kg (n=62), Group 4 — over 45 kg (n=26) (Table 2).

General clinical examination of the dogs was performed according to a standard methodology, including collection of patient’s medical history, and physical examination. Echocardiography was performed using Philips Epiq 5 and Philips HD15 ultrasound systems (“Philips”, USA) with phased array transducers operating at frequencies of 1–5 MHz, 2–9 MHz, and 4–12 MHz. The animals were positioned in the right and left lateral positions. To ensure direct contact between the transducer and the skin, the dogs’ coats were shaved, and 30% isopropyl alcohol and ultrasound gel were applied. During the examination, the structural and functional characteristics of the heart were assessed, including the presence of thrombi in the cardiac chambers, tumors in the heart, pericardium and thoracic cavity, pericardial thickening, and the presence of free fluid in the pericardial cavity.

To obtain the material for cytological examination, ultrasound-guided pericardiocentesis was performed through the 3rd–5th intercostal spaces on the right. The patient was given general anesthesia (by intravenous injection of Propofol at a dose of 4–8 mg/kg) and positioned in the left lateral decubitus position. To prepare the surgical site, the coat was shaved, and the skin around the needle insertion site was cleaned with 70% ethyl alcohol.

Cytological examination of pericardial effusion was performed in 61 dogs. Pericardial fluid obtained by pericardiocentesis was transferred to the EDTA tube. For cell count in the Goryaev chamber, 20 μl of fluid was removed from the tube and mixed with 380 μl of a 3–5% acetic acid solution stained with methylene blue. The EDTA tube was then centrifuged for 5 min at 2000 rpm. Direct brush smears were prepared from the sediment. The smears were stained for 20–30 min according to Romanowsky staining technique. The resulting smears were examined at 100×, 200×, and 1000× magnification using an Olympus CX43 microscope (“Olympus”, Japan).

A histological examination was performed in 23 patients. The dogs had recurrent cardiac tamponade with pericardial effusion accumulated two or more times. Patients underwent pericardiectomy to collect the pericardial and cardiac tumor specimens for histological examination, or euthanasia followed by pathological autopsy with histological examination. A sample for the study (a 3–5 mm thick section) was excised from the obtained biopsy specimen that had been fixed in 10% buffered formalin (BF). The resulting samples were dehydrated and impregnated with paraffin using a Histo-Tek VP1 histoprocessor (“Sakura Seiki Co.” LLC, Japan). Paraffin blocks were then prepared, from which series of sections (2–4 μm thick) were sliced using a HistoCore Biocut rotary microtome (“Leica Biosystems”, Germany).

The resulting preparations were then stained with hematoxylin and eosin to visualize tissue components. But prior to this, the preparations were deparaffinized (sequentially through a paraffin solvent (xylene) and alcohols of descending concentration). After staining, the prepared samples were cleared in xylene and mounted under coverslips using a mounting medium. The prepared histological slides were examined using an Olympus CX43 microscope at 100×, 200×, 400×, and 1000× magnification.

Research Results. Based on the anamnesis and clinical examination of 200 patients with pericardial effusion, cardiac tamponade was diagnosed in 64 animals (32.0%). In this group of animals, one or more of the leading clinical symptoms were identified: increase in volume of the abdomen due to the accumulation of ascitic fluid (50.0%), sudden weakness/syncope (37.5%), progressing weakness/decreased activity (62.5%), and dyspnea caused by hydrothorax (32.8%). The remaining patients had nonspecific symptoms (Table 3).

Physical examination of dogs with cardiac tamponade revealed a number of characteristic clinical symptoms: pallor of the mucous membranes (15.6%), muffled heart sounds (75.0%), weak pulse waveform (35.9%), arrhythmic pulse (21.8%), and paradoxical pulse (7.8%) (Table 4). One or more components of Beck’s triad (edema of the neck, dewlap, and peripheral edemas) were recorded in 3 dogs (1.5%). Jugular venous distention was not detected, possibly because of the difficulty to identify the characteristic signs due to the thick coats of the patients. In 27 of 64 patients with cardiac tamponade, the tonometry data had been obtained before pericardiocentesis. Hypotension with mean systolic blood pressure of 80 mmHg and lower was detected in 12 dogs (42.0%).

In all dogs (n=200) with pericardial effusion, echocardiography revealed a ring-shaped anechoic space (rim) of varying diameter around the heart. Echocardiographic signs of cardiac tamponade were: right atrial collapse in 42 dogs (65.6%), right ventricular diastolic collapse in 20 dogs (31.2%), reduced cardiac chambers due to insufficient filling with blood in 5 dogs (7.8%), paradoxical septal motion during respiration in 8 dogs (12.5%), dilation of caudal vena cava and reduction of its collapsibility amplitude to less than 50% during inhalation in 34 animals (53.1%) (Table 5).

Data analysis enabled establishing a relationship between clinical symptoms and structural and functional parameters of the heart: of 34 dogs with signs of dilation of caudal vena cava and reduction of its collapsibility amplitude to less than 50% during inhalation (diagnosed by echocardiography), 29 patients had ascites, 15 dogs — progressing weakness, and 13 dogs — dyspnea. Of 42 patients with right atrial collapse, 22 dogs had ascites, 23 had progressing weakness, and 19 dogs had dyspnea (Table 6).

Pericardiocentesis was performed in 64 dogs with cardiac tamponade, whereas the cytological examination of the obtained fluid for further diagnostics was performed in 61 patients. By the cytomorphological nature of effusion, the following types were identified: transudate (1.6%), modified transudate (4.9%), septic exudate (1.6%), hemorrhagic effusion, which in turn was divided into effusion corresponding to: acute hemorrhage (4.9%), chronic hemorrhage (77%) and hemorrhagic effusion with cytological signs of a tumor (9.8%) (Table 7).

Histological examination of the pericardium was performed in 23 patients with recurrent cardiac tamponade. In 16 dogs, cardiac or pericardial neoplasms were diagnosed by echocardiography, and 2 patients were suspected of having pericardial neoplasia (Table 8).

Discussion and Conclusion. Transthoracic echocardiography is a reliable method for detecting pericardial pathologies and the presence of free fluid in the pericardial cavity. Free fluid in the pericardial cavity is visualized on ultrasonogram as an anechoic rim around the heart. Upon accumulation of a large volume of fluid, a characteristic “floating” or “swinging” heart pattern is observed due to its pendulum-like movements [8].

In the presence of pericardial effusion, recognition of cardiac tamponade syndrome is of major clinical importance; it is diagnosed by comparing the characteristic clinical picture and echocardiographic signs [9]. Of 200 patients participating in the study, cardiac tamponade was diagnosed in 64 animals (32.0%). The diagnostic criteria for diagnosing cardiac tamponade obtained by the authors adhere to the data published in other studies [10]. The most common echocardiographic signs of cardiac tamponade were: right atrial collapse — detected in 65.6% (n=42), dilation of caudal vena cava and reduction of its collapsibility amplitude (to less than 50% during inhalation) — detected in 53.1% (n=34), right ventricular diastolic collapse— detected in 31.2% (n=20).

A comparison of physical examination data and echocardiographic signs revealed that symptoms such as the increase in the volume of the abdomen due to ascitic fluid accumulation, progressing weakness, and exercise tolerance decrease are diagnosed in patients with echocardiographic signs of dilation of caudal vena cava and reduction of its collapsibility amplitude to less than 50% and right atrial collapse. This is explained by impaired venous return in systemic circulation against the background of right ventricular end-diastolic pressure increase resulting from the pericardial fluid pressure. In the presence of all the above listed echocardiographic signs of cardiac tamponade (Table 5), including the paradoxical septal motion and reduction of cardiac chambers due to decrease in venous blood flow, the persistent arterial hypotension (with mean systolic blood pressure of 80 mmHg or lower) was observed in animals. The primary diagnostics of cardiac tamponade is based on a constellation of patient’s clinical symptoms and physical examination results. Echocardiography is a confirmatory and expert method that allows visualization of pericardial effusion, assessment of its volume, identification of the signs of cardiac compression and, in some cases, differentiation of the causes of effusion.

When assessing the effusion, the hemorrhagic nature of the fluid was most often diagnosed (91.8%), which is consistent with the literature data [11][12], whereas signs of acute hemorrhagic effusion were detected only in 4.9% of patients. The most common cause of hemorrhagic effusion in dogs was cardiac neoplasia – 55.7% of cases, of which cardiac hemangiosarcoma was histologically diagnosed in 22%, which, in turn, is considered one of the common neoplastic causes of pericardial effusions [13][14]. In 26% of cases, histological confirmation of a basal cardiac tumor  — chemodectoma was obtained. According to echocardiography, out of 200 patients, cardiac tumors were detected in 86 dogs, of which 32.5% were right heart tumors without histological confirmation, and in 53.5% of cases, basal cardiac tumors were determined. Unknown cause of effusion was the second most common cause and may also include patients with idiopathic pericardial effusion, which is consistent with the previous studies [11][15]. Modified transudate was recorded in 4.9% of cases, including in cases of congestive heart failure. Transudate caused by protein-losing enteropathy and septic exudate caused by myopericarditis were the rarest types of pericardial effusions. Cytological examination of pericardial fluid is deemed to be less informative and depends on the tumor type and effusion hematocrit [16][17]. Hemorrhagic effusion with cytological signs of a tumor was observed in 9.8% of cases, with large cell lymphoma being the most commonly diagnosed tumor, which is consistent with the foreign literature data [18][19]. In this study, in 64 dogs with cardiac tamponade, cardiac neoplasms were diagnosed in 55.7% by echocardiography. The development of cardiac tamponade in such patients may be explained not only by mechanical compression of the vessels by the tumor, but also by their bleeding, which can cause a surge in intrapericardial pressure, acute dilation of pericardial cavity and, as a consequence, cardiac tamponade.

Histological examination of cardiac and pericardial tumors excised during surgery is the “gold standard” for verifying the presence of cancer. According to the results of our study, of 16 dogs with cardiac and pericardial tumors identified by echocardiography, cardiac neoplasia was histologically confirmed in 12 animals: chemodectoma in 26% of cases and hemangiosarcoma in 22%. These types of cardiac tumors are the most common, as described above. In one patient suspected of having pericardial neoplasia, pericardial mesothelioma was diagnosed. In three dogs, histological examination of the cardiac or pericardial tumor turned to be inconclusive, and in one dog, a tumor biopsy was not performed. Pericarditis as a histological diagnosis was found in 39% of cases.

Most dogs with pericardial effusion were male, of medium to large breeds, weighing between 26 and 45 kg: mixed breeds (19%), Labrador Retrievers (6%), Golden Retrievers (4%), Cane Corso (2.5%), American Staffordshire Terriers (2.5%), German Shepherd Dogs (2%) — these breeds have been previously mentioned in other studies too [11][15][20]. Small dog breeds were represented by: French Bulldogs (9.5%), Chihuahuas (5.5%) and Yorkshire Terriers (5%). In this study, cardiac tamponade was most commonly diagnosed in mixed breeds (15.6%), French Bulldogs (14%), Golden Retrievers (7.8%), Yorkshire Terriers (6.25%), Siberian Huskies (4.6%), and Jack Russell Terriers (4.6%). It’s worth noting that in all French Bulldogs with cardiac tamponade, basal cardiac tumors were detected by echocardiography. In Golden Retrievers, Siberian Huskies, and Yorkshire Terriers, a common cause of cardiac tamponade was right heart tumor. Cases of right heart hemangiosarcoma diagnosed in Golden Retrievers are frequently described in scientific studies [21–23]. The unknown cause, was most common in Labrador Retrievers and Jack Russell Terriers.

Thus, as a result of the data analysis, the following most common clinical, physical and echocardiographic signs of cardiac tamponade were established: an increase in volume of the abdomen (50%), sudden weakness/syncope (37.5%), progressing weakness/decreased activity (62.5%), dyspnea (32.8%), pallor of the mucous membranes (15.6%), muffled heart sounds (75%), weak pulse waveform (35.9%), arrhythmic pulse (21.8%), paradoxical pulse (7.8%), right atrial collapse (65.6%), right ventricular diastolic collapse (31.2%), dilation of caudal vena cava and reduction of its collapsibility amplitude to less than 50% during inhalation (53.1%). A relationship was found between the clinical symptoms such as the increase in the volume of the abdomen due to the accumulation of ascitic fluid, progressing weakness and exercise tolerance decrease, and the echocardiographic signs of cardiac tamponade such as dilation of caudal vena cava and reduction of its collapsibility amplitude to less than 50% and right atrial collapse. The serious condition of a patient with developed arterial hypotension and cardiogenic shock will be accompanied by all EchoCG signs of cardiac tamponade, including paradoxical septal motion and weak filling of the left cardiac chambers. Based on the results of cytological examination, the hemorrhagic type of pericardial effusion was identified to be the most common one, accompanying cardiac and pericardial tumors in dogs of the older age group of medium and large breeds.