Double-chambered right ventricle (DCRV) is an uncommon congenital heart disease in humans and dogs (9,12). Anomalous muscle bands or endogenous trabecular hypertrophy may cause abnormal separation (9). Two chambers exist: an inflow chamber with high pressure, and an outflow chamber with low pressure (8-11). In humans, DCRV is characterized by pressure gradients of over 20 mmHg between the two chambers and is frequently associated with other anomalies, including Ebstein’s anomaly, membranous ventricular septal defect, and double-outlet right ventricle (9). However, such association has not been identified in dogs.

The obstruction creates variable sized orifices for the two chambers (1). This pressure overload creates the same pathophysiologic changes within the heart as pulmonic valvular stenosis (1,3,10,18). Also, the high pressure chamber can lead to tricuspid insufficiency without any structural tricuspid valvular disease (1). Using echocardiography to diagnose this anomaly by confirming the anomalous muscle bands or turbulent flow within the right ventricle (1). We herein present a case of DCRV in an old, Standard Poodle dog having low pressure gradient between two chambers.

A 12-year-old, 30-kg, intact male Standard Poodle was presented to the Veterinary Medicine Teaching Hospital of Kangwon National University for intermittent weakness, dyspnea under stressful situations, and nocturnal cough. He had no other previous illnesses except for otitis externa.

On physical examination, the dog had a slightly increased heart rate, varying from 160 to 220 bpm, with an irregular rhythm. A grade IV/VI systolic murmur was auscultated on both chest walls. Femoral pulse quality was normal, but pulse deficits were present. Systolic blood pressure was 130 mmHg. The capillary refill time was normal (<2 s), with a pink mucous membrane color. No remarkable lung sounds were noted. Blood analysis revealed mild elevations in liver panel levels (alanine aminotransferase, 149.77 U/L [reference interval, 19-70] U/L; and alkaline phosphatase, 185 [reference interval, 15-127]) and significant elevations in cardiac biochemical marker levels (cardiac troponin I, 0.22 ng/mL [reference interval, 0.03-0.12]; and N-terminal pro-brain natriuretic peptide 4169.5 pmol/L [reference interval, 0-900], vCheck canine NT-proBNP [V200 Analyzer, Bionote, Korea]). Since there was an irregularity in cardiac rhythm, electrocardiography was performed. Irregularly irregular tachyarrhythmia without significant p-waves was confirmed, and f-waves were present. The QRS complex was narrow (<0.07 s) and had a normal morphology. Based on this irregularity with f-waves, atrial fibrillation was diagnosed (Fig. 1). Thoracic and abdominal radiography revealed marked generalized cardiomegaly (vertebral heart score, 12.6 v) (Fig. 2A, B) and mild hepatomegaly (liver length: T11 vertebral length ratio, 7.71) (Fig. 2C). Two-dimensional echocardiographic examination revealed a severely enlarged right ventricle and atrium (Fig. 3A). A thin membrane dividing the right ventricle before the right ventricular outflow tract was observed (Fig. 3B). The proximal chamber ventricular free wall showed focal concentric hypertrophy with eccentric changes in the rest of the ventricular wall (Fig. 3C). Moreover, there was turbulent flow along the obstruction area (Fig. 3D). However, it was not possible to measure flow velocity across the obstructed area. The estimated pressure gradient between the two chambers was 18 mmHg. Severe tricuspid regurgitation with a velocity of 2.75 m/s due to right chamber dilation was confirmed by color and continuous-wave Doppler echocardiography (Fig. 3E, F). Pulmonic valve degeneration or regurgitation was not observed. Right ventricular systolic function decreased (fractional area change [FAC], 28.6% [reference interval, 33.9-56.3%]) (17). There was no other evidence of pulmonary hypertension other than tricuspid regurgitation.

Figure 1. Electrocardiography (ECG) results of the patient. ECG reveals irregularly irregular rhythm without noticeable p-waves. F-waves can also be identified. Accordingly, atrial fibrillation can be diagnosed.

Figure 2. Radiographs from the dog with double-chambered right ventricle (DCRV). Generalized cardiomegaly (vertebral heart score, 12.6 v = 6.4 [L] + 6.2 [S]) with increased sternal contact can be observed in thoracic radiographs (A, B). Abdominal radiograph shows hepatomegaly (liver length: T11 vertebral length ratio, 7.71) (C).

Figure 3. Echocardiographic findings. A severely enlarged right heart can be observed on the right parasternal long axis 4-chamber view (A). A thin fibromuscular structure dividing the right ventricle can be observed (white arrow) in the right parasternal short axis view (B). There is focal muscle hypertrophy before the obstructive lesion in the right parasternal short axis view. Wall thickness is 20.8 mm (C). Color Doppler echocardiography revealed turbulent flow along the obstructive area in the mid-right ventricle. Unfortunately, we were unable to evaluate flow velocity across the fibromuscular region, though estimated pressure gradient was 18 mmHg (D). Color Doppler echocardiography at the right parasternal long-axis 4 chamber view shows severe tricuspid regurgitation due to inadequate coaptation of tricuspid leaflets (E). The regurgitant flow velocity is 2.75 m/s, and the peak pressure gradient is 30.3 mmHg on continuous wave Doppler echocardiography at the left apical 4 chamber view (F). AO, aorta; LA, left atrium; LV, left ventricle; MPA, main pulmonary artery; PV, pulmonic valve; RA, right atrium; RV, right ventricle; RVO, right ventricular outflow.

Accordingly, the patient was diagnosed with DCRV, right-sided congestive heart failure, and atrial fibrillation. To control the symptoms and mild hepatomegaly, 1 mg/kg (PO q 12 h) of furosemide (Lasix; HANDOK, Seoul, Korea), 0.25 mg/kg (PO q 12 h) of pimobendan (Vetmedin; Boehringer Ingelheim, Ingelheim am Rhein, Germany), 0.5 mg/kg (PO q 12 h) of enalapril (Enalapril Maleate; CMG pharmaceutical, Seoul, Korea), 0.5 mg/kg (PO q 12 h) of diltiazem (Herben; HKinno.N, Cheongju, Korea), 0.5 mg/kg (PO q 12 h) of omeprazole (Omed; SK chemicals, Seongnam, Korea), and SAMe (S-adenosylmethionine; Zentonil; Vetoquinol, Lure, France) were prescribed.

After treatment, according to the owner, the dog experienced no further weakness or dyspnea. A physical examination performed again revealed an irregular heart rate of 140-160 bpm. Atrial fibrillation was still present on electrocardiography; however, the average heart rate in the hospital was 130 bpm. Radiographic evaluation showed slightly decreased cardiac size (Fig. 4A) and reduced hepatomegaly (vertebral heart score, 11.8 v; and liver length: T11 vertebral length ratio, 6.82) (Fig. 4B). Echocardiography confirmed elevated right ventricular systolic function (FAC, 44.2%). Overall, the results were good and the same medications were prescribed.

Figure 4. Radiographs after 3 months of medical therapy. Generalized cardiomegaly is slightly decreased in the thoracic radiograph (vertebral heart score, 11.8 v = 5.6 v [L] + 6.2 v [S]) (A). Abdominal radiograph shows reduced hepatomegaly (liver length: T11 vertebral length ratio, 6.8) (B).

DCRV is one of the most uncommon congenital heart diseases in dogs, with a known prevalence of 1%-1.8% among dogs with congenital heart disease (2,12). The two main classifications of this congenital anomaly can be made according to the differences in the structure that divides the right ventricle (13). Type 1 is caused by anomalous muscle bands, whereas type 2 is caused by trabecular hypertrophy (13). Moreover, the obstructive lesion is described as “high” when it is close to the pulmonary valve and “low” when it is close to the apex (4,5). There have been some discussions regarding the terminology used to name this anomaly. Primary infundibular stenosis, mid-ventricular stenosis, and DCRV are the nomenclatures used (1). These can be distinguished based on the location and characteristics of the obstruction. The cardiac remodeling and hemodynamics of DCRV are influenced by pressure overload in the right ventricle, resulting in severe right-sided congestive heart failure, eventually like pulmonary stenosis (3,10,18). However, some differences exist between these congenital diseases, including breed prevalence and treatment implications. DCRV is usually associated with other malformations (2,9,12); however, in this case, only isolated DCRV was observed.

Treatment options to control this mid-ventricular obstruction include surgical resection, catheter-based procedures, and conservative medical therapy (10,18,20). Of these modalities, surgery is the most successful (10,15,18,19). However, availability and affordability are major obstacles for this approach (10,15). Balloon dilation of the obstruction has been tried in several cases (6,10,14,15,20), though not all cases showed the best results. Some patients have favorable long-term results (6,20). The medical management of DCRV is almost the same as that of pulmonic stenosis (9,18). To minimize cardiac workload and excessive oxygen consumption, beta-blockers are commonly prescribed when there is no evidence of congestion.

Few studies have reported DCRV in dogs, most of whom die at a young age (3,12). Symptoms are progressive, resulting exercise intolerance, cough, cyanosis, panting, syncope, and sudden death in humans, dogs, and cats (3,7). However, if patients undergo surgery or balloon dilation, the prognosis is good (10,20). In this case, the patient was diagnosed at 12 years of age without significant congestive signs. The pressure gradient across the ostium was believed to be 18 mmHg, which is much less than that in other cases (3,6,10,20). In humans, when a patient does not present with significant coexisting defects, surgical intervention may not be necessary until a pressure gradient of 40 mmHg is reached (5). One study reported that a 71-year-old patient with DCRV refused surgery and was kept on beta-blockers only (16). However, DCRV is typically progressive, resulting in increasing pressure gradients and right ventricular functional impairment if not treated in a timely manner (3,10). Therefore, surgery may be required in some patients later in the clinical course of the disease.

Few studies have reported DCRV in dogs, and its epidemiology in the aforementioned population has not been fully elucidated. To our knowledge, this study is the first to report a very old Standard Poodle dog with DCRV that was managed with conservative medical therapy for over a year.

DCRV is a rare congenital disease in dogs. When an old dog presents with clinical signs of heart disease, especially of the right-sided heart, DCRV should be considered because there are some differences between DCRV and pulmonic stenosis. Although surgery is the best treatment option for DCRV, supportive medical therapy could be the second best option depending on the severity of the pressure gradient across the obstruction, as in this case.

This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science (No. NRF-2020R1C1C1007282).

The authors have no conflicting interests.