This study evaluated the changes in cardiac chamber dimensions and functions and right pulmonary artery distensibility (RPAD) index in dogs with canine heartworm infection using B-mode and M-mode echocardiography. Echocardiography was performed using an ultrasound machine with 5 MHz microconvex scanner in 15 untreated heartworm-positive dogs, aged 3.8±2.62 years old and weighing 9.78± 4.99 kg. Chamber measurements and ventricular systolic function were measured using B-mode and M-mode in parasternal views, while pulmonary hypertension was measured and evaluated indirectly based on RPAD index and left ventricle eccentricity index using B-mode in parasternal views. Left and right ventricular dimensions and ejection fraction in heartworm-infected dogs were lower than expected normal values based on their body weights. Left ventricular function indices, such as fractional shortening, cardiac output and end-diastolic volume index were within normal values. Pulmonary hypertension was also evident based on RPAD index, and heartworm-infected dogs showed right ventricular wall enlargement and right-sided chamber dilation. The results suggest that left and right ventricular dimensions, ejection fraction and RPAD index in dogs with canine heartworm infection may differ from expected values based on their body weights and from established reference values.