Right Heart Enlargement in Heartworm‐infected Dogs

Radiographically, the size of the right ventricle of 54 heartworm‐infected dogs was graded subjectively as normal, 1 +, 2 +, and 3 + and served to classify the dogs into groups A, B, C, and D. With M‐mode echocardiograms, right ventricular enlargement ratios (RVE ratios) were determined for each dog by dividing the measured right ventricular internal diastolic dimension (RVIDd) by the expected RVIDd of a normal dog of the same body weight. The normal RVIDd relation to body weight was derived from 25 healthy heartworm‐free control dogs. These two variables were linearly related with a correlation coefficient r 2 = 0.59. Means and standard deviations of the RVE ratio were calculated for each group. Despite wide ranges and considerable overlap, the ratio was significantly different (t‐test, p < 0.005) between groups with the exception of groups A and B, and C and D. The incidence of electrocardiographic signs of right ventricular hypertrophy was also determined for each group; it was 38% in group C and 62% in group D. From this data, it was concluded that M‐mode echocardiography is a very sensitive technique for the documentation of right ventricular dilatation in heartworm‐infected dogs, correlating with radiographic impressions of right ventricular enlargement.