Echocardiographic Assessment of the Left Ventricular Outflow Tract in the Boxer

Background: Soft, variable ejection murmurs are common in Boxers and are associated with increased left ventricular outflow tract (LVOT) ejection velocities. Whether these murmurs are physiologic or indicate mild aortic stenosis is controversial. Ejection velocity is impacted by LVOT area and ventricular stroke volume (SV), suggesting that these variables are pertinent to murmur development. Hypothesis: Boxers with ejection murmurs have a smaller LVOT and equivalent SV indices, compared with values in dogs without murmurs. Animals: Three age-and weight-matched groups of dogs—15 Boxers with soft ejection murmurs (group I); 15 Boxers without murmurs (group II); and 15 nonBoxer dogs without murmurs (group III) — were studied. Methods: All dogs underwent 2-dimensional and Doppler echocardiographic examinations. The LVOT size at multiple levels; LVOT ejection velocity, stroke distance, and SV index; and right ventricular SV index were determined and compared by analysis of variance. Results: Indexed LVOT areas in Boxer groups were not different, but were significantly smaller than those of non-Boxer dogs. Ejection velocities and stroke distances were significantly different across all groups, with group I having the highest and group III having the lowest values. Doppler SV indices (ml/m2) for group-I versus group-II Boxers were 70±16(SD) versus 62±12 for the LVOT (P= .27) and 58±12 versus 48±9 for the right ventricle (P= .14). Conclusions and clinical importance: These data suggest that a relatively smaller LVOT in Boxers predisposes them to increased ejection velocity and development of murmurs. The contribution of SV to the genesis of these often labile murmurs requires additional study.