Dynamic left ventricular outflow tract obstruction (DLVOTO) is common in cats with hypertrophic obstructive cardiomyopathy but is rare in dogs. Diastolic dysfunction is expected to play a major role in the pathophysiology of this disease but due to the paucity of reported canine cases this remains speculative.

This study aimed to:

1.  Evaluate the clinical features, echocardiographic findings and characterise the myocardial function, using standard 2D, M mode, Doppler echocardiography and pulsed wave tissue Doppler imaging (pw-TDI) of dogs with naturally occurring DLVOTO, and

2.  To investigate the clinical outcome and response to therapy in these dogs.

Five cases were identified. Two cases were retrospectively reviewed and three cases were prospectively evaluated. 3/5 dogs were terrier breeds. One dog had frequent syncope, but 4/5 were asymptomatic. All were normotensive and congenital heart disease was excluded. All had variable intensity left systolic heart murmur. Doppler echocardiography revealed concentric left ventricular hypertrophy (LVH), systolic anterior motion of the mitral valve and increased LV outflow velocities with scimitar shaped aortic Doppler flow profile in all case. The syncopal patient also had significant dynamic right ventricular outflow tract obstruction. Pw-TDI interrogation of the interventricular septum revealed E'/A' reversal in all but one patient. 3/5 received treatment with a β₁-adrenergic antagonist and 2/5 received no medication. Echocardiographic re-evaluation was performed in 4 patients showing regression of LVH and partial or total resolution of the DLVOTO in all patients Improvement in diastolic function (normalisation of the reversed PW-TDI E'/A' ratio) was seen in the treated group. One untreated dog showed normal echocardiographic values at 3 months. This patient showed the fastest improvement but had normal PW-TDI values on both initial and follow up evaluation. One untreated dog normalised after two years, and the two treated dogs achieved this within 6 months. The frequency of syncope was significantly reduced in the collapsing dog, but no repeat echo was possible.

This study supports evidence for the possible self-limiting nature of the disease as spontaneous resolution is reported here in two cases. Despite this, treatment with a β₁-adrenergic antagonist appears to result in faster improvement. Presence of Pw-TDI abnormalities constitute further evidence for significant diastolic dysfunction associated with the hypertrophy. Analysis of diastolic function in cases of canine DLVOTO provides valuable information that can be used to add significant prognostic information, monitoring of disease progression and response to treatment.