Dissertation Abstract

Diagnostic imaging of cardio-pulmonary structures in normal dogs and dogs with mitral regurgitation

The general aim of this thesis was to evaluate two different imaging techniques used to evaluate the heart of dogs with mitral regurgitation, one for measurements of left atrial size and one for general cardiac size and to compare cardiopulmonary imaging with plasma levels of natriuretic peptides. A more specific aim was to study the variability in objective measurements of left atrial and cardiac size and subjective evaluation of general cardiomegaly, left atrial enlargement and radiologic signs of congestive heart failure among groups of observers with varying experience in thoracic radiology. The experience levels included were; experienced radiologists, experienced small animal clinicians, trainees for a small animal specialist degree and veterinary students. General heart enlargement can be monitored by objective measurements but there is no way to objectively estimate left atrial size on radiographs. An established index method for measurements of left atrial size on M-mode echocardiogram was compared to an index that uses the two-dimensional (2-D) image for dimensional measurements. It was shown that the 2-D index can be used as a good predictor of left atrial size. The 2-D index had a greater sensitivity for left atrial enlargement compared to the M-mode index.

Plasma levels of natriuretic peptides (ANP, NT-proANP and BNP) were studied as endogenous markers for increase in left atrial and ventricular size and left ventricular dysfunction. The plasma levels of the studied peptides did not increase in any significant way until there was radiologic and echocardiographic evidence of cardiac enlargement. However, the peptides were shown to be efficient in separating compensated from decompensated heart failure.

In the objective measurement of heart size the interobserver variation of the vertebral heart size method had a mean variation among the 16 observers of around 1.0 VHS units. No difference in mean VHS values was found among the four groups of observers. The subjective evaluation had a large discrepancy amongst both the observers and between the observers and the gold standard. In general the observers overdiagnosed signs of congestive heart failure and underdiagnosed left atrial and general heart enlargement. Accuracy in the subjective interpretation of general cardiomegaly, left atrial enlargement and radiologic signs of congestive heart failure was not in any conclusive way affected by observer experience. Objective measurements of cardiac size were independent of observer experience but dependent on individual observers´ selection of reference points for measurements.

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