M‐Mode Echocardiographic Ratio Indices in Normal Dogs, Cats, and Horses: A Novel Quantitative Method

Abstract

A novel method for quantitative echocardiographic interpretations is introduced based on the calculation of ratio indices in which each raw M-mode measurement is divided by the aortic root dimension (Ao). ''Aorta-based'' indices were calculated with the animal's measured aortic root dimension (Ao m ) as the length standard. Conversely, ''weight-based'' indices employed an idealized estimate of aortic dimension (Ao w ) with a weighted least squares linear regression against the cube root of body weight (Ao w ϭ kW 1/3 ). Use of these indices circumvented undesirable statistical characteristics inherent in linear regression of echocardiographic dimensions against body weight and, to a lesser extent, body surface area. Compared with the regressions, ratio indices resulted in substantial refinement of the predictive range for each M-mode measurement in dogs, particularly with decreasing body size. Weight-based indices outperformed aorta-based indices in this regard. To refine the predictive range, neither type of index was clearly advantageous in cats compared with the simple average method typically employed for that species. Several of the raw Mmode measurements, however, were correlated with body weight in cats and horses, indicating the need for an appropriate correction for body size in these species. The ratio index method was suitable for this purpose. Summary statistics derived from normal dogs (n ϭ 53), cats (n ϭ 32), and horses (n ϭ 17) are presented for each index, including novel clinical indices calculated from area ratios. The latter were designed to represent body size-adjusted left ventricular stroke area (ie, volume overload) and myocardial wall area (ie, hypertrophy).Key words: Cardiac; Echocardiography; Heart. E chocardiographic studies have become routine for the diagnosis and evaluation of heart disease in veterinary medicine. Two-dimensional echocardiography (2DE) is used to visualize complex anatomic relationships, and Mmode echocardiography is used predominantly for quantification of cardiac dimensions. Increasingly, 2DE also is used for quantification. [1][2][3][4][5] Numerous quantitative evaluations of M-mode echocardiographic and 2DE recordings have been published. Echocardiographic interpretations by cardiologists, however, often are accomplished by visual inspection of the 2DE in real time and not necessarily by reference to published normal dimensions. This implies that these interpretations are the result of comparisons between structures visible on the 2DE in conjunction with extensive experience and knowledge of cardiac pathophysiology. An experienced echocardiographer is distinguished by a practiced eye and intimate familiarity with these relationships.Quantitative echocardiographic measurements still are required to establish whether cardiac dimensions are appropriate for the size of an individual. Published regressions of M-mode dimensions against body mass, however, might not agree with the visual interpretation of the cardiologist. Confidence intervals of these regressions ar...

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