Allometric Scaling of M‐Mode Cardiac Measurements in Normal Adult Dogs

Abstract

Indices for M-mode measurements in dogs usually have been based on the assumption that a linear relationship exists between these measurements and body weight (BW) or body surface area (BSA). The relationships between the geometry of 3-dimensional objects do not support this assumption. The purposes of this study were to retrospectively examine M-mode data from a large number of dogs of varying sizes and breeds that were examined by a large number of ultrasonographers, to use the allometric equation to determine the appropriate BW exponent required to predict these cardiac dimensions, and to determine normal mean values and prediction intervals for common M-mode variables. Linear regression analyses of data from 494 dogs (2.2-95 kg) revealed a good correlation between M-mode measurements and BW after logarithmic transformation of the data (r 2 ϭ .55-.88). Most variables were most closely related to an index of body length, BW 1/3 , although the exponent that best predicted diastolic and systolic left ventricular wall thicknesses was closer to 0.25. No variable indexed well to BW or BSA. With these data, appropriate mean values and prediction intervals were calculated for normal dogs, allowing veterinarians to correctly and appropriately index M-mode values. The equations developed from this study appear to be applicable to adult dogs of most breeds.Key words: Canine; Echocardiography; Heart; Reference range. M-mode echocardiography is commonly used to measure linear cardiac dimensions of cardiac chambers, walls, and great vessels. Because adult dogs range in weight from Ͻ2 to Ͼ90 kg, any definition of normal heart size must take into account the variation caused by differences in body size. Consequently, it is important to identify equations that accurately describe the relationships between body size and cardiac dimensions. Several authors have published equations, nomograms, or tables that are intended to be used as reference ranges for M-mode measurements for adult dogs of varying size and breed. [1][2][3][4][5][6][7][8][9][10][11] Critics have questioned the usefulness of some of these reference ranges because of small sample sizes, lack of data points for the extremes of body size, wide prediction intervals, and the use of inappropriate statistical methods. 5,[10][11][12][13][14][15] In addition, many authors have assumed that a linear relationship exists between linear cardiac dimensions and either body weight (BW) or body surface area (BSA), which might not be true. Evidence also has been presented suggesting that a general equation might never be accurate because breed is a factor Davis, CA 95616-8747; e-mail: cccornell@ucdavis.edu. Submitted March 7, 2003; Revised July 1 and October 31, 2003; Accepted December 11, 2003. Copyright that can cause M-mode measurements in dogs of the same weight to differ. 13The statistical methods used to produce the previously mentioned reference ranges were not described in every case, but in 2 studies in which the authors detailed their methods, reference ranges we...

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