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For indexing cardiac measures in small animal models, tibia length (TL) is a recommended surrogate for body weight (BW) that aims to avoid biases because of disease-induced BW changes. However, we question if indexing by TL is mathematically correct. This study aimed to investigate the relation between TL and BW, heart weight, ventricular weights, and left ventricular diameter to optimize the current common practice of indexing cardiac parameters in small animal models. In 29 healthy Wistar rats (age 5–34 wk) and 116 healthy Black 6 mice (age 3–17 wk), BW appeared to scale nonlinearly to TL 1  but linearly to TL 3 . Formulas for indexing cardiac weights were derived. To illustrate the effects of indexing, cardiac weights between the 50% with highest BW and the 50% with lowest BW were compared. The nonindexed cardiac weights differed significantly between groups, as could be expected ( P &lt; 0.001). However, after indexing by TL 1 , indexed cardiac weights remained significantly different between groups ( P &lt; 0.001). With the derived formulas for indexing, indexed cardiac weights were similar between groups. In healthy rats and mice, BW and heart weights scale linearly to TL 3 . This indicates that not TL 1  but TL 3  is the optimal surrogate for BW. New formulas for indexing heart weight and isolated ventricular weights are provided, and we propose a concept in which cardiac parameters should not all be indexed to the same measure but one-dimensional measures to BW 1/3  or TL 1 , two-dimensional measures to BW 2/3  or TL 2 , and three-dimensional measures to BW or TL 3 .    NEW &amp; NOTEWORTHY In healthy rats and mice, body weight (BW) scales linearly to tibia length (TL) to the power of three (TL 3 ). This indicates that for indexing cardiac parameters, not TL 1  but TL 3  is the optimal surrogate for BW. New formulas for indexing heart weight and isolated ventricular weights are provided, and we propose a concept of dimensionally consistent indexing. This concept is proposed to be widely applied in small animal experiments.

A novel method optimizing the normalization of cardiac parameters in small animal models: the importance of dimensional indexing